A flood is an overflow of an expanse of water that submerges land.[1] The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water.[2] In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries.[3] While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area. Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods often cause damage to homes and businesses if they are placed in natural flood plains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding. The word "flood" comes from the Old English flod, a word common to Germanic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float; also compare with Latin fluctus, flumen). Deluge myths are mythical stories of a great flood sent by a deity or deities to destroy civilization as an act of divine retribution, and are featured in the mythology of many cultures. A flood is an overflow of an expanse of water that submerges land.[1] The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water.[2] In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries.[3] While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area. Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods often cause damage to homes and businesses if they are placed
in natural flood plains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding. The word "flood" comes from the Old English flod, a word common to Germanic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float; also compare with Latin fluctus, flumen). Deluge myths are mythical stories of a great flood sent by a deity or deities to destroy civilization as an act of divine retribution, and are featured in the mythology of many cultures. [edit]Principal [edit]Riverine
types and causes
Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of
a river's channel. Causes include heavy rains frommonsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction.
Fast kinds: include flash floods resulting from convective precipitation
(intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, orglacier. [edit]Estuarine
Commonly caused by a combination of sea tidal surges caused by storm-force winds.
A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category. [edit]Coastal
Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or
hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category. [edit]Catastrophic
Caused by a significant and unexpected event e.g. dam breakage, or as a result of
another hazard (e.g. earthquake or volcanic eruption). [edit]Muddy
A muddy flood is generated by run off on crop land.
A muddy flood is produced by an accumulation of runoff generated on cropland. Sediments are then detached by runoff and carried as suspended matter or bed load. Muddy runoff is more likely detected when it reaches inhabited areas. Muddy floods are therefore a hill slope process, and confusion with mudflows produced by mass movements should be avoided. [edit]Other
Floods can occur if water accumulates across an impermeable surface (e.g. from A series of storms moving over the same area. Dam-building beavers can flood low-lying urban and rural areas, often causing
rainfall) and cannot rapidly dissipate (i.e. gentle orientation or low evaporation).
Physical damage - Can damage any type of structure, including bridges, cars, Casualties - People and livestock die due to drowning. It can also lead to epidemics and
buildings, sewerage systems, roadways, and canals.
Water supplies - Contamination of water. Clean drinking water becomes scarce. Diseases - Unhygienic conditions. Spread of water-borne diseases. Crops and food supplies - Shortage of food crops can be caused due to loss of entire
harvest.[4] However, lowlands near rivers depend upon river silt deposited by floods in order to add nutrients to the local soil.
Trees - Non-tolerant species can die from suffocation.[5]
[edit]Tertiary/long-term effects Economic - Economic hardship, due to: temporary decline in tourism, rebuilding costs, food shortage leading to price increase ,etc. [edit]Control A tsunami (Japanese: 津波 [tsɯnami], lit. 'harbor wave';[1] English pronunciation: /suːˈnɑːmi/ sooNAH-mee)
or tidal wave is a series of water waves (called a tsunami wave train[2]) caused by
the displacement of a large volume of a body of water, usually an ocean, but can occur in large lakes. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded.[3] Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions. Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices),landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami. The Greek historian Thucydides was the first to relate tsunami to submarine earthquakes,[4][5] but understanding of tsunami's nature remained slim until the 20th century and is the subject of ongoing research. Many early geological, geographical, andoceanographic texts refer to tsunamis as "seismic sea waves." Some meteorological conditions, such as deep depressions that cause tropical cyclones, can generate a storm surge, called ameteotsunami, which can raise tides several metres above normal levels. The displacement comes from low atmospheric pressure within the centre of the depression. As these storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such a storm surge inundated Burma in May 2008.
Etymology
The term tsunami comes from the Japanese, meaning "harbor" (tsu, 津) and "wave" (nami, 波). (For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in the Japanese.[6]) Tsunami are sometimes referred to as tidal waves. In recent years, this term has fallen out of favor, especially in the scientific community, because tsunami actually have nothing to do with tides. The once-popular term derives from their most common appearance, which is that of an extraordinarily high tidal bore. Tsunami and tides both produce waves of water that move inland, but in the case of tsunami the inland movement of water is much greater and lasts for a longer
period, giving the impression of an incredibly high tide. Although the meanings of "tidal" include "resembling"[7] or "having the form or character of"[8] the tides, and the term tsunami is no more accurate because tsunami are not limited to harbours, use of the termtidal wave is discouraged by geologists and oceanographers. There are only a few other languages that have a native word for this disastrous wave. In the Tamil language, the word is aazhi peralai. In the Acehnese language, it is ië beuna or alôn buluëk [9] (Depending on the dialect. Note that in the fellow Austronesian language of Tagalog, a major language in the Philippines, alon means "wave".) On Simeulue island, off the western coast of Sumatra in Indonesia, in the Defayan language the word is smong, while in the Sigulai language it is emong.[10]
Generation mechanisms
The principal generation mechanism (or cause) of a tsunami is the displacement of a substantial volume of water or perturbation of the sea.[11] This displacement of water is usually attributed to either earthquakes, landslides, volcanic eruptions, or more rarely by meteorites and nuclear tests.[12][13] The waves formed in this way are then sustained by gravity. It is important to note that tides do not play any part in the generation of tsunamis, hence referring to tsunamis as 'tidal waves' is inaccurate.
Seismicity generated tsunamis
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position.[14] More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, due to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami.
Drawing of tectonic plate boundary beforeearthquake.
Overriding plate bulges under strain, causingtectonic uplift.
Plate slips, causingsubsidence and releasing energy into water.
The energy released produces tsunami waves.
Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres (12 in) above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas. On April 1, 1946, a magnitude-7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 metres (46 ft) high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska. Examples of tsunami at locations away from convergent boundaries include Storegga about 8,000 years ago, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis came from earthquakes which destabilized sediments, causing them to flow into the ocean and generate a tsunami. They dissipated before traveling transoceanic distances. The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of the sediments, an earthquake or a release of gas hydrates (methane etc.) The 1960 Valdivia earthquake (Mw 9.5) (19:11 hrs UTC), 1964 Alaska earthquake (Mw 9.2), and 2004 Indian Ocean earthquake (Mw 9.2) (00:58:53 UTC) are recent examples of powerfulmegathrust earthquakes that generated tsunamis (known as teletsunamis) that can cross entire oceans. Smaller (Mw 4.2) earthquakes in Japan can trigger tsunamis (called local andregional tsunamis) that can only devastate nearby coasts, but can do so in only a few minutes. In the 1950s, it was discovered that larger tsunamis than had previously been believed possible could be caused by giant landslides. These phenomena rapidly displace large water volumes, as energy from falling debris or expansion transfers to the water at a rate faster than the water can absorb. Their existence was confirmed in 1958, when a giant landslide inLituya Bay, Alaska, caused the highest wave ever recorded, which had a height of 524 metres (over 1700 feet). The wave didn't travel far, as it struck land almost immediately. Two people fishing in the bay were killed, but another boat amazingly managed to ride the wave. Scientists named these waves megatsunami. Scientists discovered that extremely large landslides from volcanic island collapses can generate megatsunami, that can travel trans-oceanic distances.
Characteristics
When the wave enters shallow water, it slows down and its amplitude (height) increases.
The wave further slows and amplifies as it hits land. Only the largest waves crest.
While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but due to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft).[15] This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage. As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously, producing a distinctly visible wave. Since the wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like a fast moving tidal bore.[16] Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front. When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed 'run up'. Run up is measured in metres above a reference sea level.[16] A large tsunami
may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run up.[17] About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, and bolides.
Drawback
If the first part of a tsunami to reach land is a trough—called a drawback—rather than a wave crest, the water along the shoreline recedes dramatically, exposing normally submerged areas. A drawback occurs because the water propagates outwards with the trough of the wave at its front. Drawback begins before the wave arrives at an interval equal to half of the wave's period. Drawback can exceed hundreds of metres, and people unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed. During the Indian Ocean tsunami, the sea withdrew and many people went onto the exposed sea bed to investigate.[citation needed] Photos show people walking on the normally submerged areas with the advancing wave in the background.[citation needed] Few survived.[citation needed]
Scales of intensity and magnitude
As with earthquakes, several attempts have been made to set up scales of tsunami intensity or magnitude to allow comparison between different events.[18]
Intensity scales
The first scales used routinely to measure the intensity of tsunami were the Sieberg-Ambraseys scale, used in the Mediterranean Sea and the Imamura-Iida intensity scale, used in thePacific Ocean. The latter scale was modified by Soloviev, who calculated the Tsunami intensity I according to the formula
where Hav is the average wave height along the nearest coast. This scale, known as the Soloviev-Imamura tsunami intensity scale, is used in the global tsunami catalogues compiled by the NGDC/NOAA and the Novosibirsk Tsunami Laboratory as the main parameter for the size of the tsunami.
Magnitude scales
The first scale that genuinely calculated a magnitude for a tsunami, rather than an intensity at a particular location was the ML scale proposed by Murty & Loomis based on the
potential energy.[18] Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely used. Abe introduced the tsunami magnitude scale Mt, calculated from,
where h is the maximum tsunami-wave amplitude (in m) measured by a tide gauge at a distance R from the epicenter, a, b & D are constants used to make the Mt scale match as closely as possible with the moment magnitude scale.[19]
Warnings and predictions
See also: Tsunami warning system
One of the deep water buoys used in theDART tsunami warning system
Drawbacks can serve as a brief warning. People who observe drawback (many survivors report an accompanying sucking sound), can survive only if they immediately run for high ground or seek the upper floors of nearby buildings. In 2004, ten-year old Tilly Smith of Surrey, England, was on Maikhao beach in Phuket, Thailand with her parents and sister, and having learned about tsunamis recently in school, told her family that a tsunami might be imminent. Her parents warned others minutes before the wave arrived, saving dozens of lives. She credited her geography teacher, Andrew Kearney. In the 2004 Indian Ocean tsunami drawback was not reported on the African coast or any other eastern coasts it reached. This was because the wave moved downwards
on the eastern side of the fault line and upwards on the western side. The western pulse hit coastal Africa and other western areas. A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, andseismologists analyse each earthquake and based on many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and automated systems can provide warnings immediately after an earthquake in time to save lives. One of the most successful systems uses bottom pressure sensors that are attached to buoys. The sensors constantly monitor the pressure of the overlying water column. This is deduced through the calculation:
where P = the overlying pressure in newtons per metre square, ρ = the density of the seawater= 1.1 x 103 kg/m3, g = the acceleration due to gravity= 9.8 m/s2 and h = the height of the water column in metres. Hence for a water column of 5,000 m depth the overlying pressure is equal to
or about 5500 tonnes-force per square metre. Regions with a high tsunami risk typically use tsunami warning systems to warn the population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs indicate evacuation routes. In Japan, the community is well-educated about earthquakes and tsunamis, and along the Japanese shorelines the tsunami warning signs are reminders of the natural hazards together with a network of warning sirens, typically at the top of the cliff of surroundings hills.[20] The Pacific Tsunami Warning System is based in Honolulu, Hawaiʻi. It monitors Pacific Ocean seismic activity. A sufficiently large earthquake magnitude and other information triggers a tsunami warning. While the subduction zones around the Pacific are seismically active, not all earthquakes generate tsunami. Computers assist in analysing the tsunami
risk of every earthquake that occurs in the Pacific Ocean and the adjoining land masses.
Tsunami hazard sign A tsunami warning atBamfield, British sign on Columbia
The monument to the victims of tsunami Tsunami memorial
a seawall in Kamakura atLaupahoehoe, Hawai inKanyakumari beach , Japan, 2004. i
A seawall at Tsu, Japan
Tsunami Evacuation Route signage alongU.S. Route 101, in Washington
As a direct result of the Indian Ocean tsunami, a re-appraisal of the tsunami threat for all coastal areas is being undertaken by national governments and the United Nations Disaster Mitigation Committee. A tsunami warning system is being installed in the Indian Ocean.
Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors relay information in real time. Based on these pressure readings and other seismic information and the seafloor's shape (bathymetry) and coastal topography, the models estimate the amplitude and surge height of the approaching tsunami. All Pacific Rim countries collaborate in the Tsunami Warning System and most regularly practice evacuation and other procedures. In Japan, such preparation is mandatory for government, local authorities, emergency services and the population. Some zoologists hypothesise that some animal species have an ability to sense subsonic Rayleigh waves from an earthquake or a tsunami. If correct, monitoring their behavior could provide advance warning of earthquakes, tsunami etc. However, the evidence is controversial and is not widely accepted. There are unsubstantiated claims about the Lisbon quake that some animals escaped to higher ground, while many other animals in the same areas drowned. The phenomenon was also noted by media sources in Sri Lanka in the 2004 Indian Ocean earthquake.[21][22] It is possible that certain animals (e.g., elephants) may have heard the sounds of the tsunami as it approached the coast. The elephants' reaction was to move away from the approaching noise. By contrast, some humans went to the shore to investigate and many drowned as a result. It is not possible to prevent a tsunami. However, in some tsunami-prone countries some earthquake engineering measures have been taken to reduce the damage caused on shore. Japan built many tsunami walls of up to 4.5 metres (15 ft) to protect populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunami. However, their effectiveness has been questioned, as tsunami often overtop the barriers. For instance, the Okushiri, Hokkaidō tsunami which struck Okushiri Island of Hokkaidō within two to five minutes of the earthquake on July 12, 1993 created waves as much as 30 metres (100 ft) tall—as high as a 10-story building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami, but it did not prevent major destruction and loss of life.[23]
Natural factors such as shoreline tree cover can mitigate tsunami effects. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed because trees such as coconut palms and mangroves absorbed the tsunami's energy. In one striking example, the village of Naluvedapathy in India's Tamil Nadu region suffered only minimal damage and few deaths because the wave broke against a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records.[24] Environmentalists have suggested tree planting along tsunami-prone seacoasts. Trees require years to grow to a useful size, but such plantations could offer a much cheaper and longerlasting means of tsunami mitigation than artificial barriers.
Mitigation
Natural barriers
A report published by the United Nations Environment Programme (UNEP) suggests that the tsunami of 26th December 2004 caused less damage in the areas where natural barriers were present, such as mangroves, coral reefs or coastal vegetation. A Japanese study of this tsunami in Sri Lanka used satellite imagery modelling to establish the parameters of coastal resistance as a function of different types of trees. [25]
History
The Samoan tsunami of September 2009
A devastated Marina beach in Chennaiafter the Indian Ocean Tsunami
Main article: Historic tsunami Destructive tsunamis have been recorded throughout history, for example there were 26 that caused 200 or more deaths in the last century alone.
[26]
Of these, many were recorded in the Asia–Pacific region, particularly
around Japan and Indonesia.
Ancient history
As early as 426 B.C. the Greek historian Thucydides inquired in his book History of the Peloponnesian War about the causes of tsunami, and was the first to argue that ocean earthquakes must be the cause.[4][5] The cause, in my opinion, of this phenomenon must be sought in the earthquake. At the point where its shock has been the most violent the sea is driven back, and suddenly recoiling with redoubled force, causes the inundation. Without an earthquake I do not see how such an accident could happen.[27] The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15-19) described the typical sequence of a tsunami, including an incipient earthquake, the sudden retreat of the sea and a following gigantic wave, after the 365 A.D. tsunami devastated Alexandria.[28][29]
2004 Indian Ocean tsunami
Main article: 2004 Indian Ocean earthquake and tsunami The 2004 Indian Ocean earthquake and tsunami killed over 200,000[30] people with many bodies either being lost to the sea or unidentified. According to an article in Geographical magazine (April 2008), the Indian Ocean tsunami of December 26, 2004 was not the worst that the region could expect. Professor Costas Synolakis of the Tsunami Research Center at the University of Southern California co-authored a paper in Geophysical Journal International which suggests that a future tsunami in the Indian Ocean basin could affect locations such asMadagascar, Singapore, Somalia, Western Australia, and many others.
As a weapon
b
Storm surge
From Wikipedia, the free encyclopedia
This article is about the meteorological terminology. For the fictional character, see Storm Surge (Transformers).
Impact of a storm surge
A storm surge is an offshore rise of water associated with a low pressure weather system, typically tropical cyclones and strong extratropical cyclones. Storm surges are caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also has a small secondary effect, as can the bathymetry of the body of water. It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems. The term "storm surge" in casual (non-scientific) use is storm tide; that is, it refers to the rise of water associated with the storm, plus tide, wave run-up, and freshwater flooding. When referencing storm surge height, it is important to clarify the usage, as well as the reference point.National Hurricane Center tropical cyclone reports reference storm
surge as water height above predicted astronomical tide level, and storm tide as water height above NGVD-29. Most casualties during a tropical cyclone occur during the storm surge. In areas where there is a significant difference between low tide and high tide, storm surges are particularly damaging when they occur at the time of a high tide. In these cases, this increases the difficulty of predicting the magnitude of a storm surge since it requires weather forecasts to be accurate to within a few hours. Storm surges can be produced by extratropical cyclones, such as the Night of the Big Wind of 1839 and the Storm of the Century (1993), but the most extreme storm surge events typically occur as a result of tropical cyclones. Factors that determine the surge heights for landfalling tropical cyclones include the speed, intensity, size of the radius of maximum winds (RMW), radius of the wind fields, angle of the track relative to the coastline, the physical characteristics of the coastline and the bathymetry of the water offshore. The SLOSH (Sea, Lake, and Overland Surges from Hurricanes) model is used to simulate surge from tropical cyclones.[1] Additionally, there is an extratropical storm surge model that is used to predict those effects.[2] The Galveston Hurricane of 1900, a Category 4 hurricane that struck Galveston, Texas, drove a devastating surge ashore; between 6,000 and 12,000 lives were lost, making it the deadliest natural disaster ever to strike the United States.[3] The deadliest storm surge caused by an extratropical cyclone in the twentieth century was the North Sea flood of 1953, which killed a total of over 2,000 people in the UK and the Netherlands [edit]Mechanics
Schematic of a storm surge.
At least five processes can be involved in altering tide levels during storms: the pressure effect, the direct wind effect, the effect of the Earth's rotation, the effect of waves, and the rainfall effect.[4] The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low atmospheric pressure and fall in regions of high atmospheric pressure. The rising water level will counteract the low atmospheric pressure such that the total pressure at some plane beneath the water surface remains constant. This effect is estimated at a 10 mm (0.39 in) increase in sea level for every millibar drop in atmospheric pressure.[4]
Strong surface winds cause surface currents perpendicular to the wind direction, by an effect known as the Ekman Spiral. Wind stresses cause a phenomenon referred to as "wind set-up", which is the tendency for water levels to increase at the downwind shore, and to decrease at the upwind shore. Intuitively, this is caused by the storm simply blowing the water towards one side of the basin in the direction of its winds. Because the Ekman Spiral effects spread vertically through the water, the effect is inversely proportional to depth. The pressure effect and the wind set-up on an open coast will be driven into bays in the same way as the astronomical tide.[4] The Earth's rotation causes the Coriolis effect, which bends currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When this bend brings the currents into more perpendicular contact with the shore it can amplify the surge, and when it bends the current away from the shore it has the effect of lessening the surge.[4] The effect of waves, while directly powered by the wind, is distinct from a storm's wind-powered currents. Powerful wind whips up large, strong waves in the direction of its movement.
[4]
Although these surface waves are responsible for very little water transport in open water, they
may be responsible for significant transport near the shore. When waves are breaking on a line more or less parallel to the beach they carry considerable water shoreward. As they break, the water particles moving toward the shore have considerable momentum and may run up a sloping beach to an elevation above the mean water line which may exceed twice the wave height before breaking.[5] The rainfall effect is experienced predominantly in estuaries. Hurricanes may dump as much as 12 in (300 mm) of rainfall in 24 hours over large areas, and higher rainfall densities in localized areas. As a result, watersheds can quickly surge water into the rivers that drain them. This can increase the water level near the head of tidal estuaries as storm-driven waters surging in from the ocean meet rainfall flowing from the estuary.[4] Surge and wave heights on shore are affected by the configuration and bathymetry of the ocean bottom. A narrow shelf, or one that has a steep drop from the shoreline and subsequently produces deep water in close proximity to the shoreline tends to produce a lower surge, but a higher and more powerful wave. This situation well exemplified by the southeast coast ofFlorida. The edge of the Floridian Plateau, where the water depths reach 91 metres (299 ft), lies just 3,000 m (9,800 ft) offshore of Palm Beach, Florida; just 7,000 m (23,000 ft) offshore, the depth increases to over 180 m (590 ft).[6] The 180 m (590 ft) depth contour followed southward from Palm Beach County lies more than 30,000 m (98,000 ft) to the east of the upper Keys. Conversely, coastlines along North America such as those along the Gulf of Mexico coast from Texas to Florida, and Asia such as the Bay of Bengal, have long, gently sloping shelves and
shallow water depths. On the Gulf side of Florida, the edge of the Floridian Plateau lies more than 160 kilometres (99 mi) offshore of Marco Island in Collier County. Florida Bay, lying between the Florida Keys and the mainland, is also very shallow; depths typically vary between 0.3 m (0.98 ft) and 2 m (6.6 ft).[7] These areas are subject to higher storm surges, but smaller waves. This difference is because in deeper water, a surge can be dispersed down and away from the hurricane. However, upon entering a shallow, gently sloping shelf, the surge can not be dispersed away, but is driven ashore by the wind stresses of the hurricane. Topography of the land surface is another important element in storm surge extent. Areas where the land lies less than a few meters above sea level are at particular risk from storm surge inundation.[4] For a given topography and bathymetry the surge height is not solely affected by peak wind speed; the size of the storm also affects the peak surge. With any storm the piled up water has an exit path to the sides and this escape mechanism is reduced in proportion to the surge force (for the same peak wind speed) as the storm covers more area. [edit]Measuring
surge
Surge can be measured directly at coastal tidal stations as the difference between the forecast tide and the observed rise of water.[8] Another method of measuring surge is by the deployment of pressure transducers along the coastline just ahead of an approaching tropical cyclone. This was first tested for Hurricane Rita in 2005.[9] These types of sensors can be placed in locations that will be submerged, and can accurately measure the height of water above them.[10] After surge from a cyclone has receded, teams of surveyors map high water marks (HWM) on land, in a rigorous and detailed process that includes photos and written descriptions of the marks. HWM denote the location and elevation of flood waters from a storm event. When HWM are analyzed, if the various components of the water height can be broken out so that the portion attributable to surge can be identified, then that mark can be classified as storm surge. Otherwise, it is classified as storm tide. HWM on land are referenced to a vertical datum (a reference coordinate system). During evaluation, HWM are divided into four categories based on the confidence in the mark; only HWM evaluated as "excellent" are used by NHC in post storm analysis of the surge.[11] Two different measures are used for storm tide and storm surge measurements. Storm tide is measured using a geodetic vertical datum (NGVD 29 or NAVD 88). Since storm surge is defined as the rise of water beyond what would be expected by the normal movement due to tides, storm surge is measured using tidal predictions, with the assumption that the tide prediction is well-known and only slowly varying in the region subject to the surge. Since tides are a localized phenomenon, storm surge can only be measured in relationship to a nearby tidal station. Tidal
bench mark information at a station provides a translation from the geodetic vertical datum to mean sea level (MSL) at that location, then subtracting the tidal prediction yields a surge height above the normal water height.[8][11] [edit]Records The highest storm tide noted in historical accounts was produced by the 1899 Cyclone Mahina, estimated at 13 meters (43 ft) at Bathurst Bay, Australia, but research published in 2000 noted the majority of this was likely wave run-up, due to the steep coastal topography.[12] In the United States, one of the greatest recorded storm surges was generated by 2005'sHurricane Katrina, which produced a maximum storm surge of more than 8 meters (25 ft) in the communities of Waveland, Bay St. Louis, Diamondhead, and Pass Christian inMississippi, with a storm surge height of 8.5 m (27.8 ft) in Pass Christian.[13][14][15] Another record storm surge occurred in this same area from Hurricane Camille in August 1969, with the highest storm tide of record noted from a HWM as 7.5 m (24.6 ft), also found in Pass Christian.[16] The worst storm surge, in terms of loss of life, was the 1970 Bhola cyclone and in general the Bay of Bengal is particularly prone to tidal surges.[17] [edit]SLOSH
Example of a SLOSH run
See also: Tropical cyclone forecasting The National Hurricane Center in the US, forecasts storm surge using the SLOSH model, which stands for Sea, Lake and Overland Surges from Hurricanes. The model is accurate to within 20 percent.[18] SLOSH inputs include the central pressure of a tropical cyclone, storm size, the cyclone's forward motion, its track, and maximum sustained winds. Local topography, bay and
river orientation, depth of the sea bottom, astronomical tides, as well as other physical features are taken into account, in a predefined grid referred to as a SLOSH basin. Overlapping SLOSH basins are defined for the southern and eastern coastline of the continental U.S.[19] Some storm simulations use more than one SLOSH basin; for instance, Katrina SLOSH model runs used both the Lake Ponchartrain / New Orleans basin, and the Mississippi Sound basin, for the northern Gulf of Mexico landfall. The final output from the model run will display the maximum envelope of water, or MEOW, that occurred at each location. To allow for track or forecast uncertainties, usually several model runs with varying input parameters are generated to create a map of MOMs, or Maximum of Maximums.[20] And for hurricane evacuation studies, a family of storms with representative tracks for the region, and varying intensity, eye diameter, and speed, are modeled to produce worst-case water heights for any tropical cyclone occurrence. The results of these studies are typically generated from several thousand SLOSH runs. These studies have been completed by USACE, under contract to the Federal Emergency Management Agency, for several states and are available on their Hurricane Evacuation Studies (HES) website.[21] They include coastal county maps, shaded to identify the minimum SSHS category of hurricane that will result in flooding, in each area of the county.[22] [edit]Mitigation Although meteorological surveys alert about hurricanes or severe storms, in the areas where the risk of coastal flooding is particularly high, there are specific storm surge warnings. These have been implemented, for instance, in Holland,[23] Spain,[24][25] the United States,[26]
[27]
and Great Britain.[28]
A prophylactic method introduced after the North Sea Flood of 1953 is the construction of dams and floodgates (storm surge barriers). They are open and allow free passage but close when the land is under threat of a storm surge. Major storm surge barriers are the Oosterscheldekering and Maeslantkering in the Netherlands which are part of the Delta Works project, and the Thames Barrier protecting London. Another modern development (in use in the Netherlands) is the creation of housing communities at the edges of wetlands with floating structures, restrained in position by vertical pylons.
[29]
Such wetlands can then be used to accommodate runoff and surges without causing damage
to the structures while also protecting conventional structures at somewhat higher low-lying elevations, provided that dikes prevent major surge intrusion. For mainland areas, storm surge is more of a threat when the storm strikes land from seaward, rather than approaching from landwards.[30]
[edit]Notes
Limnic eruption
From Wikipedia, the free encyclopedia
Lake Nyos, silty after a limnic eruption
A limnic eruption, also referred to as a lake overturn, is a rare type of natural disaster in which carbon dioxide (CO2) suddenly erupts from deep lake water, suffocating wildlife, livestock and humans. Such an eruption may also cause tsunamis in the lake as the rising CO2 displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption. Lakes in which such activity occurs may be known as limnically active lakes or exploding lakes. Some features of limnically active lakes include:
CO2-saturated incoming water A cool lake bottom indicating an absence of direct volcanic interaction with lake waters An upper and lower thermal layer with differing CO2 saturations Proximity to areas with volcanic activity
Scientists have recently determined, from investigations into the mass casualties in the 1980s at Lake Monoun and Lake Nyos, that limnic eruptions and volcanic eruptions, although indirectly related, are actually separate types of disaster events.[1] [edit]Historical
occurrences
Cow killed by the limnic eruption at Lake Nyos
To date, this phenomenon has been observed only twice. The first was in Cameroon at Lake Monoun in 1984, causing the asphyxiation and death of 37 people living nearby[2]. A second, deadlier eruption happened at neighbouring Lake Nyos in 1986, this time releasing over 80 millioncubic meters of CO2 and killing between 1,700 and 1,800 people, again by asphyxiation[3]. Due to the nature of the event, it is hard to determine if limnic eruptions have happened elsewhere. However, a third lake — Lake Kivu — containing massive amounts of dissolved CO2 exists on the border between the Democratic Republic of the Congo and Rwanda. Sample sediments from the lake were taken by Professor Robert Hecky from the University of Michigan which showed that an event caused living creatures in the lake to go extinct approximately every thousand years, and caused nearby vegetation to be swept back into the lake. The Messel pit fossil deposits of Messel, Germany, show evidence of a limnic eruption there in the early Eocene. Among the victims are perfectly preserved insects, frogs, turtles, crocodiles, birds, anteaters, insectivores, early primates and paleotheres. [edit]Causes
For a limnic eruption to occur, the lake must be nearly saturated with gas. In the two known cases, the major component was CO2, however, in Lake Kivu, scientists are concerned about the concentrations of methane gas as well. This CO2 may come from volcanic gas emitted from under the lake or from decomposition of organic material. Before a lake is saturated, it behaves like an unopened carbonated beverage (soft drink): the CO2 is dissolved in the water. In both the lake and the soft drink, CO2 dissolves much more readily at higher pressure. This is why bubbles in a can of soda only form after the drink is open; the pressure is released and the CO2 comes out of solution. In the case of lakes, the bottom is at a much higher pressure; the deeper it is, the higher the pressure at the bottom. This means that huge amounts of CO2 can be dissolved in large, deep lakes. Also, CO2 dissolves more readily in cooler water, such as that at the bottom of a lake. A small rise in watertemperature can lead to the release of a large amount of CO2. Once the lake is saturated with CO2, it is very unstable. A trigger is all that is needed to set off an eruption. In the case of the 1986 eruption at Lake Nyos, landslides were the suspected triggers, but an actual volcanic eruption, an earthquake, or even wind and rain storms are other possible triggers. In any case, the trigger pushes some of the saturated water higher in the lake, where the pressure is insufficient to keep the CO2 in solution. Bubbles start forming and the water is lifted even higher in the lake (buoyancy), where even more of the CO2 comes out of solution. This process forms a column of gas. At this point the water at the bottom of this column is pulled up by suction, and it too loses its CO2 in a runaway process. This eruption pours CO2 into the air and can also displace water to form a tsunami. There are several reasons this type of eruption is very rare. First, there must be a source of the CO2, so only regions with volcanic activity are at risk. Second, temperate lakes, such as North America's Great Lakes, turn over each spring and fall as a result of seasonal air temperature changes, mixing water from the bottom and top of the lake, so CO2 that builds up at the bottom of the lake is brought to the top where the pressure is too low for it to stay in solution and it escapes into the atmosphere. Finally, a lake must be quite deep to have enough pressure to dissolve large volumes of CO2. So only in deep, stable, tropical, volcanic lakes such as Lake Nyos are limnic eruptions possible. [edit]Consequences Once an eruption occurs, a large CO2 cloud forms above the lake and expands to the neighbouring region. Because CO2 is denser than air, it has a tendency to sink to the ground while pushing breathable air up. As a result, life forms that need to breathe oxygen suffocate once the CO2 cloud reaches them, as there is very little oxygen in the cloud. The CO2 can make
human bodily fluids very acidic, potentially causing CO2 poisoning. As victims gasp for air they actually hurt themselves more by sucking in the CO2 gas. At Lake Nyos, the gas cloud descended from the lake into a nearby village where it settled, killing nearly everyone. In this eruption, some people as far as 25 km (15.5 miles) from the lake died. A change in skin color on some bodies led scientists to think that the gas cloud may have contained a dissolved acid such as hydrogen chloride as well, but that hypothesis is disputed.
[4]
Many victims were found with blisters on their skin. This is believed to have been caused
by pressure ulcers, which likely formed from the low levels of oxygen present in the blood of those asphyxiated by the carbon dioxide.[5] Thousands of cattle and wild animals were also asphyxiated, but no official counts were made. On the other hand, vegetation nearby was mostly unaffected except for that which grew immediately adjacent to the lake. There the vegetation was damaged or destroyed by a 5-meter (16.4 ft.) tsunami from the violent eruption. [edit]A
possible solution: Degassing lakes
A siphon used by French scientists in an attempt to degas Lake Nyos.
Efforts have been under way for several years to develop a solution to remove the gas from these lakes and prevent a build-up that could lead to another catastrophe. A team of French scientists began experimenting at Lake Monoun and Lake Nyos in 1990 using siphons to degas the waters of these lakes in a controlled manner. A pipe is positioned vertically in the lake with its upper end above the water's surface. Water saturated with CO2 enters the bottom of the pipe and rises to the top. The lower pressure at the surface allows the gas to come out of solution. Interestingly, only a small amount of water has to initially be mechanically pumped through the pipe to start the flow. As the saturated water rises, the CO2 comes out of solution and forms bubbles. The natural buoyancy of the bubbles draws the water up the pipe at high velocity causing a large fountain at the surface. The degassifying water acts as a pump, drawing more water into the bottom of the pipe, and creating a self-sustaining flow. This is the same process that leads to a natural eruption, but in this case it is controlled by the size of the pipe.
Each pipe has a limited pumping capacity and several would be required for both Lake Monoun and Lake Nyos to degas a significant fraction of the deep lake water and render the lakes safe. The deep lake waters are slightly acidic due to the dissolved CO2 which causes corrosion to the pipes and electronics, necessitating ongoing maintenance. There are also fears that the CO2 from the pipes could settle on the surface of the lake forming a thin layer of unbreathable air and thus causing problems for wildlife. In January 2001, a single pipe was installed on Lake Nyos. A second pipe was installed at Lake Monoun in late 2002. These two pipes are thought to be sufficient to prevent an increase in CO2 levels, removing approximately the same amount of gas as that naturally entering at the lake bed. In January 2003, an 18-month project had been given approval to fully degas Lake Monoun.[6] The project appears to have been subsequently cancelled.[citation needed] [edit]Lake
Kivu's potential danger
Satellite image of Lake Kivu.
Lake Kivu is not only 2,000 times larger than Lake Nyos — it is also located in a far more densely populated area, with over two million people living along its shores. Fortunately, it has not reached a high level of CO2 saturation yet. If the water were to become heavily saturated, it could become an even greater risk to human and animal life, as it is located very close to a
potential trigger, Mount Nyiragongo, an active volcano that erupted in January 2002. It is also located in an active earthquake zone and close to other active volcanoes. While the lake could be degassed in a manner similar to Lake Monoun and Lake Nyos, due to the size of the lake and the volume of gas involved such an operation would be expensive, running into millions of dollars. A scheme initiated in 2010 to utilise methane trapped in the lake as a fuel source to generate electricity in Rwanda has led to a degree of CO2 degassing.
[7]
During the procedure for extracting the flammable gas used to fire power stations on the
shore, some CO2 is removed in a process known as scrubbing. It is as yet unclear whether enough of the gas will be removed in this way to ensure the danger of a limnic eruption posed by Lake Kivu will be completely eliminated
Tropical Storm Allison
From Wikipedia, the free encyclopedia
This article is about the Atlantic tropical storm of 2001. For other storms of the same name, see Tropical Storm Allison (disambiguation). Tropical Storm Allison
tropical storm (SSHS)
Tropical Storm Allison on June 5, 2001
Formed Dissipat ed
June 4, 2001 June 18, 2001
Highest winds Lowest 1000 mbar (hPa; 29.53 inHg) pressure Fatalitie 41 direct, 14 indirect s Damage $5.5 billion (2001 USD) $6.82 billion (2010 USD) Areas Texas (particularly affected aroundHouston), Louisiana, most of the Eastern United States Part of the 2001 Atlantic hurricane season
Tropical Storm Allison was a tropical storm that devastated southeast Texas in June of the 2001 Atlantic hurricane season. The first storm of the season, Allison lasted unusually long for a June storm, remaining tropical or subtropical for 15 days. The storm developed from atropical wave in the northern Gulf of Mexico on June 4, 2001, and struck the upper Texas coast shortly thereafter. It drifted northward through the state, turned back to the south, and reentered the Gulf of Mexico. The storm continued to the east-northeast, made landfall on Louisiana, then moved across the southeast United States and Mid-Atlantic. Allison was the first storm since Tropical Storm Frances in 1998 to strike the northern Texas coastline.[1] The storm dropped heavy rainfall along its path, peaking at over 40 inches (1,000 mm) in Texas. The worst flooding occurred in Houston, where most of Allison's damage occurred: 30,000 became homeless after the storm flooded over 70,000 houses and destroyed 2,744 homes. Downtown Houston was inundated with flooding, causing severe damage to hospitals and businesses. Twenty-three people died in Texas. Throughout its entire path, Allison caused $5.5 billion ($6.7 billion 2008 USD) in damage and 41 deaths. Aside from Texas, the places worst hit were Louisiana and southeastern Pennsylvania. Following the storm, President George W. Bush designated 75 counties along Allison's path as disaster areas (the first time he had to do so), which enabled the citizens affected to apply for aid. Allison is the only tropical storm to have its name retired without ever having reached hurricane strength. [edit]Meteorological
history
Storm path
A tropical wave moved off the coast of Africa on May 21, 2001. It moved westward across theAtlantic Ocean, retaining little convection on its way. After moving across South America and the southwestern Caribbean Sea, the wave entered the eastern North Pacific Ocean on June 1. A low-level circulation developed on June 2, while it was about 230 miles (370 km) southsoutheast of Salina Cruz, Mexico. Southerly flow forced the system northward, and the wave moved inland on June 3. The low-level circulation dissipated, though the mid-level circulation persisted. It emerged into the Gulf of Mexico on June 4, and developed deep convection on its eastern side.[2] Early on June 5, satellite imagery suggested that a tropical depression was forming in the northwest Gulf of Mexico, which was furthered by reports of wind gusts as high as 60 mph (95 km/h) just a few hundred feet above the surface, towards the east side of the system.[3]
Tropical Storm Allison on June 5 at peak strength
At 1200 UTC on June 5, the disturbance developed a broad, low-level circulation, and was classified as Tropical Storm Allison, the first storm of the 2001 Atlantic hurricane season. Some intensification was projected, though it was expected to be hindered by cool offshore sea surface temperatures.[4] Due to the cold-core nature of the center, Allison initially
contained subtropical characteristics. Despite this, the storm quickly strengthened to attain peak sustained winds of 60 mph (95 km/h), with tropical storm-force winds extending up to 230 miles (370 km) east of the center, and a minimum central pressure of 1000 mbar.[2] The storm initially moved very little, and the presence of several small vortices from within the deep convection caused difficulty in determining the exact center location.[5] Later in the day, several different track forecasts arose. One scenario had the cyclone tracking westward in to Mexico. Another projected the storm moving east towards southern Louisiana. At the time, it was noted that little rain or wind persisted near the center, but rather to the north and east.[6] Under the steering currents of a subtropical ridge that extended in an east–west orientation across the southeast United States,[5]Allison weakened while nearing the Texas coastline, and struck near Freeport, Texas with 50 mph (80 km/h) winds.[2] Inland, the storm rapidly weakened, and the National Hurricane Center discontinued advisories early on June 6.
[7]
Shortly after being downgraded to a tropical depression, surface observations showed an
elongated circulation with a poorly defined center, which had reformed closer to the deep convection.[8]
Tropical Storm Allison with an eye-like feature over Mississippi
The depression drifted northward until reaching Lufkin, Texas, where it stalled due to a high pressure system to its north.[2] While stalling over Texas, the storm dropped excessive rainfall, peaking at just over 40 inches (1,033 mm) in northwestern Jefferson County.[9] On June 7, the subtropical ridge off Florida weakened, while the ridge west of Texas intensified. This steered Tropical Depression Allison to make a clockwise loop, and the storm began drifting to the southwest. As the center reached Huntsville, Texas, a heavy rain band began to back build from Louisiana westward into Liberty County, Texas, which had caused additional flooding.[10] At the
time, the system had a minimum central pressure of about 1004 mb and maximum sustained winds of about 10 mph (16 km/h).[11] Late on June 9 and early on June 10, Allison's remnants once again reached the Gulf of Mexico and emerged over open waters.[12] The low once again became nearly stationary about 60 mi (100 km) south of Galveston, Texas, and despite more favorable upper-level winds, it showed no signs of redevelopment.[13] Due to dry air and moderate westerly wind shear, the storm transformed into a subtropical cyclone. While the subtropical depression moved eastward, a new low level circulation redeveloped to the east, and Allison quickly made landfall on Morgan City, Louisiana on June 11.[2] At around the same time, the surface center reformed to the eastnortheast of its previous location, aligning with the mid-level circulation.
[14]
Strong thunderstorms redeveloped over the circulation, and Allison strengthened into a
subtropical storm over southeastern Louisiana.[2] The storm intensified further to attain sustained winds of 45 mph (70 km/h) and a minimum barometric pressure of about 1000 mb near Mclain, Mississippi, accompanied by a well-defined eye-like feature.[15]
Tropical Storm Allison over North Carolina
The storm was officially downgraded to a subtropical depression at 0000 UTC on June 12. Somewhat accelerating, the depression tracked to the east-northeast through Mississippi, Alabama, Georgia, and South Carolina before becoming nearly stationary near Wilmington, North Carolina.[2] The depression drifted through North Carolina and sped to the northeast for a time in response to an approaching cold front.[16]Though satellite and radar imagery show the system was well-organized, the system slowed and moved erratically for a period of time,[17]executing what appeared to be a small counterclockwise loop.[18] The storm began tracking in a generally northeasterly direction, and crossed into the southern Delmarva Peninsula on June 16.[19] The subtropical remnants reached the Atlantic on June 17, and while located east of Atlantic City, New Jersey, winds began to restrengthen, and
heavy rains formed to the north of the circulation. The low was interacting with a frontal boundary, and started merging with it, as it accelerated to the northeast at 13 mph (21 km/h).
[20]
The remnants of Allison briefly reintensified to a subtropical
storm through baroclinic processes, though it became extratropical while south of Long Island.
[2]
By later on June 17, the low was situated off the coast of Rhode Island, spreading a swath of
precipitation over New England.[21] The remnants of the tropical storm were then absorbed by the frontal boundary by June 18, and eventually passed south of Cape Race, Newfoundland on June 20, where the extratropical cyclone dissipated.[2] [edit]Preparations Main article: Effects of Tropical Storm Allison in Texas Shortly after the storm formed, officials in Galveston County, Texas issued a voluntary evacuation for the western end of Galveston Island, as the area was not protected by the Galveston Seawall.[2] The ferry from the island to the Bolivar Peninsula was closed, while voluntary evacuations were issued in Surfside in Brazoria County.[22] When the National Hurricane Centerissued the first advisory on Allison, officials issued Tropical Storm Warnings from Sargent, Texas to Morgan City, Louisiana.[23] After the storm made landfall, flash flood watches and warnings were issued for numerous areas in eastern Texas.[24] During the flood event, the National Weather Service in Houston issued 99 flash flood warnings with an average lead time of 40 minutes. With an average lead time of 24 minutes, the National Weather Service in Lake Charles, Louisiana issued 47 flash flood warnings. With an average lead time of 39 minutes, the National Weather Service in New Orleans/Baton Rouge issued 87 flash flood warnings, of which 30 were not followed by a flash flood.[25] In Tallahassee, Florida, a shelter opened the day prior to Allison's movement northward through the area, seven staff members housing 12 people. Two other shelters were on standby. Teams informed citizens in the Florida Panhandle of flood dangers.[26] [edit]Impact Tropical Storm Allison was a major flood disaster throughout its path from Texas to the MidAtlantic. The worst of the flooding occurred in Houston, Texas, where over 35 inches (890 mm) of rain fell. Allison killed 41 people, of which 27 drowned. The storm also caused over $5 billion in damage (2001 USD, $6.4 billion 2007 USD), making Allison the deadliest and costliest tropical storm on record in the United States.[2]
[edit]Texas Main article: Effects of Tropical Storm Allison in Texas Combined with waves on top, areas of Galveston Island experienced a wall of water 8 feet (2.5 m) in height, creating overwash along the coastline. The storm caused winds of up to 43 mph (69 km/h) at the Galveston Pier. While Allison was stalling over Texas, it dropped very heavy rainfall across the state.[2] Minimal beach erosion was reported.[27] Flash flooding continued for days,[10] with rainfall amounts across the state peaking at just over 40 inches (1,033 mm) in northwestern Jefferson County. In the Port of Houston, a total of 36.99 inches (940 mm) was reported.[9] Houston experienced torrential rainfall in a short amount of time. The six-day rainfall in Houston amounted to 38.6 inches (980 mm).[28] The deluge of rainfall flooded 95,000 automobiles and 73,000 houses throughout Harris County.[1] Tropical Storm Allison destroyed 2,744 homes, leaving 30,000 homeless with residential damages totaling to $1.76 billion (2001 USD, $2.05 billion 2007 USD).[28]
The Southwest Freeway, near Downtown Houston, lies under water due to flooding from Tropical Storm Allison
Several hospitals in the Texas Medical Center, the largest medical complex in the world, experienced severe damage from the storm, which hit quickly and with unexpected fury on a Friday evening. The Baylor College of Medicine experienced major damage, totaling $495 million (2001 USD, $577 million 2007 USD). The medical school lost 90,000 research animals, 60,000 tumor samples, and 25 years of research data.The University of Texas Health Science Center at Houston, across the street, lost thousands of laboratory animals. Throughout the Medical Center, damage totaled to over $2 billion (2001 USD, $2.3 billion 2007 USD).[28]
Buffalo Bayou and White Oak Bayou at Main Street after Tropical Storm Allison hitHouston
The underground tunnel system, which connects most large office buildings in downtown Houston, was submerged, as were many streets and parking garages adjacent to Buffalo Bayou. At the Theatre District, also in downtown, the Houston Symphony, Houston Grand Opera, and Alley Theater lost millions of dollars of costumes, musical instruments, sheet music, archives and other artifacts. By midnight on June 9 nearly every freeway and major road in the city was under several feet of water, forcing hundreds of motorists to abandon their vehicles for higher ground.[28] Despite massive flooding damage to entire neighborhoods there were no drowning deaths in flooded homes. In the area, there were 12 deaths from driving, 6 from walking, 3 from electrocution, and 1 in an elevator.[1] Elsewhere in Texas, a man drowned when swimming in a ditch in Mauriceville.[25] Damage totaled to $5.2 billion (2001 USD, $6 billion 2007 USD) throughout Texas.[29] [edit]Louisiana
Flooding in Chackbay, Louisiana
While making its first landfall, Allison's large circulation dropped severe rains on southwest Louisiana.[30] Days later, Allison hit the state as a subtropical storm, dropping more heavy rains to the area. Rainfall totals peaked at 29.86 inches (758 mm) in Thibodaux, the highest rainfall total in Louisiana from a tropical cyclone since another Tropical Storm Allison in 1989.[31] Most of the southeastern portion of the state experienced over 10 inches of rain (255 mm).[9] Winds were generally light, peaking at 38 mph (61 km/h) sustained in Lakefront with gusts to 53 mph (85 km/h) in Bay Gardene. The storm produced a storm surge of 2.5 feet (0.75 m) in Cameron as it was making landfall in Texas.[2] While moving northward through Texas, the outer bands of the storm produced anF1 tornado near Zachary, damaging several trees and a power line. A man was killed when a damaged power line hit his truck.[32] When Allison first made landfall, heavy rainfall flooded numerous houses and businesses. Minor wind gusts caused minor roof damage to 10 houses in Cameron Parish, while its storm surge flooded portions of Louisiana Highway 82.[33] When the system returned, more rainfall occurred, flooding over 1,000 houses in St. Tammany Parish,[34] 80 houses in Saint Bernard Parish,[35] and hundreds of houses elsewhere in the state. The flooding also forced 1,800 residents from their homes in East Baton Rouge Parish.[36] The deluge left numerous roads impassable, while runoff resulted in severe river flooding. The Bogue Falaya River in Covington crested past its peak twice to near record levels.[34] The Amite and Comite Rivers reached their highest levels since 1983. In addition, the levee along the Bayou Manchac broke, flooding roadways and more houses.[36] Damage in Louisiana totaled to $65 million (2001 USD, $75 million 2007 USD).[28] [edit]Southeast
United States
Rainfall totals from Allison
In Mississippi, Allison produced heavy rainfall of over 10 inches (255 mm) in one night,[37] while some areas in the southwestern portion of the state received over 15 inches (380 mm).[9] The flooding damaged numerous houses and flooded many roadways.[37] Thunderstorms from the storm produced four tornadoes,[2] including one in Gulfport, Mississippi that damaged 10 houses.[38] Severe thunderstorms in George Countydamaged 15 houses, destroyed 10, and injured five people.[39] Damage in Mississippi totaled to over $1 million (2001 USD, $1.2 million 2007 USD).[37][38][39] Rainfall in Alabama was moderate, with areas near Mobile experiencing more than 10 inches (255 mm).[9] Heavy rainfall closed several roads in Crenshaw County.
[40] [41]
The storm, combined with a high pressure, produced coastal flooding in southern Alabama.
Allison produced an F0 tornado in southwest Mobile County that caused minor roof
damage[42] and another F0 tornado in Covington Countythat caused minor damage to six homes and a church.[42] The storm, combined with a high pressure system, produced a strong pressure gradient, resulting in strong rip currents off the coast of Florida. The currents prompted sirens, which are normally used for storm warnings, to be activated in Pensacola Beach.[43] The rip currents killed 5 off the coast of Florida.[44] Outer rain bands from the storm dropped heavy rainfall across the Florida Panhandle of over 11 inches (280 mm) in one day. The Tallahassee Regional Airport recorded 10.13 inches (257 mm) in 24 hours, breaking the old 24 hour record set in 1969.[45] Throughout the state, Allison destroyed 10 homes and damaged 599, 196 severely, primarily in Leon County.[46] Including the deaths from rip currents, Allison killed eight people in Florida[2] and caused $20 million (2001 USD, $23 million 2007 USD) in damage.[45] Over Georgia, the storm dropped heavy rainfall of 10 inches (255 mm) in 24 hours in various locations. The deluge caused rivers to crest past their banks, including the Oconee River at Milledgeville which peaked at 33.7 feet (10.3 m). The rainfall, which was heaviest across the southwestern portion of the state, washed out several bridges and roads, and flooded many other roads. Georgia governor Roy Barnes declared a state of emergency for seven counties in the state.[47] The storm also spawned two tornadoes.[2] In South Carolina, Allison's outer bands produced 10 tornadoes[2] and several funnel clouds, though most only caused minor damage limited to a damaged courthouse, snapped trees[48] and downed power lines.[49] Allison produced from 12 to 16 inches (305 to 406 mm) of rainfall in North Carolina, closing nearly all roads in Martin County and damaging 25 homes.[50] The severe flooding washed out a bridge in eastern Halifax County[51] and flooded numerous cars.[31] Wet roads caused nine traffic accidents throughout the state.[2]
[edit]Mid-Atlantic
and Northeast United States
Damage from flooding in Pennsylvania
In Virginia, Allison produced light rainfall, with the southeastern and south-central portions of the state experiencing over 3 inches (76 mm).[9] A tree in a saturated ground fell over and killed one person.[52] Allison also produced one tornado in the state.[2] Washington, D.C. experienced moderate rainfall from the storm, totaling to 2.59 inches (66 mm) in Georgetown.[53] In Maryland, rainfall from Tropical Depression Allison totaled to 7.5 inches (190 mm) in Denton, closing eleven roads and causing washouts on 41 others. The Maryland Eastern Shore experienced only minor rainfall from one to two inches (25 to 50 mm). Damage was light, and no deaths were reported.[54] In Delaware, the storm produced moderate rainfall, peaking at 4.2 inches (106 mm) in Greenwood. No damage was reported.[55] Allison, in combination with an approaching frontal boundary, dropped heavy rainfall across southeastern Pennsylvania, peaking at 10.17 inches (258 mm) in Chalfont in Bucks County and over 3 inches (76 mm) in portions of Philadelphia. The rainfall caused rivers to rise, with the Neshaminy Creek in Langhorne peaking at 16.87 feet (5.1 m). Several other rivers and creeks in southeastern Pennsylvania crested at over 10 feet (3 m). The rainfall downed numerous weak trees and power lines, leaving 70,000 without power during the storm. The flooding washed out several roads and bridges, including a few SEPTA rail lines. In addition, the rainfall destroyed 241 homes and damaged 1,386 others. Flooding at a Dodge dealership totaled 150 vehicles. Hundreds of people were forced to be rescued from damaged buildings from flood waters. The flooding dislodged a clothes dryer in the basement of the "A" building of the Village Green Apartment Complex in Upper Moreland Township, breaking a natural gas line. The gas leak resulted in an explosion and an ensuing fire that killed six people. Firefighters were unable to render assistance as the building was completely surrounded by floodwaters. Additionally, one man drowned in his vehicle in a river.[56] Damage in Pennsylvania totaled to $215 million (2001 USD, $250 million 2007 USD).[28]
In New Jersey, the storm produced heavy rainfall, peaking at 8.1 inches (205 mm) in Tuckerton. The rains also caused river flooding, including the north branch of the Metedeconk River inLakewood which crested at 8 feet (2.5 m). The flooding, severe at places, closed several roads, including numerous state highways.[57] Gusty winds of up to 44 mph (71 km/h) in Atlantic City downed weak trees and power lines, leaving over 13,000 without power. Several people had to be rescued from high waters, though no fatalities occurred in the state. Overall damage was minimal.[58] Tropical Storm Allison caused flash flooding in New York, dropping up to 3 inches (75 mm) of rain in one hour in several locations and peaking at 5.73 inches (146 mm) in Granite Springs. The rains also caused river flooding, including the Mahwah River which crested at 3.79 feet (1.2 m). Allison's rainfall damaged 24 houses and several stores, while the flooding closed several major highways in the New York City area. Overall damage was light, and no fatalities occurred in New York due to Allison.[59] Similarly, rainfall in Connecticut peaked at 7.2 inches (183 mm) in Pomfret,[60] closing several roads and causing minor damage to numerous houses. The Yantic River at Yantic crested at 11.1 feet (3.4 m),[60] while a state road was closed when a private dam in Hampton failed from the rainfall.[60] In Rhode Island, Allison produced up to 7.1 inches (180 mm) of rainfall in North Smithfield, washing out several roads and houses, and destroying a log house in Foster.[61] An isolated severe thunderstorm in the outer bands of Allison produced an F1 tornado in Worcester and Middlesex Counties in Massachusetts, impacting over 100 trees and damaging one house and one small camper. A microburst in Leominster and another in Shirley damaged several trees. Lightning from the storm hit two houses, causing significant damage there but little elsewhere. Allison also produced moderate rainfall in the state, mainly ranging from 3 to 5 inches (75 to 125 mm). The rainfall caused drainage and traffic problems. Damage in Massachusetts totaled to $400,000 (2001 USD, $466,000 2007 USD).[62] [edit]Aftermath Main article: Effects of Tropical Storm Allison in Texas Within weeks of the disaster, President George W. Bush declared 75 counties in Texas,
[63]
southern Louisiana,[64] southern Mississippi,[65] northwestern Florida,[66] and southeastern
Pennsylvania as disaster areas.[67] The declarations allowed affected citizens to receive aid for temporary housing, emergency home repairs, and other serious disaster-related expenses. The Federal Emergency Management Agency (FEMA) also provided 75% for the cost of debris
removal, emergency services related to the disaster, and repairing or replacing damaged public facilities, such as roads, bridges and utilities.[63]
Aid from the American Red Cross
A few weeks after Allison, FEMA opened six disaster recovery centers across southeast Texas, which provided recovery information to those who applied for disaster assistance.
[68]
The American Red Cross and the Salvation Army opened 48 shelters at the peak of need for
people driven from their homes, which served nearly 300,000 meals. The National Disaster Medical System deployed a temporary hospital to Houston with 88 professionals, aiding nearly 500 people.[69] Thirty-five volunteer services provided aid for the flood victims in Texas, including food, clothing, and volunteers to help repair the houses.[70] After nearly 50,000 cars were flooded and ruined, many people attempted to sell the cars across the country without telling of the car's history.[71] Following the extreme flooding, a mosquito outbreak occurred, though FEMA provided aid to control the problem.[72] By six months after the storm, around 120,000 Texas citizens applied for federal disaster aid, totaling to $1.05 billion (2001 USD, $1.22 billion 2007 USD).[73] Like in Texas, a mosquito outbreak occurred in Louisiana. Only pesticides acceptable to the US Environmental Protection Agency and the US Fish and Wildlife Service were allowed to be used.[74] FEMA officials warned homeowners of the dangers of floodwaters, including mold, mildew, and bacteria.[75] By three months after the storm, just under 100,000 Louisiana citizens applied for federal aid, totaling to over $110 million (2001 USD, $128 million 2007 USD). $25 million (2001 USD, $29 million 2007 USD) of the total was for business loans, while an additional $8 million was for public assistance for communities and state agencies.[76] More than 750 flood victims in Florida applied for governmental aid, totaling to $1.29 million (2001 USD, $1.5 million 2007 USD).[77] In Pennsylvania, 1,670 flood victims applied for federal aid, totaling to $11.5 million (2001 USD, $13.4 million 2007 USD). $3.4 million (2001 USD, $4 million
2007 USD) of the total was to replace a SEPTA rail bridge over the Sandy Run in Fort Washington.[78]
Tropical cyclone
From Wikipedia, the free encyclopedia
"Hurricane" redirects here. For other uses, see Hurricane (disambiguation).
Hurricane Isabel (2003) as seen from orbit duringExpedition 7 of the International Space Station. The eye, eyewall and surrounding rainbands that are characteristics of tropical cyclones are clearly visible in this view from space.
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated airrises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows. The characteristic that separates tropical cyclones from other cyclonic systems is that any height in the atmosphere, the center of a tropical cyclone will be warmer than its surrounds; a phenomenon called "warm core" storm systems. The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, withcounterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. The opposite direction of spin is a result of the Coriolis force. Depending on its location and strength, a
tropical cyclone is referred to by names such ashurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge as well as spawning tornadoes. They develop over large bodies of warm water, and lose their strength if they move over land due to increased surface friction and loss of the warm ocean as an energy source. This is why coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere, and to maintain a relatively stable and warm temperature worldwide. Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. The background environment is modulated by climatological cycles and patterns such as the Madden-Julian oscillation, El Niño-Southern Oscillation, and the Atlantic multidecadal oscillation. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. It is not possible to artificially induce the dissipation of these systems with current technology.
Physical structure
See also: Eye (cyclone)
Structure of a tropical cyclone
All tropical cyclones are areas of low atmospheric pressure in the Earth's atmosphere. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.[1] Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation, which occurs when moist air is carried upwards and its water vapor condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.[2]
Eye and center
A strong tropical cyclone will harbor an area of sinking air at the center of circulation. If this area is strong enough, it can develop into a large "eye". Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent.[3] The eye is normally circular in shape, and may range in size from 3 kilometres (1.9 mi) to 370 kilometres (230 mi) in diameter.[4]
[5]
Intense, mature tropical cyclones can sometimes exhibit an outward curving of the eyewall's
top, making it resemble a football stadium; this phenomenon is thus sometimes referred to as the stadium effect.[6] There are other features that either surround the eye, or cover it. The central dense overcast is the concentrated area of strong thunderstorm activity near the center of a tropical cyclone;[7] in weaker tropical cyclones, the CDO may cover the center completely.[8] The eyewall is a circle of strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[3]Eyewall replacement cycles occur naturally in intense tropical cyclones. When cyclones reach peak intensity they usually have an eyewall and radius of maximum winds that contract to a very small size, around 10 kilometres (6.2 mi) to 25 kilometres (16 mi). Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. When the inner eyewall weakens, the tropical cyclone weakens (in other words, the maximum sustained winds weaken and the central pressure rises.) The outer eyewall replaces the inner one completely at the end of the cycle. The storm can be of the same intensity as it was previously or even stronger after the eyewall replacement cycle finishes. The storm may strengthen again as it builds a new outer ring for the next eyewall replacement.[9]
Size descriptions of tropical cyclones
ROCI
Type
Less than 2 degrees latitude
Very small/midget
2 to 3 degrees of latitude
Small
3 to 6 degrees of latitude
Medium/Avera ge
6 to 8 degrees of latitude
Large antidwarf
Over 8 degrees of latitude
Very large[10]
Size
One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. If the radius is less than two degrees of latitude or 222 kilometres (138 mi), then the cyclone is "very small" or a "midget". A radius between 3 and 6 latitude degrees or 333 kilometres (207 mi) to 670 kilometres (420 mi) are considered "average-sized". "Very large" tropical cyclones have a radius of greater than 8 degrees or 888 kilometres (552 mi).[10] Use of this measure has objectively determined that tropical cyclones in the northwest Pacific Ocean are the largest on earth on average, withAtlantic tropical cyclones roughly half their size.[11] Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius at which its relative vorticity field decreases to 1×10−5 s−1 from its center.[12]
[13]
Mechanics
Tropical cyclones form when the energy released by the condensation of moisture in rising air causes a positive feedback loop over warm ocean waters.[14]
A tropical cyclone's primary energy source is the release of the heat of condensation from water vapor condensing, with solar heatingbeing the initial source for evaporation. Therefore, a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth.[15] In another way, tropical cyclones could be viewed as a special type of mesoscale convective complex, which continues to develop over a vast source of relative warmth and moisture. While an initial warm core system, such as an organized thunderstorm complex, is necessary for the formation of a tropical cyclone, a large flux of energy is needed to lower atmospheric pressure more than a few millibars (0.10 inch of mercury). The inflow of warmth and moisture from the underlying ocean surface is critical for tropical cyclone strengthening.[16] A significant amount of the inflow in the cyclone is in the lowest 1 kilometre (3,300 ft) of the atmosphere.[17] Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy;[18] the faster winds and lower pressure associated with them in turn cause increased surface evaporation and thus even more condensation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation.[19] This positive feedback loop, called the Wind-induced surface heat exchange, continues for as long as conditions are favorable for tropical cyclone development. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The rotation of the Earth causes the system to spin, an effect known as the Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.[20][21][22] What primarily distinguishes tropical cyclones from other meteorological phenomena is deep convection as a driving force.[23] Because convection is strongest in a tropical climate, it defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere.[23] To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the
needed atmospheric moisture to keep the positive feedback loop running. When a tropical cyclone passes over land, it is cut off from its heat source and its strength diminishes rapidly.[24]
Chart displaying the drop in surface temperature in the Gulf of Mexico as HurricanesKatrina and Rita passed over
The passage of a tropical cyclone over the ocean causes the upper layers of the ocean to cool substantially, which can influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean and the warm surface waters. This effect results in a negative feedback process which can inhibit further development or lead to weakening. Additional cooling may come in the form of cold water from falling raindrops (this is because the atmosphere is cooler at higher altitudes). Cloud cover may also play a role in cooling the ocean, by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.[25] Scientists estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day,[19] equivalent to about 1 PW (1015 watt). This rate of energy release is equivalent to 70 times the world energy consumption of humans and 200 times the worldwide electrical generating capacity, or to exploding a 10-megaton nuclear bomb every 20 minutes.[19][26] In the lower troposphere, the most obvious motion of clouds is toward the center. However tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine.[15] This outflow produces high, cirrus clouds that spiral away from the center. The clouds thin as they move outwards from the center of the system and are evaporated. They may be thin enough for the sun to be visible through them. These high cirrus clouds may be the first signs of an approaching tropical cyclone.[27] As air parcels are lifted
within the eye of the storm the vorticity is reduced, causing the outflow from a tropical cyclone to have anti-cyclonic motion.
Major basins and related warning centers
Main articles: Tropical cyclone basins and Regional Specialized Meteorological Center
Basins and WMO Monitoring Institutions[28]
Basin
Responsible RSMCs and TCWCs
North Atlantic
National Hurricane Center (United States)
North-East Pacific
National Hurricane Center (United States)
North-Central Pacific
Central Pacific Hurricane Center (United States)
North-West Pacific
Japan Meteorological Agency
North Indian Ocean India Meteorological Department
South-West Indian Ocean
Météo-France
Australian region
Bureau of Meteorology† (Australia) Meteorological and Geophysical Agency† (Indonesia) Papua New Guinea National Weather Service†
Southern Pacific
Fiji Meteorological Service Meteorological Service of New Zealand†
†
: Indicates a Tropical Cyclone Warning Center
There are six Regional Specialized Meteorological Centers (RSMCs) worldwide. These organizations are designated by the World Meteorological Organization and are responsible for tracking and issuing bulletins, warnings, and advisories about tropical cyclones in their designated areas of responsibility. Additionally, there are six Tropical Cyclone Warning Centers (TCWCs) that provide information to smaller regions.[29]The RSMCs and TCWCs are not the only organizations that provide information about tropical cyclones to the public. The Joint Typhoon Warning Center (JTWC) issues advisories in all basins except the Northern Atlantic for the purposes of the United States Government.[30] The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues advisories and names for tropical cyclones that approach the Philippines in the Northwestern Pacific to protect the life and property of its citizens.[31]The Canadian Hurricane Center (CHC) issues advisories on hurricanes and their remnants for Canadian citizens when they affect Canada.[32] On 26 March 2004, Cyclone Catarina became the first recorded South Atlantic cyclone and subsequently struck southern Brazil with winds equivalent to Category 2 on the Saffir-Simpson Hurricane Scale. As the cyclone formed outside the authority of another warning center, Brazilian meteorologists initially treated the system as an extratropical cyclone, although subsequently classified it as tropical.[33]
Formation
Main article: Tropical cyclogenesis
Map of the cumulative tracks of all tropical cyclones during the 1985–2005 time period. ThePacific Ocean west of the International Date Linesees more tropical cyclones than any other basin, while there is almost no activity in the Atlantic Ocean south of the Equator.
Map of all tropical cyclone tracks from 1945 to 2006. Equal-area projection.
Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active whilst November is the only month with all the tropical cyclone basins active.[34]
Times
In the Northern Atlantic Ocean, a distinct cyclone season occurs from June 1 to November 30, sharply peaking from late August through September.[34] The statistical peak of the Atlantic hurricane season is 10 September. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.[35] The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November.[34] In the Southern Hemisphere, the tropical cyclone year begins on July 1 and runs all year round and encompasses the tropical cyclone seasons which run from November 1 until the end of April with peaks in mid-February to early March.[34][36]
Season lengths and seasonal averages[34][37]
Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.
The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood.[38] While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the following conditions. In most situations,water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft);
[39]
waters of this temperature cause the overlying atmosphere to be unstable enough to sustain
convection and thunderstorms.[40] Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.[39] High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the
atmosphere, conditions are more favorable for disturbances to develop.[39] Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.[39] Tropical cyclones generally need to form more than 555 km (345 mi) or 5 degrees of latitude away from the equator, allowing the Coriolis effect to deflect winds blowing towards the low pressure center and creating a circulation.[39] Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.
[39]
Low-latitude and low-level westerly wind bursts associated with the Madden-Julian
oscillation can create favorable conditions for tropical cyclogenesis by initiating tropical disturbances.[41]
Locations
Most tropical cyclones form in a worldwide band of thunderstorm activity called by several names: the Intertropical Front (ITF), theIntertropical Convergence Zone (ITCZ), or the monsoon trough.[42][43][44] Another important source of atmospheric instability is found intropical waves, which cause about 85% of intense tropical cyclones in the Atlantic ocean, and become most of the tropical cyclones in the Eastern Pacific basin.[45][46][47] Tropical cyclones move westward when equatorward of the subtropical ridge, intensifying as they move. Most of these systems form between 10 and 30 degrees away of the equator, and 87% form no farther away than 20 degrees of latitude, north or south.[48][49] Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5 degrees of the equator, where the Coriolis effect is weakest.[48] However, it is possible for tropical cyclones to form within this boundary as Tropical Storm Vamei did in 2001 and Cyclone Agni in 2004.[50][51]
Movement and track
Steering winds
See also: Prevailing winds Although tropical cyclones are large systems generating enormous energy, their movements over the Earth's surface are controlled by large-scale winds—the streams in the Earth's atmosphere. The path of motion is referred to as a tropical cyclone's track and has been compared by Dr. Neil Frank, former director of the National Hurricane Center, to "leaves carried along by a stream".[52] Tropical systems, while generally located equatorward of the 20th parallel, are steered primarily westward by the east-to-west winds on the equatorward side of the subtropical ridge—a persistent high pressure area over the world's oceans.[52] In the tropical North Atlantic and
Northeast Pacific oceans, trade winds—another name for the westward-moving wind currents— steer tropical waves westward from the African coast and towards the Caribbean Sea, North America, and ultimately into the central Pacific ocean before the waves dampen out.[46]These waves are the precursors to many tropical cyclones within this region.[45] In the Indian Ocean and Western Pacific (both north and south of the equator), tropical cyclogenesis is strongly influenced by the seasonal movement of the Intertropical Convergence Zone and the monsoon trough, rather than by easterly waves.[53] Tropical cyclones can also be steered by other systems, such as other low pressure systems, high pressure systems, warm fronts, and cold fronts.
Coriolis effect
Infrared image of a powerful southern hemisphere cyclone, Monica, near peak intensity, showing clockwise rotation due to the Coriolis effect
The Earth's rotation imparts an acceleration known as the Coriolis effect, Coriolis acceleration, or colloquially, Coriolis force. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents.[54] The poleward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more poleward. The westerly winds on the equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is poleward. Thus, tropical cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis effect.[21] The Coriolis effect also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds – that force is theheat of condensation.[19]
Interaction with the mid-latitude westerlies
See also: Westerlies
Storm track of Typhoon Ioke, showing recurvature off the Japanese coast in 2006
When a tropical cyclone crosses the subtropical ridge axis, its general track around the highpressure area is deflected significantly by winds moving towards the general low-pressure area to its north. When the cyclone track becomes strongly poleward with an easterly component, the cyclone has begun recurvature.[55] A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve offshore of Japan to the north, and then to the northeast, if the typhoon encounters southwesterly winds (blowing northeastward) around a low-pressure system passing over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by extratropical cyclones in this manner, which move from west to east to the north of the subtropical ridge. An example of a tropical cyclone in recurvature was Typhoon Ioke in 2006, which took a similar trajectory.[56]
Landfall
See also: List of notable tropical cyclones and Unusual areas of tropical cyclone formation Officially, landfall is when a storm's center (the center of its circulation, not its edge) crosses the coastline.[57] Storm conditions may be experienced on the coast and inland hours before landfall; in fact, a tropical cyclone can launch its strongest winds over land, yet not make landfall; if this occurs, then it is said that the storm made a direct hit on the coast.[57] As a result of the narrowness of this definition, the landfall area experiences half of a land-bound storm by the time the actual landfall occurs. For emergency preparedness, actions should be timed from when a certain wind speed or intensity of rainfall will reach land, not from when landfall will occur.[57]
Multiple storm interaction
Main article: Fujiwhara effect When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara.[58]
Dissipation
Factors
Tropical Storm Franklin, an example of a strongly sheared tropical cyclone in the Atlantic Basin during 2005
A tropical cyclone can cease to have tropical characteristics in several different ways. One such way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly losing strength.[59] Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two, or evolve into extratropical cyclones. There is a chance a tropical cyclone could regenerate if it managed to get back over open warm water, such as with Hurricane Ivan. If it remains over mountains for even a short time, weakening will accelerate.[60] Many storm fatalities occur in mountainous terrain, as the dying storm unleashes torrential rainfall,[61] leading to deadly floods and mudslides, similar to those that happened with Hurricane Mitch in 1998.[62]Additionally, dissipation can occur if a storm
remains in the same area of ocean for too long, mixing the upper 60 metres (200 ft) of water, dropping sea surface temperatures more than 5 °C (9 °F).[63] Without warm surface water, the storm cannot survive.[64] A tropical cyclone can dissipate when it moves over waters significantly below 26.5 °C (79.7 °F). This will cause the storm to lose its tropical characteristics (i.e. thunderstorms near the center and warm core) and become a remnant low pressure area, which can persist for several days. This is the main dissipation mechanism in the Northeast Pacific ocean.[65] Weakening or dissipation can occur if it experiences vertical wind shear, causing the convection and heat engine to move away from the center; this normally ceases development of a tropical cyclone.
[66]
Additionally, its interaction with the main belt of the Westerlies, by means of merging with a
nearby frontal zone, can cause tropical cyclones to evolve into extratropical cyclones. This transition can take 1–3 days.[67] Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane/typhoon force) winds and drop several inches of rainfall. In the Pacific ocean and Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane or typhoon-force wind speeds when they reach the west coast of North America. These phenomena can also affect Europe, where they are known as European windstorms; Hurricane Iris's extratropical remnants are an example of such a windstorm from 1995.[68] Additionally, a cyclone can merge with another area of low pressure, becoming a larger area of low pressure. This can strengthen the resultant system, although it may no longer be a tropical cyclone.
[66]
Studies in the 2000s have given rise to the hypothesis that large amounts of dust reduce the
strength of tropical cyclones.[69]
Artificial dissipation
In the 1960s and 1970s, the United States government attempted to weaken hurricanes through Project Stormfury by seeding selected storms with silver iodide. It was thought that the seeding would cause supercooled water in the outer rainbands to freeze, causing the inner eyewall to collapse and thus reducing the winds.[70] The winds of Hurricane Debbie—a hurricane seeded in Project Stormfury—dropped as much as 31%, but Debbie regained its strength after each of two seeding forays.[71] In an earlier episode in 1947, disaster struck when a hurricane east of Jacksonville, Florida promptly changed its course after being seeded, and smashed into Savannah, Georgia.[72] Because there was so much uncertainty about the behavior of these storms, the federal government would not approve seeding operations unless the hurricane had a less than 10% chance of making landfall within 48 hours, greatly reducing the number of possible test storms. The project was dropped after it was discovered that eyewall replacement cycles occur naturally in strong hurricanes, casting doubt on the result of the earlier attempts.
Today, it is known that silver iodide seeding is not likely to have an effect because the amount of supercooled water in the rainbands of a tropical cyclone is too low.[73] Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans.[74] Other ideas range from covering the ocean in a substance that inhibits evaporation,[75] dropping large quantities of ice into the eye at very early stages of development (so that the latent heat is absorbed by the ice, instead of being converted to kinetic energy that would feed the positive feedback loop),[74] or blasting the cyclone apart with nuclear weapons.[18] Project Cirrus even involved throwing dry ice on a cyclone.[76] These approaches all suffer from one flaw above many others: tropical cyclones are simply too large and short-lived for any of the weakening techniques to be practical.[77]
Effects
The aftermath of Hurricane Katrina in Gulfport, Mississippi.
Main article: Effects of tropical cyclones Tropical cyclones out at sea cause large waves, heavy rain, and high winds, disrupting international shipping and, at times, causing shipwrecks.[78] Tropical cyclones stir up water, leaving a cool wake behind them, which causes the region to be less favorable for subsequent tropical cyclones.[25] On land, strong winds can damage or destroy vehicles, buildings, bridges, and other outside objects, turning loose debris into deadly flying projectiles. The storm surge, or the increase in sea level due to the cyclone, is typically the worst effect from landfalling tropical cyclones, historically resulting in 90% of tropical cyclone deaths.[79] The broad rotation of a landfalling tropical cyclone, and vertical wind shear at its periphery, spawns tornadoes. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall.
[80]
Over the past two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million people worldwide. Large areas of standing water caused by flooding lead to infection, as well as contributing to mosquito-borne illnesses. Crowded evacuees in sheltersincrease the risk of disease propagation.[81] Tropical cyclones significantly interrupt infrastructure, leading to power outages, bridge destruction, and the hampering of reconstruction efforts.[81][82] Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.[83] Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the middle latitudes and polar regions.[84] The storm surge and winds of hurricanes may be destructive to human-made structures, but they also stir up the waters of coastal estuaries, which are typically important fish breeding locales. Tropical cyclone destruction spurs redevelopment, greatly increasing local property values.[85]
Observation and forecasting
Observation
Main article: Tropical cyclone observation
Sunset view of Hurricane Isidore's rainbands photographed at 7,000 feet (2,100 m)
Intense tropical cyclones pose a particular observation challenge, as they are a dangerous oceanic phenomenon, and weather stations, being relatively sparse, are rarely available on the site of the storm itself. Surface observations are generally available only if the storm is passing over an island or a coastal area, or if there is a nearby ship. Usually, real-time measurements are taken in the periphery of the cyclone, where conditions are less catastrophic and its true strength cannot be evaluated. For this reason, there are teams of meteorologists that move into the path of tropical cyclones to help evaluate their strength at the point of landfall.[86]
Tropical cyclones far from land are tracked by weather satellites capturing visible and infrared images from space, usually at half-hour to quarter-hour intervals. As a storm approaches land, it can be observed by land-based Doppler radar. Radar plays a crucial role around landfall by showing a storm's location and intensity every several minutes.[87] In-situ measurements, in real-time, can be taken by sending specially equipped reconnaissance flights into the cyclone. In the Atlantic basin, these flights are regularly flown by United States government hurricane hunters.[88] The aircraft used are WC-130 Hercules andWP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft fly directly into the cyclone and take direct and remote-sensing measurements. The aircraft also launch GPS dropsondes inside the cyclone. These sondes measure temperature, humidity, pressure, and especially winds between flight level and the ocean's surface. A new era in hurricane observation began when a remotely piloted Aerosonde, a small drone aircraft, was flown through Tropical Storm Ophelia as it passed Virginia's Eastern Shore during the 2005 hurricane season. A similar mission was also completed successfully in the western Pacific ocean. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare.[89]
A general decrease in error trends in tropical cyclone path prediction is evident since the 1970s
Forecasting
See also: Tropical cyclone track forecasting, Tropical cyclone prediction model, and Tropical cyclone rainfall forecasting Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system. The deep layer mean flow, or average
wind through the depth of the troposphere, is considered the best tool in determining track direction and speed. If storms are significantly sheared, use of wind speed measurements at a lower altitude, such as at the 700 hPa pressure surface (3,000 metres / 9,800 feet above sea level) will produce better predictions. Tropical forecasters also consider smoothing out shortterm wobbles of the storm as it allows them to determine a more accurate long-term trajectory.
[90]
High-speed computers and sophisticated simulation software allow forecasters to
produce computer models that predict tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. Combining forecast models with increased understanding of the forces that act on tropical cyclones, as well as with a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades.[91] However, scientists are not as skillful at predicting the intensity of tropical cyclones.[92] The lack of improvement in intensity forecasting is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development.
Classifications, terminology, and naming
Intensity classifications
Main article: Tropical cyclone scales
Three tropical cyclones at different stages of development. The weakest (left) demonstrates only the most basic circular shape. A stronger storm (top right) demonstrates spiral banding and increased centralization, while the strongest (lower right) has developed an eye.
Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. For example, if a tropical storm in the Northwestern Pacific reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon; if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane.[57] Neither "hurricane" nor "typhoon" is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to simply as "cyclones". Additionally, as indicated in the table below, each basin uses a separate system of terminology, making comparisons between different basins difficult. In the Pacific Ocean, hurricanes from the Central North Pacific sometimes cross the International Date Line into the Northwest Pacific, becoming typhoons (such as Hurricane/Typhoon Ioke in 2006); on rare occasions, the reverse will occur.[93] It should also be noted that typhoons with sustained winds greater than 67 metres per second (130 kn) or 150 miles per hour (240 km/h) are calledSuper Typhoons by the Joint Typhoon Warning Center.[94]
Tropical depression
A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 17 metres per second (33 kn) or 38 miles per hour (61 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression".[15]The practice of the Philippines is to name tropical depressions from their own naming convention when the depressions are within the Philippines' area of responsibility.[95]
Tropical storm
A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 metres per second (33 kn) (39 miles per hour (63 km/h)) and 32 metres per second (62 kn) (73 miles per hour (117 km/h)). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Government weather services, other than the Philippines, first assign names to systems that reach this intensity (thus the term named storm).[15]
Hurricane or typhoon
A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 33 metres per second (64 kn) or 74 miles per hour (119 km/h).[15] A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 16 kilometres (9.9 mi) to 80 kilometres (50 mi) wide in which the strongest thunderstorms and winds circulate around the storm's center. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 85 metres per second (165 kn) or 195 miles per hour (314 km/h).[96]
[hide]Tropical Cyclone Classifications (all winds are 10-minute averages)[97][98]
10Beauf minute N Indian ort sustaine Ocean scale d winds IMD (knots)
SW Indian Ocean MF
Austral SW ia Pacific BOM FMS
NW Pacific JMA
NE Pacific & NW N Atlantic Pacific NHC, CHC &C JTWC PHC
0–6
<28 knot Trop. s (32 Depressio Disturban mph; 52 n ce km/h) Tropical Tropical Depressi Depression on Tropical Tropical Tropical Depressio Depressi Low n on Deep Depressio Depressio n n
Hurricane (1) 12 64–72 knots (74–83 mph; 119–133 km/h) Very Severe Cyclonic Storm Severe Tropical Cyclone (3) Tropical Cyclone Severe Tropical Cyclone (3) Hurricane (2) Typhoon
73–85 knots (84–98 mph; 135–157 km/h)
86–89 knots (99–102 mph; 159–165 km/h)
Severe Tropica l Cyclon e (4)
Severe Tropical Cyclone (4)
Major Hurricane (3)
90–99 knots (100–114
Intense Tropical
mph; 170–183 km/h)
100–106 knots (120–122 mph; 190–196 km/h)
Cyclone
107–114 knots (123–131 mph; 198–211 km/h)
Major Hurricane (4)
115–119 knots (132–137 mph; 213–220 km/h)
>120 kn ots (140 Super mph; Cyclonic 220 Storm km/h)
Very Intense Tropical Cyclone
Severe Tropica l Cyclon e (5)
Severe Tropical Cyclone (5) Super Typhoon
Major Hurricane (5)
Origin of storm terms
Taipei 101 endures a typhoon in 2005
The word typhoon, which is used today in the Northwest Pacific, may be derived from Urdu, Persian and Arabic ţūfān ( ,)طوفانwhich in turn originates from Greek Typhon (Τυφών), a monster from Greek mythology associated with storms.[99] The related Portuguese word tufão, used in Portuguese for typhoons, is also derived from Typhon.
[100]
The word is also similar to Chinese "taifeng" ("toifung" in Cantonese) (颱風 – literally great
winds), and also to the Japanese "taifu" (台風), which may explain why "typhoon" came to be used for East Asian cyclones.[citation needed] The word hurricane, used in the North Atlantic and Northeast Pacific, is probably derived from the name of a Mayan storm god, Huracan, via the Spanish, huracán.[101] Huracan is also the source of the word Orcan, another word for a European windstorm. Another possible source is Hyrrokkin, a Jotun or giantess in Norse mythology, called upon by the Aesir to launch the ship bearing the body of the godBalder, which was too heavy for even the gods to move.[102]
Naming
Main articles: Tropical cyclone naming and Lists of tropical cyclone names Storms reaching tropical storm strength were initially given names to eliminate confusion when there are multiple systems in any individual basin at the same time, which assists in warning people of the coming storm.[103] In most cases, a tropical cyclone retains its name throughout its life; however, under special circumstances, tropical cyclones may be renamed while active. These names are taken from lists that vary from region to region and are usually drafted a few years ahead of time. The lists are decided on, depending on the regions, either by committees of the World Meteorological Organization (called primarily to discuss many other issues), or by
national weather offices involved in the forecasting of the storms. Each year, the names of particularly destructive storms (if there are any) are "retired" and new names are chosen to take their place. Different countries have different local conventions; for example, in Japan, storms are referred to by number (each year), such as 台風第 9 号 (Typhoon #9).
Notable tropical cyclones
Main articles: List of notable tropical cyclones, List of Atlantic hurricanes, and List of Pacific hurricanes Tropical cyclones that cause extreme destruction are rare, although when they occur, they can cause great amounts of damage or thousands of fatalities. The 1970 Bhola cyclone is the deadliest tropical cyclone on record, killing more than 300,000 people[104] and potentially as many as 1 million[105] after striking the densely populated Ganges Delta region ofBangladesh on 13 November 1970. Its powerful storm surge was responsible for the high death toll.
[104] [106]
The North Indian cyclone basin has historically been the deadliest basin.[81] Elsewhere, Typhoon Nina killed nearly 100,000 in China in 1975 due to a 100-year flood that
caused 62 dams including the Banqiao Dam to fail.[107] The Great Hurricane of 1780 is the deadliest Atlantic hurricane on record, killing about 22,000 people in the Lesser Antilles.[108] A tropical cyclone does need not be particularly strong to cause memorable damage, primarily if the deaths are from rainfall or mudslides. Tropical Storm Thelma in November 1991 killed thousands in the Philippines,[109] while in 1982, the unnamed tropical depression that eventually became Hurricane Paul killed around 1,000 people in Central America.[110] Hurricane Katrina is estimated as the costliest tropical cyclone worldwide,[111] causing $81.2 billion in property damage (2008 USD)[112] with overall damage estimates exceeding $100 billion (2005 USD).[111] Katrina killed at least 1,836 people after striking Louisiana and Mississippi as a major hurricane in August 2005.[112] Hurricane Andrew is the second most destructive tropical cyclone in U.S history, with damages totaling $40.7 billion (2008 USD), and with damage costs at $31.5 billion (2008 USD), Hurricane Ike is the third most destructive tropical cyclone in U.S history. The Galveston Hurricane of 1900 is the deadliest natural disaster in the United States, killing an estimated 6,000 to 12,000 people in Galveston, Texas.[113] Hurricane Mitch caused more than 10, 000 fatalities in Latin America. Hurricane Iniki in 1992 was the most powerful storm to strike Hawaii in recorded history, hitting Kauai as a Category 4 hurricane, killing six people, and causing U.S. $3 billion in damage.[114] Other destructive Eastern Pacific hurricanes include Pauline and Kenna, both causing severe damage after striking Mexico as major hurricanes.[115][116] In March 2004, Cyclone Gafilo struck
northeastern Madagascar as a powerful cyclone, killing 74, affecting more than 200,000, and becoming the worst cyclone to affect the nation for more than 20 years.[117]
The relative sizes of Typhoon Tip, Cyclone Tracy, and theContiguous United States
The most intense storm on record was Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of 870 mbar (25.69 inHg) and maximum sustained wind speeds of 165 knots (85 m/s) or 190 miles per hour (310 km/h).[118] Tip, however, does not solely hold the record for fastest sustained winds in a cyclone. Typhoon Keith in the Pacific and HurricanesCamille and Allen in the North Atlantic currently share this record with Tip.[119] Camille was the only storm to actually strike land while at that intensity, making it, with 165 knots (85 m/s) or 190 miles per hour (310 km/h) sustained winds and 183 knots (94 m/s) or 210 miles per hour (340 km/h) gusts, the strongest tropical cyclone on record at landfall.[120] Typhoon Nancy in 1961 had recorded wind speeds of 185 knots (95 m/s) or 215 miles per hour (346 km/h), but recent research indicates that wind speeds from the 1940s to the 1960s were gauged too high, and this is no longer considered the storm with the highest wind speeds on record.[96] Similarly, a surface-level gust caused by Typhoon Paka on Guam was recorded at 205 knots (105 m/s) or 235 miles per hour (378 km/h). Had it been confirmed, it would be the strongest nontornadic wind ever recorded on the Earth's surface, but the reading had to be discarded since the anemometer was damaged by the storm.[121] In addition to being the most intense tropical cyclone on record, Tip was the largest cyclone on record, with tropical storm-force winds 2,170 kilometres (1,350 mi) in diameter. The smallest storm on record, Tropical Storm Marco, formed during October 2008, and made landfall in Veracruz. Marco generated tropical storm-force winds only 37 kilometres (23 mi) in diameter.
[122]
Hurricane John is the longest-lasting tropical cyclone on record, lasting 31 days in 1994. Before the advent of satellite imagery in 1961, however, many tropical cyclones were underestimated in their durations.[123] John is also the longest-tracked tropical cyclone in the Northern Hemisphere
on record, which had a path of 7,165 miles (13,280 km). Reliable data for Southern Hemisphere cyclones is unavailable.[124]
Changes due to El Niño-Southern Oscillation
See also: El Niño-Southern Oscillation Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.[125]When the subtropical ridge position shifts due to El Nino, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September-November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E which would favor the Japanese archipelago.[126] During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long term average.
[127]
The tropical Atlantic ocean experiences depressed activity due to increased vertical wind
shear across the region during El Niño years.[128] During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China.[126]
Long-term activity trends
See also: Atlantic hurricane reanalysis
Atlantic Multidecadal Cycle since 1950, usingaccumulated cyclone energy (ACE)
While the number of storms in the Atlantic has increased since 1995, there is no obvious global trend; the annual number of tropical cyclones worldwide remains about 87 ± 10. However, the ability of climatologists to make long-term data analysis in certain basins is limited by the lack of reliable historical data in some basins, primarily in the Southern Hemisphere.[129] In spite of that, there is some evidence that the intensity of hurricanes is increasing. Kerry Emanuel stated, "Records of hurricane activity worldwide show an upswing of both the maximum wind speed in and the duration of hurricanes. The energy released by the average hurricane (again considering all hurricanes worldwide) seems to have increased by around 70% in the past 30 years or so, corresponding to about a 15% increase in the maximum wind speed and a 60% increase in storm lifetime."[130] Atlantic storms are becoming more destructive financially, since five of the ten most expensive storms in United States history have occurred since 1990. According to the World Meteorological Organization, “recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions.”[131] Pielke et al. (2008) normalized mainland U.S. hurricane damage from 1900–2005 to 2005 values and found no remaining trend of increasing absolute damage. The 1970s and 1980s were notable because of the extremely low amounts of damage compared to other decades. The decade 1996–2005 was the second most damaging among the past 11 decades, with only the decade 1926–1935 surpassing its costs. The most damaging single storm is the 1926 Miami hurricane, with $157 billion of normalized damage.[132] Often in part because of the threat of hurricanes, many coastal regions had sparse population between major ports until the advent of automobile tourism; therefore, the most severe portions of hurricanes striking the coast may have gone unmeasured in some instances. The combined effects of ship destruction and remote landfall severely limit the number of intense hurricanes in the official record before the era of hurricane reconnaissance aircraft and satellite meteorology. Although the record shows a distinct increase in the number and strength of intense hurricanes, therefore, experts regard the early data as suspect.[133] The number and strength of Atlantic hurricanes may undergo a 50–70 year cycle, also known as the Atlantic Multidecadal Oscillation. Nyberg et al. reconstructed Atlantic major hurricane activity back to the early 18th century and found five periods averaging 3–5
major hurricanes per year and lasting 40–60 years, and six other averaging 1.5–2.5 major hurricanes per year and lasting 10–20 years. These periods are associated with the Atlantic multidecadal oscillation. Throughout, a decadal oscillation related to solar irradiance was responsible for enhancing/dampening the number of major hurricanes by 1– 2 per year.[134] Although more common since 1995, few above-normal hurricane seasons occurred during 1970–94.[135] Destructive hurricanes struck frequently from 1926–60, including many major New England hurricanes. Twenty-one Atlantic tropical storms formed in 1933, a record only recently exceeded in 2005, which saw 28 storms. Tropical hurricanes occurred infrequently during the seasons of 1900–25; however, many intense storms formed during 1870–99. During the 1887 season, 19 tropical storms formed, of which a record 4 occurred after 1 November and 11 strengthened into hurricanes. Few hurricanes occurred in the 1840s to 1860s; however, many struck in the early 19th century, including a 1821 storm that made a direct hit on New York City. Some historical weather experts say these storms may have been as high as Category 4 in strength.[136] These active hurricane seasons predated satellite coverage of the Atlantic basin. Before the satellite era began in 1960, tropical storms or hurricanes went undetected unless a reconnaissance aircraft encountered one, a ship reported a voyage through the storm, or a storm hit land in a populated area.[133] The official record, therefore, could miss storms in which no ship experienced gale-force winds, recognized it as a tropical storm (as opposed to a high-latitude extra-tropical cyclone, a tropical wave, or a brief squall), returned to port, and reported the experience. Proxy records based on paleotempestological research have revealed that major hurricane activity along the Gulf of Mexico coast varies on timescales of centuries to millennia.[137]
[138]
Few major hurricanes struck the Gulf coast during 3000–1400 BC and again during the
most recent millennium. These quiescent intervals were separated by a hyperactive period during 1400 BC and 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times. This millennial-scale variability has been attributed to long-term shifts in the position of the Azores High,
[138]
which may also be linked to changes in the strength of the North Atlantic Oscillation.[139]
According to the Azores High hypothesis, an anti-phase pattern is expected to exist between the Gulf of Mexico coast and the Atlantic coast. During the quiescent periods, a more northeasterly position of the Azores High would result in more hurricanes being steered towards the Atlantic coast. During the hyperactive period, more hurricanes were
steered towards the Gulf coast as the Azores High was shifted to a more southwesterly position near the Caribbean. Such a displacement of the Azores High is consistent with paleoclimatic evidence that shows an abrupt onset of a drier climate in Haiti around 3200 14C years BP,[140] and a change towards more humid conditions in the Great Plains during the late-Holocene as more moisture was pumped up the Mississippi Valley through the Gulf coast. Preliminary data from the northern Atlantic coast seem to support the Azores High hypothesis. A 3000-year proxy record from a coastal lake in Cape Cod suggests that hurricane activity increased significantly during the past 500– 1000 years, just as the Gulf coast was amid a quiescent period of the last millennium.
Global warming
See also: Effects of global warming See also: Hurricane Katrina and global warming The U.S. National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory performed a simulation to determine if there is a statistical trend in the frequency or strength of tropical cyclones over time. The simulation concluded "the strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere".
[141]
In an article in Nature, Kerry Emanuel stated that potential hurricane destructiveness, a measure combining hurricane strength, duration, and frequency, "is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multidecadal oscillations in the North Atlantic and North Pacific, and global warming". Emanuel predicted "a substantial increase in hurricane-related losses in the twenty-first century".[142] In more recent work published by Emanuel (in the March 2008 issue of the Bulletin of the American Meteorological Society), he states that new climate modeling data indicates “global warming should reduce the global frequency of hurricanes.”[143] According to the Houston Chronicle, the new work suggests that, even in a dramatically warming world, hurricane frequency and intensity may not substantially rise during the next two centuries.[144] Similarly, P.J. Webster and others published an article in Science examining the "changes in tropical cyclone number, duration, and intensity" over the past 35 years, the period when satellite data has been available. Their main finding
Costliest U.S. Atlantic hurricanes
Total estimated property damage, adjusted for wealth normalization[132]
was although the number of cyclones decreased throughout the planet excluding the north Atlantic Ocean, there was a great increase in the number and proportion of very strong cyclones.[145] The strength of the reported effect is surprising in light of modeling studies[146] that predict only a one half category increase in storm intensity as a result of a ~2 °C (3.6 °F) global warming. Such a response would have predicted only a ~10% increase in Emanuel's potential destructiveness index during the 20th century rather than the ~75–120% increase
Ran k 1 2 3 4 5 6
Hurricane "Miami" "Galveston" Katrina "Galveston" Andrew "New England" "Cuba– Florida"
Main article: List of costliest Atlantic hurricanes
he reported.[142] Secondly, after adjusting for changes in population and inflation, and despite a more than 100% increase in Emanuel's potential destructiveness index, no statistically significant increase in the monetary damages resulting from Atlantic hurricanes has been found.[132][147] Sufficiently warm sea surface temperatures are considered vital to the development of tropical cyclones.[148] Although neither study can directly link hurricanes with global warming, the increase in sea surface temperatures is believed to be due to both global warming and natural variability, e.g. the hypothesized Atlantic Multidecadal Oscillation (AMO), although an exact attribution has not been defined.[149] However, recent temperatures are the warmest ever observed for many ocean basins.[142] In February 2007, the United Nations Intergovernmental Panel on Climate Change released its fourth assessment report on climate change. The report noted many observed changes in the climate, including atmospheric composition,
global average temperatures, ocean conditions, among others. The report concluded the observed increase in tropical cyclone intensity is larger than climate models predict. Additionally, the report considered that it is likely that storm intensity will continue to increase through the 21st century, and declared it more likely than not that there has been some human contribution to the increases in tropical cyclone intensity.[150] However, there is no universal agreement about the magnitude of the effects anthropogenic global warming has on tropical cyclone formation, track, and intensity. For example, critics such as Chris Landsea assert that man-made effects would be "quite tiny compared to the observed large natural hurricane variability".[151] A statement by the American Meteorological Society on 1 February 2007 stated that trends in tropical cyclone records offer "evidence both for and against the existence of a detectable anthropogenic signal" in tropical cyclogenesis.[152] Although many aspects of a link between tropical cyclones and global warming are still being "hotly debated",
[153]
a point of agreement is that no individual tropical cyclone or season can be
attributed to global warming.[149][153] Research reported in the 3 September 2008 issue of Nature found that the strongest tropical cyclones are getting stronger, particularly over the North Atlantic and Indian oceans. Wind speeds for the strongest tropical storms increased from an average of 140 miles per hour (230 km/h) in 1981 to 156 miles per hour (251 km/h) in 2006, while the ocean temperature, averaged globally over the all regions where tropical cyclones form, increased from 28.2 °C (82.8 °F) to 28.5 °C (83.3 °F) during this period.[154][155]
Related cyclone types
Subtropical Storm Gustav in 2002
See also: Cyclone, Extratropical cyclone, and Subtropical cyclone In addition to tropical cyclones, there are two other classes of cyclones within the spectrum of cyclone types. These kinds of cyclones, known as extratropical cyclones and subtropical cyclones, can be stages a tropical cyclone passes through during its formation or dissipation.[156] An extratropical cyclone is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses; additionally, although not as frequently, an extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone.[157] From space, extratropical storms have a characteristic "comma-shaped" cloud pattern.[158] Extratropical cyclones can also be dangerous when their low-pressure centers cause powerful winds and high seas.[159] A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wide band of latitudes, from the equator to 50°. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.[160] From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.[161]
Tropical cyclones in popular culture
Main article: Tropical cyclones in popular culture In popular culture, tropical cyclones have made appearances in different types of media, including films, books, television, music, and electronic games. The media can have tropical cyclones that are entirely fictional, or can be based on real events.[162] For example, George Rippey Stewart's Storm, a bestseller published in 1941, is thought to have influenced meteorologists into giving female names to Pacific tropical cyclones.[163] Another example is the hurricane in The Perfect Storm, which describes the sinking of the Andrea Gail by the1991 Perfect Storm.[164] Also, hypothetical hurricanes have been featured in parts of the plots of series such as The Simpsons, Invasion, Family Guy, Seinfeld, Dawson's Creek, and CSI Miami.[162][165][166][167][168][169] The 2004 film The Day After
Tomorrow includes several mentions of actual tropical cyclones as well as featuring fantastical "hurricane-like" non-tropical