East Asia and Western Pacific METEOROLOGY AND CLIMATE

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prone to boundary contamination. This is supported by the smaller mean
error (1.6 degrees) for the initial 12 hours. As for the trend, the
predicted initial temperature fall over the Hong Kong area for the
first 6 hours is not observed. This may be due to the initial surface
temperature field being ill-represented as there are only two upper
air stations within the domain to influence the initialization.
The results from the control run are apparently poor in
terms of both absolute values and trends in comparison
simulation
period. The
with the full
run. This is particularly so for the latter half of the
12-hour and 24-hour RMS errors are 2.1 and 4.8 degrees
respectively for Hong Kong. The control run predicts a sharp rise in
temperatures after day break to a maximum value of 25 degrees while
the observed temperatures actually fell steadily to 18 degrees. This
result clearly demonstrates that the solar effect is an influential
factor in local temperature forecasts.
The forecast for Hong Kong by the VFM model for the same
period are also plotted in Fig. 2a for comparison. This model predicts
a trend which is close to the actual observation but is about 3°C
lower. In view of (i) the tight temperature gradient over South China,
i.e.
about 6°C per 100 km, and (ii) the fact that VFM grid temperatures
are only mean values over an area of 111 km x 111 km, a deviation
of 3°C of the grid value
from any point measurement should be
regarded as reasonable. Nevertheless, this comparison suggests that
model resolution also plays a significant role in local temperature
forecasts.
3.2 Simulation of Surge with Bright Periods during the day
To supplement the findings in Section 3.1,another event
was selected. On March 4, 1989, a cold front reached the South China
Coast bringing the temperatures of Hong Kong down from about 18°C in
the morning to around 11°C in 24 hours' time. But, during the early
part of the day, there were some bright periods and temperature over
the Hong Kong Harbour area had even risen to a maximum of 20 degrees.
This combined solar and advection effects are simulated
using the 00-UTC upper air data from Hong Kong and a few stations in
Guangdong. The short wave radiation terms are multiplied by 0.4 to