Seasonal and Regional Distribution of Lightning Fraction Over Indian Subcontinent

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.Plain Language Summary Pre-monsoon thunderstorms exhibit more intra-cloud (IC) discharge than monsoonal thunderstorms; hence, the IC:cloud-to-ground (CG) ratio is also high in pre-monsoon. In this paper, we have shown that CG lightning is approximately 20% of total lightning in pre-monsoon whereas 25% of total lightning in monsoon all over the Indian region. A stronger vertical updraft associated with high convective available potential energy enhances the cold cloud depth. It may expand the mixed phase region, which can broaden and uplift the size of the upper positive charge center inside a thunderstorm. In contrast, the middle negative charge center remains at the same temperature level. Therefore, this process may enhance IC discharge between the upper positive charge center and the middle negative charge center, increasing the IC:CG ratio of a thunderstorm.