Spatial variation of thermodynamic indices over north-east India during pre-monsoon thunderstorm season

Abstract

During the pre-monsoon season in the Eastern and North-eastern Indian regions, thunderstorms are the primary source of precipitation. The intensity and frequency of these thunderstorms are changing over time in these areas. The knowledge of atmospheric instability associated with these pre-monsoon thunderstorms can be aptly collected using thermodynamic indices. Thermodynamic indices integrate the comprehending convective atmosphere's geographical and temporal interpretation and are crucial in thunderstorm nowcasting. The present work investigated the spatial variation of thermodynamic indices for 1987–2016 over two tropical north-eastern states of India (Assam and Tripura) using the ERA-5 reanalysis data with 0.25° × 0.25° (31 km) resolution. The analysis incorporated information on thunderstorm days (TD) and non-thunderstorm days (NTD) provided by the India Meteorological Department (IMD) for differentiating the spatial variations of thermodynamic indices. The thermodynamic indices used in this work are categorised as conditional, potential, and convective instabilities. The results reveal that indices associated with convective and potential instabilities perform better for differentiating thunderstorms. We have incorporated the Man-Kendall trend analysis to investigate the trends in the spatial variation of thermodynamic indices. The results indicate that the region is showing higher instability for thunderstorm occurrences on TD, and there is higher moisture availability for the area during the whole pre-monsoon season. The inhibition is higher over NTD days, limiting the convective activity occurrence on certain days. The severity of thunderstorms as analysed with the help of thermodynamic indices are not changing over the region.