A series of numerical simulations are conducted using the Weather Re search and Fore casting (WRF-ARW) model with a 9-km mesh to examine the physical processes responsible for the torrential rain fall associated with a mesoscale convective system (MCS) along the Mei-Yu front that caused severe dam age over south western Taiwan on 12 June 2005. In the control experiment (with full Taiwan terrain), the MCS tends to prop a gate north ward along the wind ward (western) slope, rather than being advected down stream, as it encounters the southern Central Mountain Range (CMR). The low-level convergence between the dynamically-driven re turn flow and the in coming south westerly flow is an important factor for the north ward propagation of the MCS. The values of the un sat u rated moist Froude number (Fw) and the convective avail able potential energy (CAPE) are about 0.333 and 2858 J kg^-1 for the convectively unstable basic flow. This air flow belongs to the flow-around regime as suggested by previous studies. In this flow regime, the torrential rain fall associated with the simulated MCS occurs on both the upslope side and adjacent plains over southwestern Taiwan, which agrees well with the observed rainfall distribution.

A sensitivity study of lowering the Taiwan terrain elevations to 80% (Fw~0.416) or 60% (Fw ~0.555) of that in the con troll experiment reveals that the air flow is characterized by a long-lasting orographic convective system near the mountain peaks. The simulated torrential rain fall mainly occurs along the mountain peaks of the southern CMR. The ex periment with a reduced elevation to 40% (Fw ~0.833) or 20% (Fw ~1.666) in the control experiment shows a down stream-prop a gating MCS, which falls into the flow-over regime. This study demonstrates that the north ward propagating MCS, which is mainly caused by the flow deflection by southern CMR, played a key role in producing torrential rain fall over southwestern Taiwan during 0000 - 1200 UTC 12 June 2005.