Cloud Patterns

The area of clouds associated with tropical cyclones is much larger than the region influenced by strong winds. This is because the strong winds reflect the area characterised by the steep atmospheric pressure gradient, whereas the

Tropical Cyclones Global Warming
Fig. 3.2. Satellite image of Tropical Cyclone Zoe on 27 December 2002, northeast of the main islands of Vanuatu. Note the plumes of high cirrus clouds at the outer storm extremities and the spiralling arms. Base image courtesy of NOAA.

clouds form wherever there is convection associated with the cyclone system, driven by the expanse of warm sea-surface temperatures. As a result, the cloud pattern of a tropical cyclone extends well beyond the rings of close concentric isobars shown on a weather chart, and visible and infrared satellite images are the best way of observing the arrangement of clouds (Fig. 3.2).

At their outer-most extremities, tropical cyclones are marked by high cirrus clouds. Further inwards an even layer of cirrostratus is encountered. Because convection in tropical cyclones is organised into elongated bands that are oriented in the same direction as the horizontal wind, an overhead view of cyclone cloud structure shows the familiar spiral arms that converge inwards. Towards the centre, the convective bands are tightly coiled. Near to the cyclone centre, a dense wall of cumulonimbus towers exists, causing thunderstorm activity with torrential rain and frequent lightning. The round zone of complete cloud cover is known as the central dense overcast area, or CDO. Near-circular CDOs are indicative of minimal vertical shear in the atmosphere, a condition that is favourable for the maintenance of tropical cyclone structure. The CDO contains a distinct well-centred eye, which is the hub of storm rotation.

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