On the formation of Leeside mesolows under different froude number flow regime in TAMEX

During the Mei-Yu season, when the surface front approaches and pre-frontal southwesterly flow intensifies, shallow and short-lived mesolows often form to the southeast of Taiwan on the leeside of the Central Mountain Range (CMR). Although they develop under similar synoptic settings, cases observed during the Taiwan Area Mesoscale Experiment (TAMEX) in 1987 still exhibited some case-to-case variations, particularly in the upstream Froude number (Fr) and the amount of windward-side precipitation. The present study selects two cases with more contrasting mesoscale characteristics for detailed examination, and differences found in their flow structure and associated features are discussed and compared with theoretical aspects. Results indicate that as long as Fr exceeds a certain value, two possible scenarios associated with relatively higher and lower Fr values can lead to formation of leeside mesolow through slightly different combination of processes. The case with a larger Fr (>0.5) on June 7-8 had a flow structure relatively more in favor of the "flow-over" regime under stronger prevailing winds. The airflow produced a large amount of precipitation at the windward side then sunk at the leeside together with parcels from higher levels. The adiabatic warming led to formation of the mesolow, which had no corresponding mesovortex most of the time with a higher Rossby number (Ro ≈ 2), but small and localized vortices independent from the low might appear at other favorable spots near the terrain. The second TAMEX case (June 2) had smaller Fr (<0.2) and Ro (<1) values. Due to the stronger Coriolis deflection and upstream blocking effect, the flow showed more characteristics of the "flow-around" regime. Only a relatively shallow layer below the crest of the CMR was able to climb over, and subsequently nearly no precipitation was produced. However, the stronger blocking allowed air parcels above the mountain crest to sink directly at the lee for greater depths as a transient response, and to produce mesolow at comparable strength even without the contribution from latent heat release. In this scenario with smaller Ro, the low possessed a closed circulation for longer periods during its life span. Two schematic illustrations are presented to depict the major processes found in these two mesolow cases, which show general agreement with theoretical aspects and exemplify variations in flow structure under slightly different Fr and Ro values in the real atmosphere from an observational standpoint. Cases with characteristics in between the two scenarios are also expected.