A study of a precipitation system in northeastern Taiwan during TAMEX IOP#10

Two-thirds of the land mass of Taiwan island is covered by mountains that affect precipitation systems over the island. To understand the influence of such terrain on a precipitation system was one of the objectives of TAMEX (Taiwan Area Mesoscale Experiment, Kuo and Chen, 1990). During the passage of these precipitation systems, Doppler radar readings as well as conventional data were collected. On 17 June, 1987 a precipitation system moving toward northeastern Taiwan dumped over 100 mm of rainfall per day near the mountain foothills, not far from the ocean. Over the lee side, the precipitation amount was less. The radar data results indicate that a series of cells formed about 10 km upstream of the coastal area and moved toward the mountains under the influence of an easterly wind. The zonal speed was about 4 to 8 ms(-1). The time interval for the formation of these convective cells was about 40 minutes. They intensified near the coastal area, the foot hills, and the mountain slope, but their intensity decreased on the lee-side. A two-dimensional, nonhydrostatic model with a terrain-following coordinate system was employed to study the influence of environmental wind patterns and terrain on the characteristics of a precipitation system. Simulation results indicate that a series of clouds associated with an updraft formed at the middle level, about 10 to 20 km east of the mountain foothills (near the coast line), under the influence of easterly winds in a very moist environment. Then, updrafts associated with cloud water travelled westward from the cloudy region, intensifying near the bottom of the mountains and in the coastal areas due to orographic lifting. Then, convective cells formed. As these cells continued moving westward and upward near the foothills as well as the upslope area near the mountain top, their intensity increased. But once they passed over the mountain top to the lee side, their intensity decreased. The time interval for the formation of cells was about 35 minutes and the size of the cells was about 5 to 8 km horizontally. The numerical results are qualitatively consistent with the observations. Sensitivity studies indicate that the magnitude of the wind speed influenced the formation of the cells. The low level wind profiles affected the movement of cells on the lee-side of the mountain, and the height of mountain also had an impact on the characteristics of the precipitation cells.