Description

A TC is a circular system, whose cross-section is shown schematically in Figure 13.12. There is a central eye of cloudless, calm, warm air, first described by the pirate William Dampier in 1687

when exploring parts of the Pacific. The eye is typically 25 km in diameter, but can be half or twice that. Around it is a wall of strong winds, rotating cyclonically, i.e. clockwise in the southern hemisphere. The air inside the eye is subsiding, and that is why it is cloudless, calm and warm. That is surrounded by a wall of cloud within a raging vortex of updraught. This in turn is encompassed by weaker updraughts further from the eye, creating spirals of convective rainclouds, easily seen in satellite photographs. Between the eye wall and the central subsidence there is a thin cylinder of descending air cooled by evaporation from the wall. This cooled air becomes entrained into the ascending wall near sea-level (Figure 13.12).

The updraught in the eye-wall is fed by a

150 100 50 0 50 100 150

radial distance: km

Figure 13-12 Radial cross-section of an idealised tropical cyclone. The solid lines to the right of the diagram are isotachs (m/s) showing the primary cyclonic circulation around the eye. The dashed lines on the right are isotherms (°C). The secondary, vertical circulation is shown on the left, with radial flow aloft and subsidence in the surrounding environment.

Plate 13.1 Tropical cyclone Chloe viewed from above, when located off the north-west coast of Australia, in the Kimberley region. The photograph was taken from the NOAA-12 satellite at 7.19 a.m. local time on 7 April 1995. Notice the eye of the cyclone, and the swirl of cloud from it in the upper troposphere. The distance across the solid disk of cloud around the eye is about 300 km.

Plate 13.1 Tropical cyclone Chloe viewed from above, when located off the north-west coast of Australia, in the Kimberley region. The photograph was taken from the NOAA-12 satellite at 7.19 a.m. local time on 7 April 1995. Notice the eye of the cyclone, and the swirl of cloud from it in the upper troposphere. The distance across the solid disk of cloud around the eye is about 300 km.

clockwise (i.e. cyclonic) spiralling inflow at sea-level, felt over a radius of up to 1,000 km. On the other hand, convective rainclouds extend only 500 km or less, and winds of 10-20 m/s up to about 200 km from the eye. The updraught is surmounted by an anticyclonic outflow of cirrus near the tropopause, which itself is lifted by the force of the updraughts to as much as 18 km.

Warming in the eye by subsidence from near the tropopause leads to a tall column of low-density air, and hence a very low sea-level pressure, e.g. below 950 hPa in severe cyclones. The record minimum in the Australian region is 914 hPa, measured in 1899, whilst a global record of 876 hPa was observed in the north-west Pacific in 1975. So the sea-level pressure may fall by 50 hPa in eight hours as a TC approaches.

The steep horizontal gradients of sea-level pressure imply strong surface winds (Section 12.2; Note 13.F). In fact, TCs are conventionally distinguished from tropical storms by wind speeds above 25 m/s for at least ten minutes (Table 13.2). A rough rule is that a central pressure of 950 hPa means winds up to 39 m/ s at 30°S, 46 m/s at 20°S and 56 m/s at 10°S (Note 13.F).

An analysis of TCs affecting Fiji shows that the wall of the eye characteristically involves gusts in excess of 44 m/s. The highest gust recorded at the surface in the Australian region was 69 m/s (i.e. 250 km/h) but there are even higher speeds at about 2 km elevation within the eyewall.

The energy for these winds comes from the surface of the warm ocean, collected directly as sensible heat and as latent heat in the rapid evaporation that the high speed of the surface winds induces. This heat is released in the condensation caused by the adiabatic expansion of surface air entering the central zone of low pressure, and then the consequent warming and convection within the eyewall powers the immense updraught. That lifts the heat to the upper limb of the Hadley cell, where it is exported to higher latitudes (Figure 12.16). This export of heat is important in limiting temperatures near the equator.

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