Seasonal and regional distribution of lightning fraction over Indian Sub-continent

Abstract

The three years of IITM LLN lightning observation data are used to determine the seasonal and spatial (over different geographical locations) distribution of the ratio of intra-cloud lightning (IC) to cloud-to-ground lightning (CG) in thunderstorms over the Indian sub-continent. The ratio is high (8-10) in the north-western parts and low (0.3-3) in the north-eastern parts. There is not a prominent latitudinal variation of IC and CG ratio, but a climatological seasonal variability exists all over the regions. In the Pre-monsoon (March to May), the mean ratio is observed at 3.87 with a standard deviation of 0.74, and during Monsoon (June to September), that is 3.01 with a standard deviation of 0.52. Pre-monsoon thunderstorm exhibits more IC discharge comparatively monsoonal thunderstorms; hence IC:CG ratio is also high in pre-monsoon. We have observed that CG lightning is approximately 20% of total lightning in pre-monsoon whereas 25% of total lightning in monsoon all over the Indian region. High CAPE associated with a stronger vertical updraft enhances the cold cloud depth and expands the mixed phase region, which can broaden and uplift the size of the upper positive charge center inside a thunderstorm while the middle negative charge center remains at the same temperature level. Therefore it enhances the occurrence of IC discharge between the upper positive charge center and middle negative charge center, hence increasing the IC:CG ratio of a thunderstorm. The implication of these observed results has the importance of separating CG lightning flash from total and can be used in the numerical model to give a proper prediction of CG lightning in hazard mitigation.