East Asia and Western Pacific METEOROLOGY AND CLIMATE

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al.,1984). In this paper, a general description of the model physics and
the operational results are given.
The precipitation in the atmosphere is related to two kinds of
condensation processes: large-scale ascending motion and cumulus
convection. In most numerical models, the conve'ctive condensation is
considered as a kind of sub-grid scale physics and computed by the
so-called "parameterization" method. But calculating large-scale condensation
and cumulus convection separately would not simulate the
precipitation process correctly since the two processes interact with
each other. However, the interaction is too complicated for us to give an
exact explanation. For example, removal of cumulus convection does not
decrease would rather increase the rainfall amount sometimes(Guo,1987).
On the other hand, as a "sub-grid scale" physical process, cumulus
convection should play different roles in models with different
resolutions. It is universally recognized that increasing the resolution is
efficient in improving the precipitation forecast. It is also necessary to
examine the effects of these two kinds of condensation processes on
precipitation forecasts as resolution increases. A sensitivity experiment
of sub-grid scale convection effects (Kuo's scheme) on horizontal
resolution has been carried out, and the results are shown in the IV
section of this paper.
II. DESCRIPTION OF THE MODEL PHYSICS
The precipitation scheme consists of two processes. The first one is
large scale condensation, which is similar to that presented by
Smagorinsky(1965). If the relative humidity exceeds 80% of saturation, the
excess moisture will be condensed and droped down as precipitation. At
the same time the temperature is increased to conserve the moist static
energy. The variations in temperature and specific humidity produced by
the large-scale condensation are expressed as follows
1 C
where Cp is specific heat, L is latent heat, and Rv is the gas constant of
water vapour. The re-evaporation of falling rain-drops is neglected.
The second process is convective condensation, for which the
modified Kuo's scheme (1974) is used. Earlier tests with Kuo's scheme
resulted in predicted rainfall areas much wider than those observed
(Zhang, et al.,1981). In order to concentrate the predicted rain area,
criteria controlling the convective parameterization are required, viz. the
stratification must be conditionally unstable at the beginning of
convection; and both low level moisture flux convergence and velocity
convergence must exceed some given critical values, which are empirical
constants.
The rate of convective precipitation
Re is determined by
where Mt is the moisture flux convergence in the vertical column of
the modeling atmosphere, and it can be calculated from