As the converging air spirals inwards, it becomes organised into bands of cloud that rotate slowly clockwise around the storm centre. The spiral bands of cloud are known as feeder bands because they feed heat and moisture into the central low pressure. This is essential for the continuing development of the cyclone, by providing energy into the storm system. At a certain distance from the centre of the low pressure, typically 20-40 km, the inflowing air suddenly turns upwards in a ring of intense uplift surrounding the eye (Fig. 2.3). This is called the eyewall and is the ring of strongest winds and heaviest precipitation. The upward transport of heat and moisture in the eyewall produces the vertical growth of immense thunderstorm clouds. The thunderstorms release large amounts of heat at mid and upper levels of the troposphere through the condensation of water vapour, allowing clouds to grow to the very top of the troposphere.
Once the tropopause is reached, the presence of the thermal inversion at this level in the atmosphere causes the air and clouds to spread out horizontally. This spreading pattern is known as the upper-level divergence or anticyclone. The divergence at the summit of the storm, at a height of between 9,000 and 15,000 m in the upper troposphere, appears to be another key process on which continuing cyclone development depends. Divergence aloft is essential to enable the high-level outflow of all the air that is being drawn in by convergence at the base of the storm. In effect, the upper-level divergence 'ventilates' the system. Most heat energy is exported by the anticyclone
100 centre 100
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Fig. 2.3. Cross-section through a tropical cyclone to illustrate size, cloud structure, eye and streamlines of air movement. Modified from Barry and Chorley (1982).
circulation at an altitude of about 12 km. At this altitude the atmospheric pressure is approximately 200 mb, so climatologists refer to divergence occurring in streamlines at the 200 mb layer.
The upper divergence pattern also establishes an important positive-feedback mechanism. By intensifying the central low pressure at sea level and enhancing the low-level convergence pattern, this continues the cycle to fuel the system. During this stage a tropical cyclone is considered to be mature and has acquired a quasi-steady state, with only random fluctuations in central pressure and maximum wind speed.
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