A study of the total lightning activity in two hailstorms

Abstract

Two thunderstorms which developed in the northeastern region of Spain on 17 August 2004 are analyzed. According to radar and ground observations, one of these storms produced large hail (larger than 19 mm) and therefore could be severe. Both thunderstorms exhibited strong vertical developments with cloud tops reaching 17.5 and 14 km for the severe and the nonsevere ones, respectively. The total lightning activity was monitored, thanks to the Spanish magnetic-direction-finding and time-of-arrival (MDF/TOA) network and thanks to a regional very-high-frequency (VHF) interferometer network. Both storms presented low cloud-to-ground (CG) flash rates (lower than 2 min -1) during their mature phase, while the intracloud (IC) flash rate reached 92 min -1 for the severe one. The IC/CG flash ratio was very high for both storms: 35 and 60 in average value for the severe and the nonsevere ones, respectively. The lightning characteristics exhibited low values in terms of peak current and multiplicity for negative CG (-CG) flashes and in terms of VHF source number per flash for IC flashes. The average multiplicity of the -CG flashes was 1.74 and 1.17 for the severe and the nonsevere storms, respectively, and their average peak current was - 11.7 and - 10.65 kA, respectively. When the presence of hail is suspected from high radar reflectivity values in both cells, the flash rates tended to substantially decrease. The severe storm produced positive CG (+CG) flashes during 1 hour, and at that moment, the multiplicity and peak currents of -CG flashes were very low. Lightning activity for both storms ended with a substantial increase of the -CG flash rate. The elevated charge hypothesis is consistent with the observations, especially if we consider two aspects of this hypothesis, the one about charge separation, which could be limited by the strong updrafts, and the one about the lightning initiation, which could be favored by low pressures at high altitude. Copyright 2007 by the American Geophysical Union.