Diurnal circulations and rainfall in taiwan during SoWMEX/TiMREX (2008)

Abstract

The diurnal cycle of the local circulation, rainfall, and heat and moisture budgets is investigated in Taiwan's heavy rain (mei-yu) season using data from the 2008 Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). Comparisons are made between an undisturbed (UNDIST; 22-29May) and disturbed period (DIST; 31May-4 June).Many aspects of the diurnal evolution in surface flows and rainfall were similar during both periods.At night and during early morning hours, the lowlevel southwesterly flow was deflected around Taiwan'smain topographic barrier, the CentralMountainRange (CMR), with rainfall focused near areas of enhanced offshore confluence created by downslope and landbreeze flows. During the day, the flow switched to onshore and upslope, rainfall shifted inland, and deep convection developed along the coastal plains and windward slopes. Atmospheric budget analysis indicates a day-to-evening transition of convective structure from shallow to deep to stratiform. Evaporation associated with the evening/nighttime stratiform precipitation likely assisted the nocturnal katabatic flow. Though the flow impinging on Taiwan was blocked during both periods, a very moist troposphere and strengthened low-level oncoming flow during DIST resulted in more widespread and intense rainfall that was shifted to higher elevations, which resembled a more weakly blocked regime. Correspondingly, storm cores were tilted upslope during DIST, in contrast to the more erect storms characteristic of UNDIST. There were much more lofted precipitation-sized ice hydrometeors within storms during DIST, the upslope advection of which led to extensive stratiform rain regions overlying the CMR peaks, and the observed upslope shift in rainfall. © 2013 American Meteorological Society.