Positive cloud-to-ground flashes and wind shear in Tel-Aviv thunderstorms

The occurrence of negative and positive ground flashes in Tel-Aviv, Israel (32.05 N, 34.45 E) has been monitored through daily registrations of a CGR3-SN5 lightning flash counter (Mackerras, 1985). Measurements were conducted from 1987 to 1995, with the most continuous data sets available For the period 1992-1995. The lightning data was augmented by radar measurements of the thunderclouds (starting from 1994) and by radiosonde data. It was found that the fraction R of positive lightning from the total ground flash count is highly variable, with a long-term average of 0.25. It is shown that storms having a relatively large value of R coincide with strong vertical shear of the horizontal wind component. The dependence of R on the intensity of the wind shear S in the cloud layer between the 0°C and -25°C isotherms, may be expressed by: log R = aS - b, where a=0.1504 and b=1.1471.