1Triggering Mechanisms of Boundary Layer Convections over the Taiwan Area in Spring Season

By using the Civil Aeronautics Administration (CAA) ground-based Doppler radar (25.08 deg N, 121.2 deg E) data, the kinemic and dynamic structures of boundary layer convections accompanying with the frontal system over the northern part of Taiwan were investigated. The three-dimensional conceptual models associated with this case were constructed in order to realize the possible mechanisms for triggering boundary layer convections. The preliminary results show: (1) The Mei-yu frontal systems were quite shallow (less than 1.5 km in altitude) and possessed strong vertical wind shear (more than 14 ms-1km-1). The boundary layer convections associated with the shallow fronts could be triggered by the instability of the frontal system and the abundant supply of warm, moist air related to the southwesterly flow. (2) The southwesterly low level jet (LLJ) obviously occurred at altitude between 0.5 km and 1.5 km in intensity of more than 10 m/s in use of high spatial and temporal resolution radar data. The existence of LLJ couldn’t be detected by the routine rawinsonde observations and was resulted from the strong pressure gradient force due to the high and low pressure distributions over the Taiwan Island which could be clearly delineated by the surface subjective analysis based on the hourly observation data. (3) The convective cells embedded in the frontal systems could be well organized into convective turbulences due to the following key factors. The first key factor was the confluent effect, which was resulted from the mergence of southwesterly, westerly and/or northwesterly flows and accumulated sufficient air mass and momentum at a specified region. The second one was the local cyclonic motion that made the convective cells well organized and triggered more precipitation over ground. The prevailing southwesterlies had a deceleration in the vicinity of the frontal edge and gave impulses to propagate the convective systems toward northeast within the frontal systems