The Diurnal Cycle of Convective Activity over South Asia as Diagnosed from METEOSAT-5 and TRMM Data

Bhuwan Chandra Bhatt,Munehisa Yamamoto,Tieh-Yong Koh,Kenji Nakamura

doi:10.3319/tao.2010.02.04.01(a)

Abstract

Cloud and precipitation data from satellites were used to characterize the climatological diurnal cycle of convective activity over South Asia. Spatial and temporal variability were investigated using three hourly blackbody brightness temperature (TBB) data from the METEOSAT-5 during March-April-May (pre-monsoon) and June-July-August (summer monsoon) of 1999 - 2000. We focused on three regions over South Asia: region A in the Himalayas, region B in north India and region C in the Bay of Bengal (BOB). The cloud cluster (CC) technique was used first to detect deep convection over north India during the summer monsoon when deep convection was less frequent; cloud cover frequency (CCF) was used to diagnose convection of a moderate depth. TBB thresholds of 219 and 235 K were used in the CC and CCF methods, respectively. Ten years of Tropical Rainfall Measuring Mission (TRMM) rainfall product (3B42) was analyzed as well to check the METEOSAT-5 findings. The diurnal cycle varies greatly with location. In the pre-monsoon season, there were many CC over region B especially during the late afternoon hours. But deep convective activity was weaker during the summer monsoon season as convection mostly reached the mid-troposphere only. Bimodal diurnal cycle in CC occurrence was noted in the region A during summer monsoon, with the morning mode having more of the larger clusters than the afternoon mode. The afternoon mode appears to be suppressed in the presence of strong ambient wind shear. The life cycle of cloud clusters during summer monsoon is also worthy of note: statistically speaking, there is a gradual delay in the preferred time of initiation, attainment of maximum area and dissipation as one progresses northward from regions C through B to A. More clusters survive for longer than 3 hours in region C as compared to the other two regions.

Highlights

The terrain of South Asia constitutes a highland-lowland system (Fig. 1 upper panel) that influences synoptic and mesoscale weather
The summer monsoon precipitation is well-organized in space with the heaviest rainfall over regions in the Bay of Bengal (BOB), Arabian Sea, and southern slopes of the Himalayas (Adler et al 2000; Xie et al 2006)
As much work has already been done for certain aspects of the diurnal cycle in South Asia, this study aims to demonstrate some of the more notable findings in the literature using the more recent METEOSAT-5 dataset

Summary

Introduction

The terrain of South Asia constitutes a highland-lowland system (Fig. 1 upper panel) that influences synoptic and mesoscale weather. The Himalayas and Tibetan plateau is a major highland feature, with steep slopes flanking the southern edge of the Himalayas. Widespread cloud and precipitation before and during the summer monsoon in this sub-continent has long been recognized 1992; Goswami 2000; Webster et al 2002). The summer monsoon precipitation is well-organized in space with the heaviest rainfall over regions in the Bay of Bengal (BOB), Arabian Sea, and southern slopes of the Himalayas (Adler et al 2000; Xie et al 2006). The frequency of convection is modulated by a variety of phenomena including monsoon depressions, intraseasonal active-break phases and the local diurnal circulations such as land-sea breezes and mountainvalley winds

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Conclusion