Spatial and vertical structure of precipitating clouds and the role of background dynamics during extreme precipitation event as observed by C-band Polarimetric Doppler Weather Radar at Thumba (8.50° N, 77.00° E)

Abstract. Extreme precipitation events have been cynosure for many meteorologists as well as for common men as it causes severe weather hazards and affects the densely populated regions, especially urban cities. It is now well known that these extreme events have been increasing over the Indian region during the past few years. It becomes very important to understand and assess these events, which is challenging in terms of limited observations. Very recently, the state of Kerala, India experienced extreme rainfall events during August 2018 and led to major flooding, which is regarded as one of the worst natural disasters experienced by Kerala in the last hundred years. This catastrophic event occurred during 12^th to 17^th August 2018 in which the Kerala state has received 60 % more rainfall than the normal during this period. The present study focuses on investigating the spatial and vertical structure of precipitating clouds and their microphysical properties during this extreme precipitation event using C-band Polarimetric Doppler Weather Radar (DWR) observations over Thumba (8.50° N, 77.00° E). The DWR analyses were carried out during episodes of extreme rainfall, and the time evolution of radar reflectivity structure is examined very closely to understand the structure and dynamics of this unprecedented event. The spatial and vertical structures of precipitating clouds are strongly linked with the background dynamics. Apart from the DWR observations, prevailing dynamics such as tropical easterly jet (TEJ), low-level jet (LLJ) along with vertical velocity also investigated, which showed distant signatures lead to the extreme event. It was observed that the upper level divergence existed associated with low level convergence, which aids to the development of convection. The westward equatorial waves were present in the period of 7–10 days throughout the month of August 2018. The weakening of TEJ at upper troposphere resulted in decrease of vertical shear, which favours the vertical growth of convective clouds leading to the extreme precipitation. The enhanced strength of LLJ is also contributed to the precipitation extreme. Thus, the significance of the present study lies in delineating the structure and dynamics of the extreme precipitation event using indigenously developed DWR.