Orographic Effects on Airflow and Mesoscale Weather Systems Over Taiwan

Abstract

The mountains of Taiwan affect both the airflow and mesoscale weather systems impinging on the island significantly since two-thirds of the landmass of the country is covered by rugged terrain. The orographic feature which has the most significant impact on atmospheric systems is the Central Mountain Range (CMR) which runs through Taiwan in a NNE-SSW direction with a width of about 120 km, a length of about 300 km and an average height of 2 km. The dominant peak of the CMR has a height of 3997 m above the mean sea level. Since Taiwan is surrounded by oceans, it provides a unique environment for studying the orographic effects on prevailing airflow and impinging mesoscale weather systems. In this paper, we review several prominent weather problems related to the orographic effects of the topography of Taiwan, which includes: (a) local rainfall enhancement by the CMR on prevailing winds and mesoscale convective systems, (b) the formation of mesolows and mesocyclones, (c) the effects on Mei-Yu fronts, (d) the effects on mesoscale convective systems, and (e) the influence on typhoon circulations and tracks. Understanding the first four problems is essential in improving the weather forecasting of flash floods in Taiwan during the Mei-Yu season, and is one of the major objectives of the Taiwan Area Mesoscale Experiment (TAMEX) which was a joint field experiment conducted by Taiwanese and American scientists during the period of 1 May to 29 June 1987. When a typhoon impinges on Taiwan, its circulation and track is significantly affected by the CMR. The typhoon track may be continuous or discontinuous depending upon the impinging angle, intensity, and preexisting synoptic scale pressure system. This proposes a serious forecasting problem in Taiwan. Similar problems have also been found in other parts of the world, such as Caribbean islands and Phillipines.