The Distribution of Deep Convection over Ocean and Land during the Asian Summer Monsoon

Abstract

Tle highly reflective cloud (HRC) dataset is a daily index of organized deep convection, at one degree resolution, from 17 years of polar-orbiting, satellite imagery. These data are used to analyze and discuss the climatological geographical distribution of deep convection observed over the Asian summer monsoon season and its component months (June, July, August and September). Intraseasonal variations of convection for selected regions am examined using normalized pentad time series of regional median HRC values. We also compare HRC data over two regions (western coastal India and western coastal Burma/Thailand) with the results from a two-dimensional numerical model, consisting of a simple differentially heated land-ocean system which predicts that a preponderance of deep convection occurs over the coastal zone. The Buma/Thailand regional comparison supports the model result. Comparison of the model with the western coastal India region is less conclusive, which may be due to the limitations of the model. We conclude that monsoon deep convection, and its attendant sources of latent heat momentum, and mass sources important to large-scale monsoon dynamics is localized and persistent from year to year. If, as hypothesized by others, tropical cumulonimbus activity is important to stratospheric-tropospheric exchange, this study shows the preferred areas of such exchange during the monsoon. The locations of areas with large HRC amounts are consistent with upstream lifting of low-level, conditionally unstable air by low, coastal mountains. Intraseasonal variability follows variations in sea surface temperature and low-level flow. Upper-level dynamics are also recognized as an important contribution.