Diurnal Circulations and Rainfall in Taiwan during SoWMEX/TiMREX (2008)

Abstract

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; 22–29 May) and disturbed period (DIST; 31 May–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 low-level southwesterly flow was deflected around Taiwan's main topographic barrier, the Central Mountain Range (CMR), with rainfall focused near areas of enhanced offshore confluence created by downslope and land-breeze 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.