cyclone

cyclone The term ‘cyclone’ refers to large-scale rotating weather systems which rotate with positive vorticity. Two major types of cyclones are tropical cyclones and extra- tropical cyclones. The latter are also known as depressions. The mechanisms of formation and the structures of these two forms are so different that they should be considered separately. It is unfortunate that they share the same name.

Tropical cyclones

Tropical cyclones are also known as hurricanes and typhoons. They develop over tropical oceans and can produce extremely heavy rainfall and devastating winds with sustained wind speeds sometimes in excess of 100 metres per second (m s⁻¹). Satellite pictures (Fig. 1) reveal a striking circular symmetry in tropical cyclones with a small (50 km diameter) cloud-free ‘eye’. The preferred regions for hurricane development are oceans where the winds are light, the humidity is high, and the surface water temperature is high (usually over 26 °C) over an extensive area. Since these conditions exist in some places for only part of the year, there is a tropical cyclone ‘season’, from June to November, in the tropical north Atlantic and Pacific.

The conditions required for tropical cyclones are also suitable conditions for thunderstorms, deep convective clouds with strong updraughts. Thunderstorms can be organized into a tropical cyclone in the presence of low-level convergence. The winds converging in one particular region increase the large-scale rotation in an anti clockwise sense in the northern hemisphere and in a clockwise sense in the southern hemisphere. Because this rotation is an important factor in the development of tropical cyclones, and because no rotation is imparted at the Equator, no development of tropical cyclones is found within about 5 degrees of the Equator.

As the air converges, the thunderstorms become more organized and closer together. Huge amounts of water evaporating from the warm ocean surface are carried aloft in the bands of thunderstorms. As the air rises it cools, and the water condenses releasing latent heat. The latent heat released greatly enhances the buoyancy of the air, producing even stronger updraughts, which in turn draw in more converging air at the base. There is positive feedback as more warm moist air being drawn into the base of the clouds produces even stronger updraughts. At the tropopause the air spreads out in bands moving away from the centre of the cyclone. When this divergence of air at high levels exceeds the convergence of air at low levels, the surface pressure drops, forming a low-pressure centre around which the air circulates and converges, bringing in even more warm moist air to feed the cyclone.

The clear ‘eye’ at the centre of the cyclone is formed by air subsiding as it emerges from the intense eye-wall thunderstorms. This small region of subsiding air warms as it descends through the atmosphere, inhibiting the growth of clouds and producing a clear eye. The fiercest storms are those in the eye-wall cloud with the highest rainfall rate, up to 250 mm per hour, and the strongest winds, up to 100 ms⁻¹.

Tropical cyclones move somewhat erratically, making it difficult to predict their exact course. As they move to areas where the sea is not so warm, their energy source is removed and they dissipate.

Extra-tropical cyclones

Extra-tropical cyclones are the middle latitude tropospheric circulation systems also known as depressions. The life cycle of a depression is often described by the polar front approach, in which the depression is seen as a disturbance which grows and modifies the front as it develops. Another approach is used here to illustrate the three-dimensional structure and the development mechanism by which an extra-tropical cyclone develops. Fig. 2 shows a wave depression at its most vigorous stage of development. The surface weather map shows an open warm sector with a deepening low-pressure centre. The low-pressure centre is situated below a region in the upper troposphere where a wave or trough of low pressure is lying slightly behind the surface low. This is exactly the configuration required to enable a low-pressure centre to deepen.

At the upper level there is a region of divergence. The air moving around the trough moves more slowly at the curved section and faster as the flow becomes straighter. The result of this is that some air is removed from the column of air above the low-pressure centre. However, near the surface the flow is not exactly around the centre of low pressure but slightly towards the centre. This is a region of convergence where air is added to the column. At this stage in the development of the depression the divergence at the upper level is greater than the low-level convergence, and so the net effect is to remove air from the air column and thus reduce the surface pressure. At an earlier stage the development was initiated when the upper level trough, which is moving faster than the low-level system, approached the region of the polar front and the initial divergence produced a low-pressure centre.

The circulation of the wave depression is in an anticlockwise sense. (In the southern hemisphere the circulation would be clockwise, but the cold air would be to the south and so the mechanism would be the same.) The warm air at low levels in the warm sector is lifted above the warm front. This applies not only to air at the surface, but to all the air through a substantial depth. At the same time the cold air moving southwards behind the low-pressure centre is losing height. The net effect of warm air rising and cold air sinking is to decrease the ‘centre of gravity’ of the system because the cold air is more dense than the warm air. By lowering the centre of gravity, some potential energy is removed from the system and converted to kinetic energy—the energy of motion. (When we release an object from a height and let it fall, we are converting potential energy into kinetic energy). In the wave depression the kinetic energy is manifested by the strength of the winds in the circulating system. Because it is these winds that are moving the warm air upwards and the cold air downwards, the process accelerates and feeds on itself. This unstable situation, known as baroclinic instability, continues until the warm air is lifted from the surface into the upper troposphere.

By moving warm, less dense, air into the column ahead of the upper trough the pressure ahead of the upper trough is reduced. Conversely, the cold air introduced behind the trough increases the pressure: so the trough minimum is moved from east to west. This means that the region of divergence is now no longer above the centre of low pressure at the surface. In fact, there is neither divergence nor convergence at upper levels, but convergence remains at the surface. The effect is thus to increase the surface pressure and complete the last phase in the life of the depression.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co., St. Paul, Minnesota.
Palmen, E. and and Newton C. W. (1969) Atmospheric circulation systems. Academic Press, London.