Lightning phenomenology in the Tel Aviv area from 1989 to 1996

Abstract

We present the results of a continuing survey of lightning characteristics in Tel-Aviv, Israel, for the period 1989-1996, based on daily registrations of a CGR3 lightning flash counter [Mackerras, 1985]. The lightning season in Israel lasts from October to April, and the long-term average of the annual flash density in the Tel-Aviv area was found to be 4.7±2.3 km-2 y-1. The mean intracloud/cloud-to-ground flash ratio was found to be 2.5±1.3, with maxima in the autumn months. This may be attributed to the higher altitudes of the -10°C and -25°C isotherms (which signify the locations of charge centers) and to the weaker wind shears that occur in these months. The average fraction of positive ground flashes (PGF) in Tel-Aviv thunderstorms was F=0.16±0.08. Storms that exhibited larger than average PGF fraction were found to be subjected to a strong shear of the horizontal wind. The observed empirical relation between the PGF fraction and the intensity of the wind shear W (in m s-1 km-1) was log F = 0.0305W + 0.073.