Mountain-Originated Mesoscale Precipitation System in Northern Taiwan: A Case Study 21 June 1991


In the late morning of 21 June 1991, a convective precipitation system developed in the mountains area in northern Taiwan. The convective precipitation system was organized into a northeast-southwest oriented mesoscale line feature along the western mountain slope at a later time. More than 120 mm of rainfall poured down on Taipei and Keelong cities within 3 hours. The initiation, development, organization and dissipation of the storm were detected by a C-band Doppler weather radar on the northwest coast of Taiwan. In this paper, the role of mountains in organization and propagation of the mesoscale precipitation system is studied by using the Doppler radar data.
The storm initiated at the mountain peak area, propagated down the terrain slope and brought heavy rain to the basin and plain areas. In the early stage, advection dom­inated. The precipitation system developed along the mean low-to-middle tropospheric winds into a northeast-southwest convective line. When precipitation-induced outflows occurred, the interaction tween the upslope winds and the storm-generated outflows dominated storm propagation. At this stage, the precipitation system moved northwest­ ward downslope. Reaching the foothills of the mountains, the storm was intensified dramatically by sea-breeze circulation. In the basin area, the interaction between the storm-generated outflows and the environmental low-level flows dominated the storm development. At this stage, the storm was quasi-stationary. Deep cumulonimbus clouds continually re rmed over the southwest quadrant, moved northeastward and produced copious rainfall in the same region.
The vertical structure of the convective precipitation system was also examined in this study. The strong reflectivity region in the storm was first found at an altitude of about 2 km and extended well above 12 km. The convective updrafts, inferring from the uplift of the westerly momentum in the middle troposphere associated with the pen­etrated sea-breeze circulation, tilted slightly downshear at the leading edge of the storm. The existence of convective downdrafts located just behind the updraft region was also indicated by the Doppler radar observations pronounced precipitation-induced di­vergence out ows near the surface and also confirmed by surface observations at Taipei station. The stratiform rain area was at the rear portion of the precipitation system and was not well-organized. No bright band signature was found in the reflectivity data.

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