Radar observations of intense orographic precipitation associated with typhoon Xangsane (2000)

Intense and persistent orographic rain occurred as the typhoon Xangsane (2000) moved northward off the northeastern coast of Taiwan and brought strong low-level northeasterly to north-northeasterly winds impinging on the mountainous northern coast. Measurements from two ground-based Doppler radars located in northern Taiwan were used to document the detailed aspects of precipitation distribution and intensity, and how these spatial and temporal variations of precipitation relate to the topography and upstream airflow was particularly focused in this study.

In northern Taiwan, there are two primary mountain barriers; one is the Mt. Da-Tun, a complicated three-dimensional mountain barrier with peak mountain heights of ~1000 m, and the other, Snow Mountain Range, adjacent to the east of the Mt. Da-Tun, is an approximately two-dimensional mountain range oriented roughly southwest-northeast. Analyses of radar observations revealed different precipitation patterns in the vicinity of the two distinct barriers. Two precipitation maxima were observed over the windward slopes of Mt. Da-Tun but with a sharp decrease in precipitation near mountain peaks and on the lee, leading to a local precipitation minimum located at low foothills southwest of the barrier. Another precipitation maximum was found over the first windward slope/peak of the Snow Mountain Range. Reduced precipitation was however observed over inland, higher slopes. Regions of these observed precipitation maxima were found to be closely related to the terrain forced vertical motions, although their locations of maximum were not exactly colocated. Given a stable-to-neutral environmental condition at low levels, the importance of upslope condensation on the precipitation formation over the mountain slopes is suggested for this case. In addition, the dual-Doppler synthesis was applied to obtain the kinematic information off the northern coast of Taiwan, upstream of the barriers. The intensity and direction of low-level oncoming flow were observed to be crucial for influencing the locations of enhanced precipitation relative to terrain features.