Tropical Disturbances

It was not until the 1940s that detailed accounts were given of types of tropical disturbances other than the long-recognized tropical cyclone. Our view of tropical weather systems was revised radically following the advent of operational meteorological satellites in the 1960s. Special programmes of meteorological measurements at the surface and in the upper air, together with aircraft and ship observations, have been carried out in the Pacific and Indian Oceans, the Caribbean Sea and the tropical eastern Atlantic.

Five categories of weather system may be distinguished according to their space and timescales (see Figure 11.3). The smallest, with a life span of a few hours, is the individual cumulus, 1 to 10 km in diameter, which is generated by dynamically induced convergence in the trade wind boundary layer. In fair weather, cumulus clouds are generally aligned in 'cloud streets', more or less parallel to the wind direction (see Plate 25), or form polygonal honeycomb-pattern cells, rather than scattered at random. This seems to be related to the boundary-layer structure and wind speed (see p. 97). There is little interaction between the air layers above and below the cloud base under these conditions, but in disturbed weather conditions updrafts and downdrafts cause interaction between the two layers, which intensifies the convection. Individual cumulus towers, associated with violent thunderstorms, develop particularly in the intertropical convergence zone, sometimes reaching above 20 km in height and having updrafts of 10 to 14 m s-1. In this way, the smallest scale of system can aid the development of larger disturbances. Convection is most active over sea surfaces with temperatures exceeding 27°C, but above 32°C convection ceases to increase, due to feedbacks that are not yet fully understood.

The second category of system develops through cumulus clouds becoming grouped into mesoscale convective areas (MCAs) up to 100 km across (see Figure 11.3). In turn, several MCAs may comprise a cloud cluster 100 to 1000 km in diameter. These sub-synoptic-scale systems were initially identified from satellite images as amorphous cloud areas; they have been studied primarily from satellite data over the tropical oceans (Plate 1 and Plate 24). Their definition is rather arbitrary, but they may extend over an area 2° square up to 12° square. It is important to note that the peak convective activity has passed when cloud cover

Figure 11.3 The mesoscale and synoptic structure of the equatorial trough zone (ITCZ), showing a model of the spatial distribution (above) and of the vertical structure (below) of convective elements which form the cloud clusters.

Source: From Mason (1970), by permission of the Royal Meteorological Society.

is most extensive through the spreading of cirrus canopies. Clusters in the Atlantic, defined as more than 50 per cent cloud cover extending over an area of 3° square, show maximum frequencies of ten to fifteen clusters per month near the ITC and also at 15 to 20°N in the western Atlantic over zones of high sea-surface temperature. They consist of a cluster of mesoscale convective cells with the system having a deep layer of convergent airflow (see Figure 11.3). Some persist for only one to two days, but others develop within synoptic-scale waves. Many aspects of their development and role remain to be determined. While convection has been stressed, studies in the western equatorial Pacific 'warm pool' region indicate that large rain areas in cloud clusters consist mainly of stratiform precipitation. This accounts for over 75 per cent of the total rain area and for more than half of the rain amount. Moreover, the cloud systems are not 'warm clouds' ( p. 102) but are made up of ice particles.

The fourth category of tropical weather system includes the synoptic-scale waves and cyclonic vortices (discussed more fully below) and the fifth group is represented by the planetary-scale waves. The planetary waves (with a wavelength from 10,000 to 40,000 km) need not concern us in detail here. Two types occur in the equatorial stratosphere and another in the equatorial upper troposphere. While they may interact with lower tropospheric systems, they do not appear to be direct weather mechanisms. The synoptic-scale systems that determine much ofthe 'disturbed weather' ofthe tropics are sufficiently important and varied to be discussed under the headings of wave disturbances and cyclonic storms.

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