Observation of Blue Corona Discharges and Cloud Microphysics in the Top of Thunderstorm Cells in Cyclone Fani

Abstract

Blue corona discharges are often observed at the top of thunderclouds. They are bursts of streamers, but the cloud conditions that enable them are not well known. Here we present observations by the Atmosphere-Space Interactions Monitor (ASIM) of 92 discharges during its ∼1 min pass over tropical cyclone Fani in the Bay of Bengal from 20:10:55 to 20:12:05 UTC on 30 April 2019. The discharges were observed in convective cells forming in the rainbands of the cyclone where Convective Available Potential Energy reached ∼6,000 J kg−1. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite passed over one of the cells ∼12 min after ASIM from 20:23:58 to 20:24:14 UTC. It measured the cloud microphysics related to the discharges and indicated they occurred in a convection region with the cloud top overshooting for over 20 min. The updraft lifted ice particles to lower stratospheric altitudes and formed the gullwing-shaped cirrus. The discharges are found at an average altitude of ∼16 km where the cloud environment contained ∼2 × 107 m−3 ice particles with ∼50 µm radius, resulting in a photon mean free path of ∼3 m. Around 20% of the blue corona discharges coincide with Narrow Bipolar Events indentified from the Earth Networks Total Lightning Network. Our observations suggest that the overshooting cloud top formed by deep convection and a surge in lightning activity facilitated conditions for the blue corona discharges. This work provides the first-ever estimate of important microphysical parameters related to blue corona discharges based on data measurements, establishing a reference for future empirical and theoretical studies.