Geoscience Reference
In-Depth Information
Table 11.3
Precipitation in the Koolau Mountains, Oahu, Hawaii (mm).
Location
Elevation
Source of rainfall
Trade winds
Cyclonic disturbances
28 May to 3 Sept 1957
2 to 29 Jan 1957
5 to 6 March 1957
Summit850
m
713
499
329
760 m west of summit
625 m
1210
544
370
7,600 m west of summit
350 m
329
467
334
Source
: After Mink (1960).
19°C for February and 16°C for July), a high diurnal
temperature range (averaging 9.5°C in July and 13°C
in February) and a large average cloud cover (mean 7 to
8/10ths).
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 local-
ized 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 semi-
diurnal pressure oscillation encourages convergence
and therefore convective activity in the early morning
and evening, but divergence and suppression of con-
vection 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 inter-
actions with the low-level southwesterly monsoon
current. For example, there is a nocturnal maximum
in the Malacca Straits region associated with the
3 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 pene-
trates 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.
