Mountains can provide heat and moisture to the atmosphere and act as a barrier to the prevailing wind. Thus they can influence the formation and development of precipitation systems. About two-thirds of the island of Taiwan is occupied by mountains. Therefore, it is important to understand the influence of terrain on the formation and development of precipitation systems. This kind of research has become one of the objectives of the Taiwan Area Mesoscale Experiment (TAMEX, Kuo and Chen, 1990) held in 1987.
On June 20, 1987 during TAMEX IOP 11, a Pacific high pressure system was over Taiwan. Two major precipitation systems occurred in the Taiwan area in the afternoon. According to satellite and surface data, one precipitation system was over northern Taiwan and the other over central Taiwan. Several soundings show that southern Taiwan was under the influence of southerly ow and northern Taiwan was affected by southwestern flow. The Pan-Chiao sounding in northern Taiwan indicates the level of free convection (LFC) decreased to 1 km in height at 1400 LST. Thus lifting due to topographic effects or other factors could help convection. A three-dimensional non­ hydrostatic numerical model with terrain following coordinate system was employed to study where cloud systems developed. We found that when the initial wind used in the model was from the south, water vapour convergenced in northern Taiwan. Then cloud and rain formed there. The formation of this cloud and rain system plus surface heating helped to form a convergence area. The land-sea temperature contrast seemed not to be important for the formation of this convergence area, but it could enhance the intensity of the cloud system. If wind direction was from either 165° or 225°, the cloud and precipitation system formed in northwestern and northeastern Taiwan, respectively. Upslope flow helped clouds rm in central Taiwan. The cloud and precipitation sys­tem formed in the mountainous area in central Taiwan when initial wind in the model was from 165°, 190°, and 225°. Higher wind speed om the south at low level would cause the precipitation in northern Taiwan to move northward away from Taiwan island rather than eastward as observation data indicated.