Some precipitation characteristics of an autumn torrential rainfall event in northern Taiwan as determined from dual-polarization radar data

Certain precipitation characteristics of the torrential rainfall event that occurred on 10 September 2006 in northern Taiwan were investigated. This heavy precipitation event lasted for almost six hours due to favorable synoptic conditions and the effects of local topography on convection. Results obtained show that: 1) the high pressure system in central China and the low pressure system in south-eastern Taiwan formed a convergent zone on the low levels over northern Taiwan, resulting in a sequence of (moderate) convective activities over that region. These convective cells moved westward across northern Taiwan, following the prevailing winds, and became more organized, intensifying in the Mt. Datun area due to the effect of orographical lifting. 2) The reflectivity cores of these convective cells ranged from 40 to 50 dBZ and could only reach the melting level at 5 km, similar to the convection with medium depth (CMD) found in some Mei-Yu frontal systems. 3) The values of differential reflectivity (ZDR) for the heavier rainfall near the mountains were closely related to higher horizontal reflectivity (ZHH) and the specific differential (propagation) phase shift (KDP), indicative of the existence of larger drops in that area. In the same area, the values of ZDR increased in the lower layer below the melting level. These phenomena suggest that precipitation could be enhanced by the processes of collision and coalescence, indicating the dominance of the warm rain process. 4) The retrieved 3D relative wind field reveals that the older cells in a convective system continued to provide lighter hydrometeors in the upper layer and formed a widespread stratiform region. After melting, the stratiform region sprayed hydrometeors which “seeded” the convection underneath. The sloping updraft played a feeder-like role which provided a wider spread of DSDs and set the stage for the so-called “seeder-feeder mechanism”, thereby prolonging the life span of this heavy rain producing event.