Regional variation of electrical and lightning properties of thunderclouds during the pre-monsoon season over the north-eastern and eastern part of India

Abstract

Regional variation of electrical and lightning properties of thunderclouds is investigated by using simultaneous observations of electric field mill (EFM-100) and lightning detector (LD-350) in synergy with their radiance and bulk microphysical properties. The study is carried out over Kohima in the north-eastern and over Rampurhat in the eastern part of India (20o-30o N, 85o-96o E), during the pre-monsoon season (March–May) of 2017–2018. The occurrence of EFM-100 detected thunderclouds is found to be more over Kohima (111) compared to Rampurhat (88). Whereas, the average number of lightning flash is much higher over Rampurhat (~542 flash/thundercloud) compared to Kohima (~32 flash/thundercloud). Over both the regions, the maximum occurrence of thunderclouds is found during afternoon to evening hours. The upper whisker value in the box plot shows that thunderclouds over Rampurhat are associated with higher CAPE value (Kohima: 1100 J Kg−1; Rampurhat: 5000 J Kg−1). The diurnal variation of CAPE supports relatively early occurrence of thunderclouds over Kohima. Thunderclouds over Rampurhat persists for relatively longer duration. It is observed that overall percentage occurrence of CG lightning is more over Kohima (Kohima: −CG: 28.09% & +CG: 09.74%; Rampurhat (–CG: 12.33% & +CG: 08.60%), with relatively higher −CG lightning compared to +CG, over both the stations. On the contrary, the occurrence of IC lightning is more over Rampurhat (Rampurhat: −IC: 31.75% & +IC: 47.30%; Kohima: −IC: 36.44% & +IC: 25.72%). Over Kohima occurrence of −IC is more than the +IC, whereas over Rampurhat, the occurrence of +IC is more than −IC. The overall higher occurrence of total negative and positive discharges over Kohima and Rampurhat respectively is consistence with EFM observations. The prevalence of positive/negative discharge suggests the presence of a tripole charge structure within thunderclouds over both the regions. The upper whisker value in the box plots revels higher CG flash density, IC:CG and larger log(area) over Rampurhat (Kohima: 3.0 × 10−3 flash km−2 h−1 and Rampurhat: 4.25 × 10−3 flash km−2 h−1; Kohima: 7 and Rampurhat: 15; Kohima: 10.5 km2 and Rampurhat: 11.5 km2). The lower whisker value of cloud top temperature reveals prevalence of colder thunderclouds over Rampurhat (Kohima: 220 K; Rampurhat: 184 K). The thunderclouds over Rampurhat are found to be associated with stronger updrafts and mixed-phase processes. The study reveals that, over these two regions, the electrical and lightning properties have distinct characteristics.