A Hurricanes and typhoons

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The most notorious type of cyclone is the hurricane (or typhoon). Some eighty or so cyclones each year are responsible, on average, for 20,000 fatalities, as well as causing immense damage to property and a serious shipping hazard, due to the combined effects of high winds, high seas, flooding from the heavy rainfall and coastal storm surges. Considerable attention has been given to forecasting their development and movement, so their origin and structure are beginning to be understood. Naturally, the catastrophic force of a hurricane makes it a very difficult phenomenon to investigate, but information is obtained from aircraft reconnaissance flights sent out during the 'hurricane season', from radar observations of cloud and precipitation structure (Plate F), and from satellite data (see Plate 27).

The typical hurricane system has a diameter of about 650 km, less than half that of a mid-latitude depression (Plate 23), although typhoons in the western Pacific are often much larger. The central pressure is commonly 950 mb and exceptionally falls below 900 mb. Named tropical storms are those defined as having one-minute average wind velocities of at least 18 m s-1 at the surface. If these winds intensify to at least 33 m s-1, the named storm becomes a tropical cyclone. Five hurricane intensity classes are distinguished: category (1) weak (winds of 33 to 42 m s-1); (2) moderate (43 to 49 m s-1); (3) strong (50 to 58 m s-1); (4) very strong (59 to 69 m s-1) and (5) devastating (70 m s-1 or more). Hurricane Camille, which struck coastal Mississippi in August 1969, was a category (5) storm, while Hurricane Andrew, which devastated southern Florida in August 1992, has been reclassified also as a category (5) storm. In 1997 there were eleven super-typhoons in the northwest Pacific with winds >66 ms-1. The great vertical development of cumulonimbus clouds, with tops at over 12,000 m, reflects the immense convective activity concentrated in such systems. Radar and satellite studies show that the convective cells are normally organized in bands that spiral inward towards the centre.

Although the largest cyclones are characteristic of the Pacific, the record is held by the Caribbean hurricane

'Gilbert'. This hurricane was generated 320 km east of Barbados on 9 September 1988 and moved westward at an average speed of 24 to 27 km hr-1, dissipating off the east coast of Mexico. Aided by an upper tropospheric high-pressure cell north of Cuba, Hurricane Gilbert intensified very rapidly, the pressure at its centre dropped to 888 mb (the lowest ever recorded in the western hemisphere), and maximum wind speeds near the core were in excess of 55 m s-1. More than 500 mm of rain fell on the highest parts of Jamaica in only nine hours. However, the most striking feature of this record storm was its size, being some three times that of average Caribbean hurricanes. At its maximum extent, the hurricane had a diameter of3500 km, disrupting the ITCZ along more than one-sixth of the earth's equatorial circumference and drawing in air from as far away as Florida and the Galapagos Islands.

The main tropical cyclone activity in both hemispheres is in late summer to autumn during times of maximum northward and southward shifts of the equatorial trough (Table 11.1). A few storms affect both the western North Atlantic and North Pacific areas as early as May and as late as December, and have occurred in every month in the latter area. In the Bay of Bengal, there is also a secondary early summer maximum. Floods from a tropical cyclone that struck coastal Bangladesh on 24 to 30 April 1991 caused over 130,000 deaths from drowning and left over ten million people homeless. The annual frequency of cyclones shown in Table 11.1 is only approximate, since in some cases it is uncertain whether the winds actually exceeded hurricane force. In addition, storms in the more remote parts of the South Pacific and Indian Oceans frequently escaped detection prior to the use of weather satellites.

A number of conditions are necessary, even if not always sufficient, for cyclone formation. One requirement as shown by Figure 11.8 is an extensive ocean area with a surface temperature greater than 27°C. Cyclones rarely form near the equator, where the Coriolis parameter is close to zero, or in zones of strong vertical wind shear (i.e. beneath a jet stream), since both factors inhibit the development of an organized vortex. There is also a definite connection between the seasonal position of the equatorial trough and zones of cyclone formation. This is borne out by the fact that no cyclones occur in the South Atlantic (where the trough never lies south of 5°S) or in the southeast Pacific (where the trough remains north of the equator). However, the northeast Pacific has an unexpected number of cyclonic vortices in summer. Many of these move westward near the trough line at about 10 to 15°N. About 60 per cent of tropical cyclones seem to originate 5 to 10° latitude poleward of the equatorial trough in the doldrum sectors, where the trough is at least 5° latitude from the equator. The development regions of cyclones lie mainly over the western sections of the Atlantic, Pacific

Table 11.1 Annual frequencies and usual seasonal occurrence of tropical cyclones (maximum sustained winds exceeding 25 m s-1), 1958 to 1977.

Location

Annual frequency

Main occurrence

Western North Pacific 26.3

Eastern North Pacific 13.4

Western North Atlantic 8.8

Northern Indian Ocean 6.4

Northern hemisphere total 54.6

July-October

August-September

August-October

May-June; October-November

Southwest Indian Ocean 8.4

Southeast Indian Ocean 10.3

Western South Pacific 5.9

January-March January-March January-March

Southern hemisphere total 24.5

Global total 79.1

Note: Area totals are rounded. Source: After Gray (1979).

Figure 11.8 Frequency of hurricane genesis (numbered isopleths) for a twenty-year period. The principal hurricane tracks and the areas of sea surface having water temperatures greater than 27°C in the warmest month are also shown.

Source: After Palmen (1948) and Gray (1979).

and Indian Oceans, where the subtropical high-pressure cells do not cause subsidence and stability and the upper flow is divergent. About twice per season in the western equatorial Pacific, tropical cyclones form almost simultaneously in each hemisphere near 5° latitude and along the same longitude. The cloud and wind patterns in these cyclone 'twins' are roughly symmetrical with respect to the equator.

The role of convection cells in generating a massive release of latent heat to provide energy for the storm was proposed in early theories of hurricane development. However, their scale was thought to be too small for them to account for the growth of a storm hundreds of kilometres in diameter. Research indicates that energy can be transferred from the cumulus-scale to the large-scale storm circulation through the organization of the clouds into spiral bands (see Figure 11.9 and Plate F), although the nature of the process is still being investigated. There is ample evidence to show that hurricanes form from pre-existing disturbances, but while many of these disturbances develop as closed low-pressure cells, few attain full hurricane intensity. The key to this problem is high-level outflow (Figure 11.10). This does not require an upper tropospheric anticyclone but can occur on the eastern limb of an upper trough in the westerlies. This outflow in turn allows the development of very low pressure and high wind speeds near the surface. A distinctive feature of the hurricane is the warm vortex, since other tropical depressions and incipient storms have a cold core area of shower activity. The warm core develops through the action of 100 to 200 cumulonimbus towers releasing latent heat of condensation; about 15 per cent of the area of cloud bands is giving rain at any one time. Observations show that although these 'hot towers' form less than 1 per cent of the storm area within a radius of about 400 km, their effect is sufficient to change the environment. The warm core is vital to hurricane growth because it intensifies the upper anticyclone, leading to a 'feedback' effect by stimulating the low-level influx of heat and moisture, which further intensifies convective activity, latent heat release and therefore the upper-level high pressure. This enhancement of a storm system by cumulus convection is termed conditional instability of the second kind (CISK) (cf. the basic parcel instability described on p. 94). The thermally direct circulation converts the heat increment into potential energy and a small fraction of this - about 3 per cent - is transformed into kinetic energy. The remainder is exported by the anticyclonic circulation around the 12-km (200 mb) level.

In the eye, or innermost region of the storm (see Figure 11.9 and Plate 28), adiabatic warming of descending air accentuates the high temperatures, although since high temperatures are also observed in the eye-wall cloud masses, subsiding air can be only one contributory factor. Without this sinking air in the eye, the central pressure could not fall below about 1000 mb. The eye has a diameter of some 30 to 50 km, within which the air is virtually calm and the cloud cover may be broken. The mechanics of the eye's inception are still largely unknown. If the rotating air conserved absolute angular momentum, wind speeds would become infinite at the centre, and clearly this is not the case. The strong winds surrounding the eye are more or less in cyclostrophic balance, with the small radial distance providing a large centripetal acceleration (see

Cumulonimbus Clouds Air Circulation
Figure 11.9 A model of the areal (A) and vertical (B) structure of a hurricane. Cloud (stippled), streamlines, convective features and path are shown.

Source: From Musk (1988).

p. 114). The air rises when the pressure gradient can no longer force it further inward. It is possible that the cumulonimbus anvils play a vital role in the complex link between the horizontal and vertical circulations around the eye by redistributing angular momentum in such a way as to set up a concentration of rotation near the centre.

The supply of heat and moisture combined with low frictional drag at the sea surface, the release of latent heat through condensation and the removal of the air aloft are essential conditions for the maintenance of cyclone intensity. As soon as one of these ingredients diminishes the storm decays. This can occur quite rapidly if the track (determined by the general upper tropospheric flow) takes the vortex over a cool sea surface or over land. In the latter case, the increased friction causes greater cross-isobar air motion, temporarily increasing the convergence and ascent. At this stage, increased vertical wind shear in thunderstorm cells may generate tornadoes, especially in the northeast quadrant of the storm (in the northern hemisphere). However, the most important effect of a land track is that cutting off of the moisture supply removes one of the major sources of heat. Rapid decay also occurs when cold air is drawn into the circulation or when the upper-level divergence pattern moves away from the storm.

Hurricanes usually move at 16 to 24 km hr-1, controlled primarily by the rate of movement of the upper warm core. Commonly, they recurve poleward around the western margins of the subtropical high-pressure cells, entering the circulation of the westerlies, where they die out or regenerate into extra-tropical disturbances (see Figure 11.37).

Some of these systems retain an intense circulation and the high winds and waves can still wreak havoc. This is not uncommon along the Atlantic coast of the United States and occasionally eastern Canada. Similarly, in the western North Pacific, recurved typhoons are a major element in the climate of Japan (see D, this chapter) and may occur in any month. There is an average frequency of twelve typhoons per year over southern Japan and neighbouring sea areas.

To sum up: a tropical cyclone develops from an initial disturbance, which, under favourable environmental

Models Hurricanes And Typhoon

Figure 11.10 A schematic model of the conditions conducive (left) or detrimental (right) to the growth of a tropical storm in an easterly wave; U is the mean upper-level wind speed and c is the rate of propagation of the system. The warm vortex creates a thermal gradient that intensifies both the radial motion around it and the ascending air currents, termed the solenoidal effect.

Source: From Kurihara (1985), copyright © Academic Press. Reproduced by permission.

Figure 11.10 A schematic model of the conditions conducive (left) or detrimental (right) to the growth of a tropical storm in an easterly wave; U is the mean upper-level wind speed and c is the rate of propagation of the system. The warm vortex creates a thermal gradient that intensifies both the radial motion around it and the ascending air currents, termed the solenoidal effect.

Source: From Kurihara (1985), copyright © Academic Press. Reproduced by permission.

conditions, grows first into a tropical depression and then into a tropical storm. The tropical storm stage may persist for four to five days, whereas the cyclone stage usually lasts for only two to three days (four to five days in the western Pacific). The main energy source is latent heat derived from condensed water vapour, and for this reason hurricanes are generated and continue to gather strength only within the confines of warm oceans. The cold-cored tropical storm is transformed into a warm-cored hurricane in association with the release of latent heat in cumulonimbus towers, and this establishes or intensifies an upper tropospheric anticyclonic cell. Thus high-level outflow maintains the ascent and low-level inflow in order to provide a continuous generation of potential energy (from latent heat) and the transformation of this into kinetic energy. The inner eye that forms by sinking air is an essential element in the life cycle.

Hurricane forecasting is a complex science. Recent studies of annual North Atlantic/Caribbean hurricane frequencies suggest that three major factors are involved:

1 The west phase of the Atlantic Quasi-Biennial Oscillation (QBO). The QBO involves periodic changes in the velocities of, and vertical shear between, the zonal upper tropospheric (50 mb) winds and the lower stratospheric (30 mb) winds. The onset of such an oscillation can be predicted with some confidence almost a year in advance. The east phase of the QBO is associated with strong easterly winds in the lower stratosphere between latitudes 10°N and 15°N, producing a large vertical wind shear. This phase usually persists for twelve to fifteen months and inhibits hurricane formation. The west QBO phase exhibits weak easterly winds in the lower stratosphere and small vertical wind shear. This phase, typically lasting thirteen to sixteen months, is associated with 50 per cent more named storms, 60 per cent more hurricanes and 200 per cent more major hurricanes than is the east phase.

2 West African precipitation during the previous year along the Gulf of Guinea (August to November) and in the western Sahel (August to September). The former moisture source appears to account for some 40 per cent of major hurricane activity, the latter for only 5 per cent. Between the late 1960s and 1980s the Sahel drought was associated with a marked decrease in Atlantic tropical cyclones and hurricanes, mainly through strong upper-level shearing winds over the tropical North Atlantic and a decrease in the propagation of easterly waves over Africa in August and September.

3 ENSO predictions for the following year (see G, this chapter). There is an inverse correlation between the frequency of El Niños and that of Atlantic hurricanes.

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Responses

  • ollie
    Why cyclones, hurricanes or typhoons are form at the equator from Africa to Central Pacific?
    2 years ago

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