Cloud-to-ground lightning activity in relation to the radar-derived hail kinetic energy in Switzerland

Abstract

First of its kind, this study presents relationships between radar-derived hail kinetic energy at low storm level (1.5 km MSL) and cloud-to-ground (CG) lightning stroke rates for 41 hail cells of various intensities that occurred over the Swiss Mittelland (1992–1995). Hail kinetic energy is calculated from C-band Doppler radar Constant Altitude Plan Position Indicators (CAPPIs), and locations of positive and negative CG strokes attributed to hail cells, which are approximated for every time step (5 min) by a circle including reflectivities ≥30 dBZ. Inaccuracies of CG stroke and radar measurements were corrected to obtain a complete data set. The results show that negative CG stroke totals are in good linear correspondence with total amounts of hail kinetic energy ( E KINTOT) with a correlation coefficient of 0.95. E KINTOT correlates to temporal (0.88) and spatial locations of negative CG stroke peaks (0.84) relative to maximally expected hailfall (peaks in hail kinetic energy): in 66% of the cases, negative CG stroke rates peaked on the average 22 min and 19 km prior to maximal hailfall. Totals of positive CG stroke rates are in no direct correspondence with E KINTOT. Temporal–spatial locations of positive CG stroke peaks reveal large variances with a tendency for weak isolated hail cells to occur on the average 10 min before and 3.5 km after maximal hailfall, whereas peaks of positive CG strokes lag maximal hailfall up to 30 min and 45 km in strong, large isolated cells. The study suggests that negative CG stroke rates could be used in the future to support and improve nowcasting operations for severe hailstorms in Central Europe.