Drifting snow high drifting snow low

Figure 9.4 Weather-map symbols for various kinds of precipitation.

Figure 9.5 The evolution of a thunderstorm: (a) is the early cumulus stage, (b) the mature stage and (c) the dissipating stage.

Figure 9.5 The evolution of a thunderstorm: (a) is the early cumulus stage, (b) the mature stage and (c) the dissipating stage.

spreads out to form an anvil (Figure 8.4), which heralds the 'mature' stage, lasting about 15-30 minutes. At that point, downdraughts develop under the weight of the water in the cloud, and there is heavy precipitation. Then comes the 'dissipation' stage, when downdraughts are dominant and the rainfall peters out. So the cell's static instability has been discharged, and all that is left is the cirrus cloud of the ice crstals in the anvil, which may take days to vanish. The energy released in an average summer thunderstorm is similar to that of a Nagasaki-size atomic bomb (Section 8.1).

Grouping of Cells

Thunderstorm cells group themselves in four ways:

1 Some are randomly distributed, isolated cells called air-mass thunderstorms (or heat thunderstorms), which each cover only a few square kilometres and last about an hour (Figure 9.5).

2 There are also clusters of cells, forming multicell thunderstorms, which result from downbursts. The airflow from the downburst in a cell often hits the ground as a squall of cold wind, whose forward boundary is called a gust front (Chapter 14). This pushes adjacent surface air upwards, triggering an updraught which initiates another cell, to discharge instability there. So there is a domino effect leading to a sequence of heavy showers across a region (Figure 9.6). Most multicell thunderstorms are loosely organised in a line. The youngest cells are usually at the north (equatorward) end of the line and involve vigorous convection, while stratiform precipitation from decaying cells predominates at the southern (i.e. poleward) end.

3 Cells triggered by their neighbours may be aligned in a squall line, a string of

Figure 9.6 A multi-cell thunderstorm with five cells at this moment. The cells discharged their instability and provided rain in the order shown, the downdraught in one cell triggering an updraught in the next.

thunderstorms with a common, connected gust front. The line may be several hundreds of kilometres long. Squall lines in SE Australia are usually oriented either north to south or north-west to south-east, and move ahead of a cold front (Chapter 13) with speeds up to 30 m/s, causing strong wind gusts near the surface. Or squall lines may be caused by small boundary-layer disturbances. They can endure for more than a day if the troposphere is sufficiently unstable (i.e. a CAPE exceeding 2 kJ/kg—see Section 7.3 and Figure 7.6) and if also there is a difference of at least 20 m/s between winds at the surface and at 5 km altitude, respectively. Such storms can be sufficiently intense to induce tornadoes (Section 7.5), hail and gusts of strong wind. The squall line is often followed by a broad belt of stratiform clouds (Figure 9.7) which may yield as much rain as the squall line itself.

Finally, there are mesoscale connective complexes (MCCs), which, unlike squall lines, are almost round masses of thunderstorms, with a common anvil cloud

Figure 9.7 A storm in the Amazon valley. It is shown moving from right to left, with new cells forming at the leading edge. Heavy rain falls from the central 'convective line' and ice crystals are carried towards the rear, where they grow in stratiform clouds.s

near the tropopause. By definition, the anvil of a MCC is at least 50,000 km2 in area, has a temperature below -52°C and lasts for over six hours. However, a MCC may last for several days, producing heavy rain over 12-16 hours. They are uncommon: only about twenty MCC's occur each year over Australia and the surrounding waters. They appear downwind of mountain ranges such as the Andes, i.e. to the east at midlatitudes but to the west at low (Figure 9.8), where dry easterly air coming over the mountains

Figure 9.8 Distribution of some 'mesoscale convective complexes' about the Americas between 1983 and 1985.

Plate 9.1 A developing mesoscale convective complex (MCC) over Botswana on 8 January 1984, seen from a satellite at 830 km. Isotherms of cloud-top temperature have been superimposed, derived from infra-red measurements taken at about the same time by the geostationary satellite Meteosat. Notice that the lowest temperature was about 193k (i.e. -80°C), showing that the cloud top was about 16 km high at the tropopause.

finds itself above warm moist air from the equator.

Distribution

The location of thunderstorms can be determined in at least four ways—by direct observation, from radar echoes, in satellite images, or by lightning detection. Human observations are biased by the population density and are limited in range: thunder can be heard only within 16 km or so (Note 7.M) and lightning seen only at night within about 80 km, depending on the cloudiness (Section 9-6). The difference between these distances might explain why some forty-four days of lightning are recorded annually at Sydney but only twenty-nine of thunder. A similar difference is found at Brisbane (Figure 9.9).

The intensity of the echoes seen by radar is a measure of the rate of rainfall, so thunderstorms cause strong echoes within a range of about 300 km. Figure 9.10 shows a wide area of rainfall, with a rapid movement of the centre of a storm over Sydney, comparable with Figure 9.6.

Geostationary satellites (Section 8.8) can view a whole hemisphere at once and show the progress of entire storm systems. Infra-red photographs show the tops of thunder clouds

- BRISBANE

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