The Multi-Layer Feature of Atmospheric Boundary Layer Surrounding a Subtropical Monsoon Island

Taiwan, located at western Pacific coast region, is a mountainous island bisected by the Tropic of Cancer and has 300-km north-south stretch of the Central Mountain Range (CMR) with more than 50 peaks above 3,000 m. The weather and climate of Taiwan are strongly affected by the circulation of the Eastern Asia winter and summer monsoon. These prevailing monsoon systems provide unique background environment for atmospheric boundary layer (ABL) developing surrounding this north-south stretching and steep terrain of island. In this study, six the isochronal radiosonde observations with different vegetation patterns (urban cities, airport, harbor and remote islands) surrounding CMR steep terrain of Taiwan island were collected to diagnose the vertical meteorological profiles below 4000 m from 2010 to 2011. These upper-air sounding observations operated by Central Weather Bureau of Taiwan, Air Force weather stations and the project-oriented field campaigns have distance less than 150 km to each other. All the radiosondes are the same type (Vaisala RS92-SGP) with in 1~2 seconds sampling rate, and were processed into 25 m height interval for sites' comparison. The virtual potential temperature (èv) profiles at all sites come to consistent above 2200 m to 3200 m height (free atmospheric layer) in different seasons. But there existed significant turning points in these virtual potential temperature profiles to mark the multi-layer feature of ABL. During one-week field campaign of December of 2011 (winter monsoon season), the virtual potential temperature profiles were compared to illustrate the multi-layer feature of ABL after sunset (12:00UTC). The results showed that the upslope flow of CMR made higher nocturnal boundary layer (NBL) than the ones at downslope side. The more southward of land sites, the lower of NBL. Basin-like topography at western city sites could be the major reason to cause lower NBL. Meanwhile, low-level wind shear below 1000 m was significant at ocean sites. In the other hand, ABL pattern at all sites has not much different to each other during summer monsoon weathers with weak southern air flow. This ABL feature surrounding Taiwan could be treated as the critical validation of high-resolution models' simulation to an island.