Numerical Modeling of an Orographically Enhanced Precipitation Event Associated with Tropical Storm Rachel over Taiwan

Abstract

An orographic rainfall event that occurred on 6?7 August 1999 during the passage of Tropical Storm (TS) Rachel over Taiwan is investigated by performing triply nested, nonhydrostatic numerical simulations using the Naval Research Laboratory's (NRL) Coupled Ocean?Atmosphere Mesoscale Prediction System (COAMPS) model. By examining both observational data and numerical model output, it is found that this orographic rainfall event may be separated into three distinct stages. During the first stage (0000?1200 UTC 6 August), TS Rachel was located in the South China Sea and tracked northeastward to Taiwan. Meanwhile, TS Paul was steered by the subtropical high over southwest Japan. During the second stage (1200 UTC 6 August?0000 UTC 7 August), the southwesterly monsoon current as well as the circulation of TS Rachel over southwest Taiwan strengthened and formed a low-level jet (LLJ) with high equivalent potential temperature when TS Rachel moved closer to Taiwan. In comparing the control and sensitivity (without orography) experiments, it was found that the strong LLJ triggered convective systems in the concave region of the southwest Central Mountain Range (CMR), which then produced the first episode of heavy rainfall. The second episode of heavy rainfall, which occurred during the third stage (0000?1800 7 August), was attributed to the modification of TS Rachel's own rainbands by the mountains as well as the strong southwesterly flow impinging on the mountains. The low-level convergence was propagated upstream over the sea, and the impinging flow from southwest Taiwan produced new convective cells. The orographic vertical moisture flux, which is a product of low-level horizontal velocity, mountain steepness, and moisture content, is calculated based on the fine-resolution model output. The regions of maximum moisture flux roughly coincide with the heavy-rainfall regions over the island during this event, while the regions of the general vertical moisture flux coincide with the heavy-rainfall regions over the ocean. Hence, the orographic vertical moisture flux may serve as an index for predicting this type of upslope orographic heavy rainfall. Overall, the model is able to predict the storm track, rainbands, and period of rainfall reasonably well over southern Taiwan, although the maximum rainfall may be slightly overpredicted. Nevertheless, the model results also suggest that a finer-resolution domain or vortex bogusing might be needed for the simulation of precipitation in association with a tropical storm over complex terrain.