A preliminary study of the formation of precipitation systems under undisturbed conditions during TAMEX

The effect of mountains on the occurrence of precipitation systems on Taiwan island is very significant, especially as mountain areas occupy about two-thirds of the land-mass. The mountains are, on average, about 3 km high. To investigate the formation of precipitation systems influenced by Pacific high pressure systems, we selected five cases (May 24, 25 and 26, June 19 and 20 in 1987) during a field program, TAMEX (Taiwan Area Mesoscale Experiment, Kuo and Chen, 1990). In all cases most of the rainfall took place in the afternoon when the level of free convection (LFC) was at about the 1 km height. If the average wind (below 3 km in height) was from the south (May 25 and 26), higher amounts of precipitation would be found along the sloped areas of western and eastern Taiwan. Rainfall also occurred in southern and northern Taiwan. If the average wind was from the southwest (May 24), the precipitation pattern was similar to that on May 25, except over the plains area in southwest and northeast Taiwan, where the amount was less. However, if the prevailing wind direction changed little with height and the average wind was from the south-southeast (June 19), higher rainfall amounts occurred from northwestern to central Taiwan. If the average wind was from the south and wind direction changed little with height (June 20), higher rainfall amounts took place in northern and central Taiwan. A nonhydrostatic model was used to simulate the formation of precipitation systems in all five cases. Simulation results indicated that the mixing ratio of rainwater could occur on the upstream side of a mountain slope and in the central mountain areas, where topographic lifting from the environmental wind and an upslope flow due to surface heating were evident. On the downstream side of the mountain, upward motion due to lee-side convergence and upslope motion from surface heating would also help rain form.