Identifying atmospheric conditions that accompany "sparkles", i.e. small scale, intermittent lightning discharges near the top of thunderstorms.

Abstract

Many aspects of lightning, including the initiation and propagation, remain poorly understood. The Low-Frequency Array (LOFAR) radio telescope, based in the North-East part of the Netherlands, allows to study lightning with unprecedented detail. Utilizing an array of antennas and an interferometry-based method, LOFAR can capture high resolution lightning images every ˜20 min. On June 18 2021, LOFAR image small-scale, intermittent lightning discharges near thundercloud tops, which are named "sparkles". Similar observations in the U.S. indicate that sparkling clouds are associated with strong updrafts and overshooting cloud tops. Although hypothesized to be related to charge pockets, the exact mechanisms for sparkle behaviour remain unclear. This research aims to improve the understanding of sparkles by comparing the sparkle events, on June 18 2021 in the Netherlands as imaged by LOFAR, with ambient atmospheric conditions, using data from the polarimetric C-band radar on Borkum island (Germany). A fuzzy logic hydrometeor classification algorithm provides insights into the particles aloft. Beside visual analysis, a spatio-temporal clustering algorithm distinguishes sparkles from other lightning structures and allows for a statistical comparison between the atmospheric conditions around sparkles and other lightning structures. Radar images reveal that major sparkle clouds are located above mesoscale vortices. Furthermore, sparkles occur in areas of high radar reflectivity, horizontal divergence, turbulence, and graupel. Visual inspection shows that sparkles are often located near the edge of high-reflectivity volumes and near volumes with graupel. These findings suggest that turbulent conditions in the vicinity of graupel are responsible for sparkles. Beside the possibility of mixing between oppositely charged graupel and ice crystal volumes, it is also hypothesised that either local inductive charge separation or local charge sedimentation could be responsible for sparkle behaviour. In order to test this hypotheses, one might employ a high resolution atmospheric simulation that incorporates the inductive charging mechanism. The accuracy of these results is limited by radar resolution, particularly in vertical height, and attenuation. Additionally, the statistical significance and confidence in the persistence of sparkle clouds is limited. Unexplained cloud top signatures in radar differential reflectivity values warrant further investigation.