Graupel and increased turbulence observed near small-scale intermittent lightning discharges at the top of intense thunderstorms

Abstract. Sparkles are defined as intermittent, small-scale lightning discharges near the top of thunderstorms. To increase the understanding of mechanisms that lead to sparkles, we compare high resolution lighting 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 June 18, 2021. We used a two-stage clustering approach to computationally distinguish sparkles from other lightning structures. Subsequently, we compare the radar data near sparkles to radar data near other lightning structures. The two convective systems that produced sparkles resemble, respectively, a supercell and a squall line. 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. Given the altitude of the radar data, the findings support the hypothesis that sparkles are caused by large charged hydrometeors that get lofted to relatively high altitudes and near a stratospheric charged screening layer. 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.