Diurnal Variation of Lee Vortices in Taiwan and the Surrounding Area

Abstract

Lee vortices have been frequently observed in the wake of mesoscale mountains under a low Froude number flow regime. During the Taiwan Area Mesoscale Experiment (TAMEX), a cyclonic vortex was observed to the Ice of Taiwan by a P-3 aircraft. In this paper a numerical simulation is carried out to study this event. It is shown that the numerical results are capable of recapturing the detailed features as observed by airplane and surface analysis. The simulated surface pressure, wind field, and Ice vortex are in good agreement with observations. The diurnal oscillation of cloudiness and precipitation in Taiwan is also consistent with the observations under undisturbed conditions during the TAMEX period. Under a prevailing southwesterly-to-westerly summer monsson flow, numerical results demonstrate that the observed cyclonic vortex initially develops to the southeast of Taiwan after sunset, then drifts northeastward. The diurnal forcing not only generates land/sea breezes but also controls the vortex shedding. A sensitivity test without diurnal forcing indicates that the intrinsic vortex shedding period of Taiwan island is about 54 hours under the same initial condition. Due to the influence of diurnal forcing, however, the vortex shedding period becomes 24 hours with the cyclonic vortex forming at 1700 LST and the anticyclonic vortex forming at 0500 LST. Moreover, the diurnal effect also influences the propagation of vortices, especially near the surface. A vorticity budget study is also carded out to compare with the idealized case. The results show that the tilting term is important to generate vorticity over the Central Mountain Range. On the other hand, the stretching and advection terms are responsible for carrying and enhancing the vorticity to the lee side and are directly related to the initial development of the vortex. Moreover, each term in the vorticity budget is quite complicated, due to the existence of clouds, boundary-layer forcing, and the circulation of land/sea breezes.