In Taiwan, comparing with the major hazardous Mei-Yu fronts and typhoons in summer, a synoptic condition with the relatively weak high and low pressure systems at certain locations in the autumn season may bring torrential rainfall to northern Taiwan. A detailed dual-polarimetric/Doppler radar analysis was carried out for one case under this condition to reveal the mesoscale precipitation mechanisms and microphysical characteristics over terrain. The high pressure system moving eastward off China and the low pressure system over the ocean in the southeast of of south-eastern Taiwan formed a convergent zone at the low levels, resulting in a sequence of convective activities. These convective cells moved westward, and became more organized and intense in the Mt. Datun area and the estuary of Tamsui River near the lee side of Mt. Datun. Moderate intensity convective cells were embedded in the wide, long-lasting stratiform regions. In the mountain area with intense precipitation, terrain-induced upward motion of the cells enhanced condensation, significantly increased drop counts, and acted as a feeder. The older cells in this convective system continued to provide lighter hydrometeors in the upper layer and formed a widespread stratiform region as a seeder. The wider spectrum of drop size distribution set the stage for collision and coalescence process, resulted in the larger drops formed at the low level of the mountain area. Along with the increasing concentration of raindrops, the total effect finally caused heavy rainfall over the mountain area.