A numerical study of local circulation as influenced by clouds.

Abstract

A two-dimensional numerical model which contains mountain and sea surface has been developed mainly to study the influence of cumulus convection and radiation on the local circulation. Numerical experiments were carried out for four cases. Case 1 is the basic one which contains only the computation of net radiation on the ground surface to predict ground surface temperature. Cooling or warming due to radiation in the atmosphere and the effects of cloud are not taken into consideration. Net radiative fluxes at each level including the influence of cloud are estimated in Case 2, but the process of cumulus convection is not included in this case. Case 3 is the same as Case l but the cumulus convection can be treated. Net radiation at each level containing cloud effects and cumulus convection are taken into consideration in Case 4. The experimental results show that clouds exert great influence on ground surface temperatures and the local circulation itself through the processes of condensation and radiation. The computation of net radiation in the atmosphere is very important in simulating the surface inversion layer and the thin downslope wind on the slope at nighttime. The formation of clouds in the daytime suppresses the circulation due to rapid depression of net solar radiation on the ground. However, it enhances vertical velocity over the mountain through the cumulus convection. In this case, the anti-slope wind seems to be weakened. Namely clouds have effects which oppose each other on the circulation. The high temperature in a cloud as compared with its environs and lower one at the upper adjacent layer can be simulated if both effects are taken into consideration. In addition, clouds delay the onset of downslope wind if they remain after sunset.