An orographic rainfall event occurred on 6-7 August 1999 during the passage of tropical storm (TS) Rachel over Taiwan is investigated by performing triply-nested, non-hydrostatic numerical simulations. The first spell of the heavy rainfall episode started in the southwestern concave region of the Central Mountain Range (CMR), at about 15UTC 6 August when TS Rachel was still far upstream of Taiwan. The rainfall then weakened significantly and resumed at the same location at about 03UTC 7 August. In this study, we investigate: (a) the effects of synoptic and mesoscale environments on the formation of orographic heavy rainfall, (b) the vertical moisture flux from the model output as a potential index for predicting orographic rainfall, and (c) orographic effects on the generation, propagation, and redistribution of rainfall.

Although the maximum rainfall rate is under predicted, the model (NAVY COAMPS) is able to predict the storm track and rainbands, location and period of rainfall quite accurately. Examining both observational data and numerical outputs, we found that this orographic rainfall event may be separated into three rather distinct stages. During the first stage (00 to 12UTC 6 August), the interactions of upper-level cold-core low over China coast, upper-level circulation of TS Paul located to the southwest of Japan, and lower-level circulation of TS Paul forced TS Rachel, which was located at South China Sea, to move more slowly and also counterclockwisely toward Taiwan. During the second stage (12UTC 6 to 00UTC 7 August), the southwesterly monsoon flow over southwest Taiwan was strengthened and formed a low-level jet (LLJ) with high theta-e by TS Rachel when it moved closer to Taiwan. Strong orographic lifting of the conditionally and potentially unstable LLJ triggered convective systems in the concave region of the southwest CMR, which then produced the first spell of the heavy rainfall episode. The second spell of heavy rainfall during the third stage (00 to 18UTC 7 August) was attributed to themodification of TS Rachel's own rainbands by the mountains.

The orographic vertical moisture flux, which is a product of low-level horizontal velocity, mountain steepness, and moisture content (Lin et al. 2001), is calculated based on the fine-resolution model output. The maximum regions of this flux coincide with the heavy rainfall regions over the island (i.e. mountainous area) during this event, while the maximum regions of the general vertical moisture flux (wq) coincide with the heavy rainfall regions over the ocean. Therefore, the orographic vertical moisture flux may serve as an index for predicting orographic heavy rainfall.

Orographic effects on the generation, propagation, and redistribution of the rainfall, such as the concave geometry, three-dimensionality, CAPE, and moist Froude number, will be studied by performing sensitivity tests.