East Asia and Western Pacific METEOROLOGY AND CLIMATE

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closure model (1974).
Radiation boundary conditions are imposed at the four
lateral boundaries. This is certainly not the best approach especially
for rapidly changing boundary conditions but is sufficient for the
present study which is aimed at providing indications up to 12 hours
under rather steady meteorological conditions. The ground is treated
as a no-slip boundary and the model top a material surface. The equations
are written in terms of the terrain following co-ordinates.
The model domain for this experiment is about 300 km x 300
km with grid size of 5 km. There are 16 layers for the vertical and
the model top is about 7 km above mean sea level.
3. Results and Discussion
Two typical cases of winter monsoon surges were simulated,
one under overcast conditions and the other having some bright periods
during the day. For each case, a simulation using "nearest to actual"
solar heating conditions and one with "hypothetically extreme" solar
heating conditions were made. The purpose is to highlight quantitatively
the solar effects as well as to establish the significance of
the results. In order to have a better feel of the terrain influences,
the overcast case was nested down to 2-km grid and the result was
compared with those obtained from the 5-km grid and the VFM with grid
size of 111 km.
3.1 Simulation of a Surge with Overcast Conditions
On January 11, 1989, the temperature and pressure gradients
were tight over South China. A surge of northerly winds arrived
Hong Kong around late evening. Temperatures over the territory were
between 19 to 21 degrees initially (12 UTC). The temperatures fell
steadily to around 15 degrees in the next 24 hours.
To simulate this event, the 12-UTC upper air data from
Hong Kong and a number of Chinese stations in Guangdong were used to
initialize the model.
The result of the simulation suggests a drop of about 4°C
throughout the coastal region. This compares favourably with the
actual temperature fall of the region. To simplify the validation of
this model result, time series of the actual observations (ACTUAL),
time series of the forecast by the full simulation (MODEL) and that
forecast by the control simulation using in this case clear sky
conditions (M-SUNNY) at three strategic locations, namely the Royal
Observatory (RO), Macau and Guangzhou (GZ) are plotted in Fig. 2a, 2b
and 2c. The root mean square (RMS) errors* the bias and the standard
deviation (std. dev.) are evaluated and listed in Table 1 below.