Orographic Effects on Airflow and Mesoscale Weather Systems Over Taiwan

The mountains of Tai wan affect both the airflow and mesoscale weather systems impinging on the island sig nificantly since two-thirds of the land mass of the country is covered by rugged terrain. The orographic feature whic h has the most significant impact on atmospheric systems is the Cen tral Mountain Range (CMR) which ru ns 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 3997m above the mean sea level. Since Tai wan is surrounded by oc eans, it provides a unique environment for studying the orographic effects on pre vailing ai rflow and impinging mesoscale weather systems. In thi s 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 wi nds and mesoscale convective systems, (b) the formation of rnesolows and mesocyclones, (c ) the effects on Mei-Yu fronts, (d) the effects on rnesoscale convecti ve 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 ob jectives of the Tai wan Area Mesosc ale Experiment (TAMEX) whic h was a joint field experiment conducted by Tai wanese and American sc ientists during the period of 1 May to 29 Ju ne 1987. When a typhoon impinges on Taiwan, its circulation and track is sig nificantly affected by the CMR. The typhoon track may be continuous or discontinu ous depending upon the impinging angle, intensity, and preexisting synoptic scale pressu re system. This proposes a seriou s forecasting prob lem in Taiwan. Similar problems have also been found in other parts of the world , such as Caribbean islands and Phillipines.