Environmental conditions associated with active lightning events in April around the Nansei Islands

Abstract A lot of cloud-to-ground lightning flashes sometimes occur around the Nansei Islands in April, while those April lightning bursts (ALBs) have not been investigated. This work aims at elucidating the environments contributing to ALBs through a case study and composite analysis. Carrot-shaped clouds characterize 58% of the ALBs cases, suggesting the importance of a mesoscale convective system known as the back-and-side building. The composite analysis was conducted for cases with daily cloud-to-ground (CG) flashes > 1000 d −1 (L1000) and with no daily CG flashes (NL). In both cases, westerly wind blows in the middle-troposphere. In L1000, the Nansei Islands were typically located to the west of a synoptic-scale migratory high at the surface and southerly or south-westerly wind advected the air of high equivalent potential temperature that makes the atmospheric condition more unstable to the Nansei Islands. Low-level south-to-southerly and middle-level westerly consisted of the vertical wind shear favorable for the carrot-shaped clouds. Geographically, the ALBs distribution well corresponded to the local-warm Kuroshio current, implying that the pressure adjustment mechanism over the Kuroshio current helps trigger the ALBs through the convergence. As in L1000, the pressure adjustment mechanism was active in NL; however, synoptic-scale downward motion and higher stability likely prevented the active convection.