Case study of a morning convective rainfall event over southwestern Taiwan in the Mei-Yu season under weak synoptic conditions

There exists a minor, secondary early-morning peak in mei-yu rainfall along the western coast of Taiwan, and this work investigates one such event on June 8, 2012 in southwestern Taiwan under weak synoptic conditions through both observational analysis and numerical modeling, with the main focus on the triggering mechanism of the convection. Observations indicate that the convection developed offshore around midnight near the leading edge of a moderate low-level southwesterly wind surge of 15 – 20 kts and intensified and moved onshore to produce rainfall. The cold outflow from precipitation also led to new cell development at the backside, and the rain thus lasted for several hours until approximately 07:00 LST. Numerical simulation using a cloud-resolving model at a grid size of 0.5 km successfully reproduced the event development in close agreement with the observations, once a time delay in the arrival of the southwesterly wind surge in initial/boundary conditions (from global analyses) was corrected. Aided by two sensitivity tests, the model results indicate that the convection breaks out between two advancing boundaries, one from the onshore surge of the prevailing southwesterly wind and the other from the offshore land/mountain breeze, when they move approximately 40 km apart. Additionally, both boundaries are required, as either one alone does not provide sufficient forcing to initiate deep convection in the model. These findings on the initiation of offshore convection in the mei-yu season, notably, are qualitatively similar to some cases in Florida with two approaching sea-breeze fronts (in daytime over land).