Many studies in recent times have suggested that long‐term changes in Convective Available Potential Energy (CAPE) are associated with the changes in convective activity and atmospheric energy budget and, hence, the changes in CAPE could be used as a potential indicator of climate change. Here, we analyse daily radiosonde data of 32 stations over the Indian region from 1984 to 2008 (25 years), to study the climatology and long‐term changes in CAPE, lower troposphere moisture content and upper tropospheric temperature. Using gridded daily rainfall data over this region, the relationship between average CAPE and average rainfall as well as the trends in CAPE and its effect on rainfall pattern are also examined during different seasons of this region. All the coastal and island stations, as well as some stations in the foothills of the Himalayas, show higher values of CAPE compared to other inland stations. This study clearly demonstrates that CAPE values have been systematically increasing over most parts of the Indian region during last 25 years. The increase in CAPE is found to be due to increase in low‐level moisture content and decrease in upper level temperature over the last 25 years. The seasonal variations show that the CAPE is higher during the monsoon compared to pre‐monsoon or post‐monsoon seasons and it suggests that thermodynamic conditions favourable for high CAPE together with large‐scale dynamics are necessary for organized monsoon convections over this region. In comparison with a large increase in the all‐India average of CAPE during monsoon season, which is about 38 J Kg⁻¹ year⁻¹, the all‐India summer monsoon rainfall increases about 1.3 mm year⁻¹. The systematic increasing trend in CAPE may be compensating for weakening of monsoon circulation and, thus, maintaining the monsoon rainfall over the Indian region. Copyright © 2011 Royal Meteorological Society

Four years of Indian Institute of Tropical Meteorology lightning location network lightning observation data are used to determine the seasonal and spatial (over different geographical locations) distribution of the ratio of intra‐cloud (IC) lightning to cloud‐to‐ground (CG) lightning in thunderstorms over the Indian subcontinent. The ratio is high (6–10) in the northwestern parts and low (0.5–3.5) in the northeastern parts. No prominent latitudinal variation of the IC to CG ratio exists, but a climatological seasonal variability exists over all regions. In the pre‐monsoon season (March–May), the mean ratio is observed to be 3.81 with a standard deviation of 0.79, and during the monsoon season (June–September), a value of 3.04 with a standard deviation of 0.50. Although convective available potential energy is the regulating factor, little dependency has been found between the ratio of IC to CG lightning (IC:CG ratio) and the total flash rate (f), as well as with cold cloud depths. The ratio is observed to be proportional to the total flash rate as f^0.61. The cold cloud depth is most prominently linked with the regional and seasonal IC:CG ratio. The implication of these observed results has the importance of separating CG lightning flash from total and can be used in numerical models to give a proper prediction of CG lightning in hazard mitigation.

Spatial and vertical distributions of aerosol radiative properties over Indian Continental Tropical Convergence Zone (CTCZ) up to 6 km altitude during the pre-monsoon and monsoon seasons of 2008 ha...

Indian subcontinent receives ~80% of its annual total rainfall during Indian summer monsoon (ISM) and since last 100 years the prediction of interannual variability of ISM is limited due to incomplete understanding of various processes and their localized influence on the development of varying scales of convective systems. The present study reports that the intense lightning contributed from the deepest convections occurring over Indian subcontinent are concentrated in four specific regions: (a) southwest coast, (b) Gangetic‐Brahmaputra plain, (c) Himalayan foothills and (d) southeast coast. The spatial and temporal variations of deep convections, size of “convection fractions” and associated physical mechanisms are described. It shows an annual cycle of systematic onset and withdrawal of deep convections and associated lightning depending on region and season. The southwest coast region is developing very deep convections during premonsoon and postmonsoon seasons but less lightning during monsoon. While Arabian Sea sources high moisture to this region in monsoon season, the orographic lift of low‐level westerlies by Western Ghats may effectively assist the warm rain processes and produce highest rainfall, low lightning and low area of deep convections. Highest flash rates, high number of days with lightning and higher area of convection are observed in Gangetic‐Brahmaputra plain but relatively lower rainfall compared to southwest coast region during monsoon. The sustained strong updrafts over the Gangetic‐Brahmaputra plain region are supported by the moisture source from warmer Bay of Bengal, incidence of monsoon trough and the continental heating. Significant lightning activity observed in northwest part of Himalayan foothills during winter is associated with Western Disturbances (WD) phenomena. The diurnal variation of lightning flash rate and size of “convective fraction” in these regions show two modes at 1200 and 1800 UTC and it indicates active day time surface heating and nocturnal cooling processes together contribute to total rainfall.