Effects of Terrain on the Surface Structure of Typhoons over Taiwan

Abstract

The island of Taiwan is situated in the main path of western North Pacific typhoons. Its dominant central mountain range (CMR), with a hoizontal scale comparable to the radius of a typhoon, often produces significant distortions in the typhoon circulation. A 20-year dataset from 22 surface stations is used to describe the effects of the Taiwan terrain on the surface structure of typhoons. Empirical orthogonal function analysis on the pressure field is used to identify the primary structure modes. The first mode is a uniform-sign anomaly pattern portraying the decrease in pressure as a typhoon is approaching. The second mode represents the strong terrain-induced west-east pressure gradient that is normal to the main axis of the CMR. The third mode results mainly from the west-cast pressure gradient arising from the relative location of the typhoon center to the east or west of Taiwan, but it also contains a weak south-north pressure gradient that can he attributed to the terrain. A regression technique is then used to determine the surface wind, temperature, relative humidity, and hourly rainfall associated with each pressure mode. In all cases, them fields are consistent, showing the effects of the terrain blocking or deflection and their consequent ascending and descending motions. The relative importance of each mode depends strongly on the location of the typhoon center. No dependence on the direction or speed of motion is discernible when all cases are considered. When different, persistently smooth tracks are identified, the variations due to motion direction can be recognized because the terrain effect is affected by the mean steering flow. Only two types of smooth tracks that represent clearly different steering flows intersect in an area. At the intersection, a subsequent difference in storm structure over Taiwan exists that can be explained by the difference in the steering flows associated with the two track types. The leeside secondary low that was often observed on the west coast of Taiwan is found to consist of at least two basic modes. It develops only when the typhoon center is in southeastern Taiwan or an ocean area to the east-southeast. The observed scale of this low is significantly smaller than that which can be produced by an interaction of the mean steering flow and the CMR. This smaller scale is due to a local buildup of the surface pressure south of the lee vortex, which results from the against-mountain return flow of the cyclonic circulation.