Enhanced Lightning Activity during the Merger of Super Typhoon Hinnamnor (2022) with a Southern Tropical Depression

We examined lightning activity during the merger of two tropical cyclones (TCs), Super Typhoon Hinnamnor (2022) and a tropical depression (TD) to its south. To accurately characterize the lightning activity throughout the evolution of two TCs, we propose a boundary extraction method for convective systems based on the cloud-top Blackbody Brightness Temperature (TBB) observations from Himawari-8. The overall lightning activity can be divided into three distinct periods. Compared with Period-1 (prior to the merger), Period-2 (during the merger) was marked by an explosive increase in total lightning activity (intra-cloud, IC, plus cloud-to-ground, CG flashes) in both TCs, while the proportion of CG flashes decreased significantly. Interestingly, during Period-3 (after the merger), the total lightning increased again, along with a rapid increase in the proportion of CG flashes in Hinnamnor (2022), which stabilized to around 40%, close to the average level in Period-1 (46%). In addition, Hinnamnor (2022) underwent a total of 6 eyewall lightning outbreaks during Period-2 and Period-3, with the first two occurring 4 and 6 hours before the final merger of the two TCs, respectively. Our analysis shows that when the distance between the centers of two TCs is close enough (e.g., 500 km in this case), binary cyclone interaction may affect their convective development, further leading to changes in their charge structures, resulting in the lightning activity characteristics mentioned above. These results also suggest that lightning activity may, to some extent, serve as an indicator of binary cyclone interaction.