Tropical cloud clusters

Mesoscale convective systems (MCSs) are widespread in tropical and subtropical latitudes. The mid-latitude mesoscale convective complexes discussed in Chapter 9.I are an especially severe category of MCS. Satellite studies of cold (high) cloud-top signatures show that tropical systems typically extend over a 3000 to 6000 km2 area. They are common over tropical South America and the maritime continent of Indonesia-Malaysia and adjacent western equatorial Pacific Ocean warm pool. Other land areas include Australia, India and Central America, in their respective summer seasons. As a result of the diurnal regimes of convective activity, MCSs are more frequent at sunset compared with sunrise by 60 per cent over the continents and 35 per cent more frequent at sunrise than sunset over the oceans. Most of the intense systems (MCCs) occur over land, particularly where there is abundant moisture and usually downwind of orographic features that favour the formation of low-level jets.

Mesoscale convective systems fall into two categories: non-squall and squall line. The former contain one or more mesoscale precipitation areas. They occur diurnally, for example, off the north coast of Borneo in winter, where they are initiated by convergence of a nocturnal land breeze and the northeast monsoon flow (Figure 11.11). By morning (08:00 LST), cumulonimbus cells give precipitation. The cells are linked by an upper-level cloud shield, which persists when the convection dies at around noon as a sea breeze system replaces the nocturnal convergent flow. Recent studies over the western equatorial Pacific warm pool indicate that convective cloud systems account for <50 per cent of the total in large precipitation areas (boxes 240 X 240 km), while stratiform precipitation is more widespread and yields over half of the total precipitation.

Tropical squall line systems (Figure 11.12) form the leading edge of a line of cumulonimbus cells (Plate 29). The squall line and gust front advance within the low-level flow and by forming new cells. These mature and eventually dissipate to the rear of the main line. The process is analogous to that of mid-latitude squall lines (see Figure 9.28) but the tropical cells are weaker. Squall line systems, known as sumatras, cross Malaya from the west during the southwest monsoon season giving heavy rain and often thunder. They appear to be initiated by the convergent effects of land breezes in the Malacca Straits.

In West Africa, systems known as disturbance lines (DLs) are an important feature of the climate in the summer half-year, when low-level southwesterly monsoon air is overrun by dry, warm Saharan air. The meridional airmass contrast helps to set up the lower-tropospheric African Easterly Jet (AEJ) (see Figure 11.40). The convective DLs are transported across West Africa by African easterly waves that are steered by the AEJ at around 600 mb. The waves recur with a four to eight-day period during the wet season (May to

Figure 11.11 Schematic development of a non-squall cloud cluster off the north coast of Borneo: large arrows indicate the major circulation; small arrows, the local circulation; vertical shading, the zones of rain; stars, ice crystals; and circles, melted raindrops.

Source: After Houze et al. (1981), by permission of the American Meteorological Society.

Figure 11.11 Schematic development of a non-squall cloud cluster off the north coast of Borneo: large arrows indicate the major circulation; small arrows, the local circulation; vertical shading, the zones of rain; stars, ice crystals; and circles, melted raindrops.

Source: After Houze et al. (1981), by permission of the American Meteorological Society.

Figure 11.12 Cross-section of a tropical squall line cloud cluster showing locations of precipitation and ice particle melting. Dashed arrows show the air motion generated by the squall line convection and the broad arrows the mesoscale circulation.

Source: After Houze; from Houze and Hobbs (1982)

Figure 11.12 Cross-section of a tropical squall line cloud cluster showing locations of precipitation and ice particle melting. Dashed arrows show the air motion generated by the squall line convection and the broad arrows the mesoscale circulation.

Source: After Houze; from Houze and Hobbs (1982)

October). Disturbance lines tend to form when there is divergence in the upper troposphere north of the Tropical Easterly Jet (see also Figure 11.40A). They are several hundred kilometres long and travel westward at about 50 km hr-1 giving squalls and thunder showers before dissipating over cold-water areas of the North Atlantic. Spring and autumn rainfall in West Africa is derived in large part from these disturbances. In wet years, when the AEJ is further north, the wave season is prolonged and the waves are stronger. Figure

11.13 for Kortright (Freetown), Sierra Leone illustrates the daily rainfall amounts in 1960 to 1961 associated with disturbance lines at 8°N. Here the summer monsoon rains make up the greater part of the total, but their contribution diminishes northward.

Figure 11.13 Daily rainfall at Kortwright, Freetown, Sierra Leone, October I960 to September 1961. Source: After Gregory (1965).

Figure 11.14 Regions experiencing a seasonal surface wind shift of at least 120°, showing the frequency of the prevailing octant.

Source: After Khromov.

Figure 11.14 Regions experiencing a seasonal surface wind shift of at least 120°, showing the frequency of the prevailing octant.

Source: After Khromov.

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment