Lightning and precipitation relationship in coastal thunderstorms

Some differences between lightning and precipitation characteristics of thunderstorms over sea and over land are analyzed. The rain field is observed by a meteorological radar. The cloud-to-ground (CG) lightning activity is observed by the French network METEORAGE, and the total lightning activity is observed by a Surveillance et Alerte Foudre par Interferometrie Radioelectrique (SAFIR) interferometric system. Several parameters are compared for 21 stormy days over land and over the ocean in different meteorological conditions. The parameters considered for precipitation are cumulative rainfall and rain rate. Those for the lightning activity are the density, rate, proportion, peak current, and multiplicity of CG flashes, and the density and rate of the total flashes and of the VHF sources. The whole data set is statistically analyzed, and some individual convective systems are discussed. The storm activity and some parameters like the CG peak current clearly seem to be affected by the surface characteristics (sea or land). Others parameters, like the positive CG flash percentage, are not affected. The land-sea ratio is found to be 1.8 for cumulative rainfall and 2.36 for CG flash number. Most of the thunderstorms observed over sea develop close to the coastline. A substantial decrease of lightning activity is observed during the lifetime of a hail-bearing thunder cell. The rain volume per CG flash, averaged for all situations and for the whole domain, is 68 X 10(3) m(3) per CG flash. For individual cases this volume increases with the positive CG proportion. Moreover, the low values of the rain volume per CG flash correspond to a relatively strong lightning activity and are often associated with a large cloud vertical development. Assuming that a high probability of lightning flash occurrence corresponds to particularly efficient charging processes, this last observation suggests that the role of strong vertical velocities in these electrical processes is essential.