Case Study of the Leeside Mesolow and Mesocyclone in TAMEX

Abstract

An observational study was performed on a leeside mesolow case that occurred 8 June 1987 during the Taiwan Area Mesoscale Experiment (TAMEX). The case developed as prevailing west-southwest winds strengthened and interacted with the terrain of Taiwan, with a Froude number (Fr = U/Nh) increasing from below 0.3 to over 0.5. A quasi-stationary mesolow formed to the southeast of Taiwan with no closed circulation through most of its 13-h life stages until passage of the Mei-yu front. A stationary and localized mesovortex also formed about 90 km southwest of the low center, but little adjustment was observed between the mesolow and the vortex. Results suggest that airflow at lower levels was blocked and moved around the terrain of the southern Central Mountain Range (CMR). This led to the formation of low-level jets (LLJs) both to the northwest and southeast of Taiwan. The latter branch provided shear vorticity in the background region of the vortex. Air parcels at higher levels, on the other hand, tended to climb over the mountain and caused precipitation on the windward slope, then subsided at the lee side. The subsidence produced the mesolow through adiabatic warming and drying, which was strongest between 1 and 2 km. Latent heat release at the windward side was estimated to contribute a maximum of about 55%–60% of the total warming. Eventually, as the Mei-yu front moved southward along the eastern coast of Taiwan, the mesolow merged with the front and transformed into a migratory mesocyclone along the front.