A numerical study of precipitation characteristics over Taiwan island during the winter season

Two-thirds of the land mass of Taiwan island is mountainous, which affects the airflow and precipitation systems over the island. In this study, we di scuss the characteristics of precipitation systems when the prevailing wind direction is from the north-east during winter. Observations indicate that rainfall amounts were higher in northeastern Taiwan (the upstream side of the mountains) and that a rainfall shadow occurred in southwestern Taiwan. Simulation results from a non-hydrostatic model indicate that airflow was d eflected in eastern Taiwan, while relatively high (low) pressure areas form ed in eastern (western) Taiwan. A higher mixing ratio of rainfall occurred over northeastern Taiwan while lighter rainfall occurred in the eastern, an d northwestern areas and the southern tip of Taiwan. This was consistent wi th the observational data except for the southern tip of Taiwan. Uplift due to the topography near the mountainous areas, as well as low level converg ence near the coastal areas (due to the deceleration of an easterly wind in northeastern Taiwan), helped form the mixing ratio of rain. Transportation of the mixing ratio of rainfall, due to low level westward flow and upper level eastward flow, caused it to cover a larger area. The mixing ratio of rainfall formed in the upper mountainous areas in northeastern Taiwan if th e upstream moisture content was reduced significantly. A temperature invers ion at low levels resulted in a decrease in relative humidity and an increa se in stability, requiring that the mixing ratio of rainfall should develop closer to the mountainous areas. If a low level wind blew parallel to the orientation of the mountains (NNE-SSW), a higher mixing ratio of rainfall c ould occur in the mountainous areas of western Taiwan.