Numerical Simulations of the Boundary Layer Jet off the Southeastern Coast of China

A strong coastal boundary layer jet (CBLJ) (similar to 8ms(-1)) off the southeastern coast of China (around 28 degrees N, 122 degrees E) is found from the July 2006-11 hourly model data simulated by the Advanced Research Weather Research and Forecasting Model (WRF-ARW) with a 9-km horizontal grid. The southerly CBLJ has a jet core at the 925-hPa level, located along the western periphery of the west Pacific subtropical high (WPSH). The CBLJ is mainly contributed by large-scale enhancement by diurnal forcing and orographic effects by the coastal terrain along the southeastern China coast and the terrain of Taiwan. Although the geostrophic winds offshore are faster in the afternoon due to the larger east-west pressure gradient caused by land surface heating over the China plain, the CBLJ has a nocturnal (similar to 0200 LST) maximum. In the afternoon hours, easterly ageostrophic winds driven by differential land-sea thermal heating develop at low levels. After sunset, with the disappearance of land surface heating, the ageostrophic winds offshore veer southward by the Coriolis force and combine with the southerly geostrophic flow resulting in a nocturnal maximum in the CBLJ. Furthermore, from two model sensitivity experiments (NoTW and LowFJ), it is apparent that the terrain of Taiwan and Fujian exerts a secondary influence (1-2ms(-1)) on the strength of the CBLJ. The orographic blocking by the terrain of Taiwan and Fujian is more significant with a larger (similar to 1ms(-1)) southerly wind component north of the Taiwan Strait at night than in the afternoon hours.