DIURNAL-VARIATION OF LEE VORTICES IN TAIWAN AND THE SURROUNDING AREA

Lee vortices have been frequently observed in the wake of mesoscale mo untains under a low Froude number flow regime. During the Taiwan Area Mesoscale Experiment (TAMEX), a cyclonic vortex was observed to the le e 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 resul ts are capable of recapturing the detailed features as observed by air plane and surface analysis. The simulated surface pressure, wind field , and lee vortex are in good agreement with observations. The diurnal oscillation of cloudiness and precipitation in Taiwan is also consiste nt with the observations under undisturbed conditions during the TAMEX period. Under a prevailing southwesterly-to-westerly summer monsoon f low, numerical results demonstrate that the observed cyclonic vortex i nitially develops to the southeast of Taiwan after sunset, then drifts northeastward. The diurnal forcing not only generates land/sea breeze s but also controls the vortex shedding. A sensitivity test without di urnal forcing indicates that the intrinsic vortex shedding period of T aiwan island is about 54 hours under the same initial condition. Due t o the influence of diurnal forcing, however, the vortex shedding perio d becomes 24 hours. with the cyclonic vortex forming at 1700 LST and t he anticyclonic vortex forming at 0500 LST. Moreover, the diurnal effe ct also influences the propagation of vortices, especially near the su rface. A vorticity budget study is also carried out to compare with th e idealized case. The results show that the tilting term is important to generate vorticity over the Central Mountain Range. On the other ha nd, the stretching and advection terms are responsible for carrying an d enhancing the vorticity to the lee side and are directly related to the initial development of the vortex. Moreover. each term in the vort icity budget is quite complicated, due to the existence of clouds, bou ndary-layer forcing, and the circulation of land/sea breezes.