Physical Interpretation and Implications of Convective Impulses in Thunderstorms Based on Lightning and Polarimetric Radar Observations

Abstract. Convective impulses (CIs) occur in thunderstorms and are strongly associated with severe convection, often contributing to hazardous weather events. However, the underlying physical mechanisms governing CIs remain poorly understood. In this study, multiple CI events were identified in two selective isolated thunderstorms and analyzed from the perspective of the cloud life cycle using polarimetric radar and lightning observations. We investigated the roles of environmental conditions and cloud microphysics in CI events. Our results indicate a pronounced increase in supercooled liquid water and graupel content prior to CI occurrence. The breakup of raindrops is closely linked to the observed increase in supercooled raindrops, suggesting that the fragmentation of large raindrops below the melting layer contributes to raindrop multiplication at subfreezing temperatures. These smaller raindrops subsequently freeze into graupel-like particles, releasing latent heat that may enhance convection and/or lightning activity. This hypothesized physical mechanism is further supported by idealized numerical simulations, which demonstrate that the updraft intensity varies with the efficiency of raindrop breakup. Additionally, large raindrops involved in the breakup process originate from the coalescence of raindrops during the initial pre-CI event, whereas graupel melting or shedding plays a role in subsequent pre-CI events. This improves the understanding of CIs following precipitation loading, melting, and evaporation within a near-stationary thunderstorm cell. These findings reveal a likely physical mechanism contributing to CI events and offer new insights into thunderstorm microphysics and dynamics.