LIGHTNING PHENOMENOLOGY IN THE TEL-AVIV AREA FROM 1989 TO 1996

We present the results of a continuing survey of lightning characteris tics in Tel-Aviv, Israel, for the period 1989-1996, based on daily reg istrations of a CGR3 lightning flash counter [Mackerras, 1985]. The li ghtning season in Israel lasts from October to April, and the long-ter m average of the annual flash density in the Tel-Aviv area was found t o 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. Th is may be attributed to the higher altitudes of the -10 degrees C and -25 degrees C isotherms (which signify the locations of charge centers ) and to the weaker wind shears that occur in these months. The averag e fraction of positive ground flashes (PGF) in Tel-Aviv thunderstorms was F=0.16+/-0.08. Storms that exhibited larger than average PGF fract ion were found to be subjected to a strong shear of the horizontal win d. The observed empirical relation between the PGF fraction and the in tensity of the wind shear W (in m s(-1) km(-1)) was log F = 0.0305W 0.073.