Cloud‐to‐ground lightning activity and radar observations of storms in the pyrénées range area

Abstract

AbstractThis paper analyses radar observations and cloud‐to‐ground (CG) lightning flash activity of several thunderstorms occurring over the Pyrénées range during the night of 23‐24 August 1993 and the afternoon of 24 August 1993. the available data consisted of the European meteorological bulletin, the plan position indicator radar scans at three elevations performed by the 5.5 cm wavelength Rodin radar of Toulouse, and the characteristics of the CG lightning strikes.The first thunderclouds began to develop at around 2200 UTC 23 August. Up to 0100 UTC 24 August, we observed three isolated convective cells. We also studied a mesoscale convective system (MCS) occurring during the afternoon of 24 August, and chose to investigate three convective cells embedded within it. These different thunderclouds developed over the Spanish side of the Pyrénées and moved north‐eastwards over the mountains. the locations of CG flashes in relation to the reflectivity fields revealed two kinds of CG lightning activity: flashes associated with intense rainfall, and those located in stratiform cloud areas far from the intense precipitating zone. the flash density was larger within the first type and the positive flash proportion was larger in the second type. the amount of large precipitating particles was characterized by the area A(τ) covered by reflectivities larger than τ dBZ, with τ taken as 33 dBZ. the study of the time evolution of A (33) at three altitudes showed that, for the lightning activity associated with intense rainfall, strong reflectivities at high altitude (4.5 km, and sometimes 7.5 km) coincided with large lightning flash rates, though it was the time evolution of the vertical profile of the area covered by precipitation that drove the CG lightning flash rate (or vice‐versa). the maximum flash rate coincided with the fall of large precipitating particles. Furthermore, a time lag was observed at the ground between the lightning flash rate and the rainfall rate. the time evolutions of the flash rate and A (τ) at the lowest elevation of the radar beam (0.6°) confirmed this delay. the correlation coefficient between these two parameters for the isolated convective cells reached 0.81 in the best case. the correlation between the lightning activity and precipitation of the whole MCS was less good but, nevertheless, the volume of precipitation per CG lightning flash was of the same order of magnitude for all the cells or MCSs observed. It ranged from 3.2 × 103 to 46.8 × 103 m3 flash‐1, in accordance with the results of other authors.