Pre-convective environment of pre-monsoon thunderstorms around Chennai – A thermodynamical study

Abstract

Pre-convective environment around Chennai during March – May 2003 has been studied using 0000 UTC upper air (RS/RW) and 0600 and 0900 UTC surface meteorological data. The study revealed the following results : (i) prevalence of positive (negative) dew point anomaly at 850 hPa, convective instability exceeding –6° K/km (convective stability, i.e., lapse rate less than –3° K/km) between 1000 and 700 hPa and positive (negative) relative humidity anomaly in the layer 850-500 hPa at 0000 UTC are associated with strong (no) convection (ii) SSEly to NNWly at 850 hPa is favourable for strong convection whereas ENEly to SSEly winds at 850 hPa are associated with no convection (iii) George’s K index, Deep Convective Index and Showalter’s stability index show better forecasting skill over the method of persistency (iv) Galway’s lifted index, SWEAT index, humidity index and Boyden index do not have forecasting skill over persistency and hence they are considered unsuitable for forecasting thunderstorm during pre-monsoon season (March – May) over Chennai and (v) forecast based on 0000 UTC convective available potential energy (CAPE) and convective inhibition energy (CINE) does not throw any light in improving our forecasting skill.