Radar data shows graupel and increased turbulence near small-scale intermittent lightning discharges at the top of intense thunderstorms

Sparkles are the intermittent, small-scale lightning discharges that occur near the top of intense thunderstorms. To increase the understanding of mechanisms that lead to sparkles, we compare high resolution lightning data from the LOw Frequency ARray (LOFAR) to data from a meteorological radar. The study focuses on the thunderstorms that crossed the northeast of the Netherlands on 18 June 2021. We use a two-stage clustering approach to computationally distinguish sparkles, defined as small and isolated lightning structures, from other lightning activity. Subsequently, we compare the radar data near sparkles to radar data near the other lightning activity. From radar images, we conclude that the convective systems that produced sparkles exhibit supercellular characteristics. Consistent with previous studies, we find that sparkles were present at high altitudes when radar reflectivity values were relatively high. Such values are associated with strong updrafts, lofting of graupel, and overshooting cloud tops. We confirm with a fuzzy-logic hydrometeor classification algorithm that graupel is often present near sparkles. Near sparkles, radar data also shows enhanced spectral width values and heterogeneous patterns in the radial velocity. This likely represents enhanced turbulence. Our observations match hypotheses to explain the small extent of sparkles, namely folding of a charged screening layer, and fragmentation of existing charge pockets. Additionally, we hypothesize that inductive charging, enhanced by turbulence, could play a role in the formation of sparkles.