Diurnal variations

Diurnal weather variations are particularly evident at coastal locations in the trade wind belt and in the Indonesia-Malaysian Archipelago. Land and sea breeze regimes (see Chapter 6C.2) are well developed, as the heating of tropical air over land can be up to five times that over adjacent water surfaces. The sea breeze normally sets in between 08:00 and 11:00 hours, reaching a maximum velocity of 6 to 15 m s-1 about 13:00 to 16:00 and subsiding around 20:00. It may be up to 1000 to 2000 m in height, with a maximum velocity at an elevation of 200 to 400 m, and it normally penetrates some 20 to 60 km inland.

On large islands under calm conditions the sea breezes converge towards the centre so that an afternoon maximum of rainfall is observed. Under steady trade winds, the pattern is displaced downwind so that descending air may be located over the centre of the island. A typical case of an afternoon maximum is illustrated in Figure 11.57B for Nandi (Viti Levu, Fiji) in the southwest Pacific. The station has a lee exposure in both wet and dry seasons. This rainfall pattern is commonly believed to be widespread in the tropics, but over the open sea and on small islands a night-time maximum (often with a peak near dawn) seems to occur, and even large islands can display this nocturnal regime when there is little synoptic activity. Figure 11.57A illustrates this nocturnal pattern at four small island locations in the western Pacific. Even large islands may show this effect, as well as the afternoon maximum associated with sea breeze convergence and convection.

There are several theories concerning the nocturnal rainfall peak. Recent studies point to a radiative effect, involving more effective nocturnal cooling of cloud-free areas around the mesoscale cloud systems. This favours subsidence, which, in turn, enhances low-level convergence into the cloud systems and strengthens the ascending air currents. Strong cooling of cloud tops, relative to their surroundings, may also produce localized destabilization and encourage droplet growth by mixing of droplets at different temperatures (see Chapter 5.D). This effect would be at a maximum near dawn. Another factor is that the sea-air temperature difference, and consequently the oceanic heat supply to the atmosphere, is largest at about 03:00 to 06:00 hours. Yet a further hypothesis suggests that the semidiurnal pressure oscillation encourages convergence and therefore convective activity in the early morning and evening, but divergence and suppression of convection around midday.

Measurements by the Tropical Rainfall Measurement Mission (TRMM) satellite programme indicate that during 1998 to 1999, rainfall at night or in the early morning over the ocean area 30°N to 30°S, 80°E to 10°W and passive microwave estimates indicate a rainfall peak at 04:00 to 07:00 LST. Over land areas there is an afternoon convective maximum. In Amazonia, the diurnal maximum is at 16:00 to 18:00 LST and over monsoon India at 12:00 to 15:00 LST, compared with a broad maximum between 01:00 and 14:00 LST over the northern Bay of Bengal.

The Malayan peninsula displays very varied diurnal rainfall regimes in summer. The effects of land and sea breezes, anabatic and katabatic winds and topography greatly complicate the rainfall pattern by their interactions with the low-level southwesterly monsoon current. For example, there is a nocturnal maximum in the Malacca Straits region associated with the

00 04 08 12 16 20 24

LOCAL TIME (hr)

Figure 11.57 Diurnal variation of rainfall intensity for tropical islands in the Pacific. (A) Large and small islands in the western Pacific. (B) Wet and dry seasons for Nandi (Fiji) in the southwest Pacific (percentage deviation from the daily average).

Sources: (A) After Gray and Jacobson (1977). (B) After Finkelstein, in Hutchings (1964).

LOCAL TIME (hr)

Figure 11.57 Diurnal variation of rainfall intensity for tropical islands in the Pacific. (A) Large and small islands in the western Pacific. (B) Wet and dry seasons for Nandi (Fiji) in the southwest Pacific (percentage deviation from the daily average).

Sources: (A) After Gray and Jacobson (1977). (B) After Finkelstein, in Hutchings (1964).

convection set off by the convergence of land breezes from Malaya and Sumatra (cf. p. 274). However, on the east coast of Malaya the maximum occurs in the late afternoon to early evening, when sea breezes extend about 30 km inland against the monsoon south-westerlies, and convective cloud develops in the deeper sea breeze current over the coastal strip. On the interior mountains the summer rains have an afternoon maximum due to the unhindered convection process. In northern Australia, sea breeze phenomenon apparently extends up to 200 km inland from the Gulf of Carpentaria by late evening. During the August to November dry season, this may create suitable conditions for the bore-like 'Morning Glory' - a linear cloud roll and squall line that propagates, usually from the northeast, on the inversion created by the maritime air and nocturnal cooling. Sea breezes are usually associated with a heavy buildup of cumulus cloud and afternoon downpours. On large islands under calm conditions the sea breezes converge towards the centre so that an afternoon maximum of rainfall is observed. Under steady trade winds, the pattern is displaced downwind so that descending air may be located over the centre of the island.

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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.

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