A diagnostic study of cloud physics and lightning flash rates in a severe pre-monsoon thunderstorm over northeast India

Numerical simulations of a thunderstorm event that occurred in the pre-monsoon season over the North-Eastern region of India (NEI) and Bangladesh are performed using Weather Research and Forecasting (WRF) Advanced Research Weather Research and Forecasting Model (ARW, version 3.8.1). Doppler weather radar indicates severe convective activity lasted for more than 10 hr. These extremely deep convective clouds with minimum cloud-top temperature −70 °C at 19 km were triggered by the mixing of a moist air mass transported from the Bay of Bengal in the south and dry air transported from the northwest. A cyclonic circulation observed over the Tibetan plateau is likely to be associated with the strong southerly low-level wind over NEI, as the plateau acts as a source of heat-lows during the pre-monsoon season. The coexistence of ice particles and supercooled water in the storms resulted in a large number of lightning flashes during the storm as observed from the Tropical Rainfall Measuring Mission Lightning Imaging Sensor (TRMM-LIS). Co-location of supercooled cloud water droplets helps in forming graupel through riming that plays a vital role in these convective systems. Lightning flashes calculated from WRF simulation using the Morrison microphysical and cloud top height based dynamical lightning parametrization scheme was found comparable with the observed flashes from TRMM-LIS. Since the WRF model could simulate the thunderstorm, we recommend using this state-of-the-art regional model in thunderstorm and lightning predictions for northeast India which would be useful in preparedness for such extreme events.