The Probability of Hail in Relation to Radar Echo Heights on the South African Highveld

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  • 1 National Physical Research Laboratory, CSIR, Pretoria, South Africa
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Abstract

Thunderstorms in the vicinity of Johannesburg and Pretoria have been tracked by an S-band radar since 1971. A total of 181 individual cells which occurred during 1974/75 was selected for this study. Of these, the numbers that did and did not produce hail on the ground were 77 and 104, respectively. The mean of the maximum heights of hail-producing cells was found to be 6.3 km AGL for the 40 dBZ contour and 9.1 km for echo tops (23 dBZ). These parameters in rain-producing cells reached only 4.5 and 6.4 km, respectively. The mean temperature in the free atmosphere at echo top height was −37°C for hail-producing cells and −20°C for rain-producing cells. No strict correlation was found for the temperature distribution and for the position of the freezing level versus the occurrence of hail on the ground There is at least a 50% chance for thunderstorm cells exceeding 7 km to produce hail on the ground. Probability graphs relating the occurrence of hail on the ground to heights of the 23 and 40 dBZ reflectivity contours as well as to temperatures at maximum echo-top heights were plotted. Such graphs could be used for the evaluation of hail suppression experiments. The bi-modal distribution of the probability graph of hail can be explained in terms of differences in the height distributions of the 40 dBZ contour of cells producing hailstones ≤1 cm and >1 cm in diameter.

Abstract

Thunderstorms in the vicinity of Johannesburg and Pretoria have been tracked by an S-band radar since 1971. A total of 181 individual cells which occurred during 1974/75 was selected for this study. Of these, the numbers that did and did not produce hail on the ground were 77 and 104, respectively. The mean of the maximum heights of hail-producing cells was found to be 6.3 km AGL for the 40 dBZ contour and 9.1 km for echo tops (23 dBZ). These parameters in rain-producing cells reached only 4.5 and 6.4 km, respectively. The mean temperature in the free atmosphere at echo top height was −37°C for hail-producing cells and −20°C for rain-producing cells. No strict correlation was found for the temperature distribution and for the position of the freezing level versus the occurrence of hail on the ground There is at least a 50% chance for thunderstorm cells exceeding 7 km to produce hail on the ground. Probability graphs relating the occurrence of hail on the ground to heights of the 23 and 40 dBZ reflectivity contours as well as to temperatures at maximum echo-top heights were plotted. Such graphs could be used for the evaluation of hail suppression experiments. The bi-modal distribution of the probability graph of hail can be explained in terms of differences in the height distributions of the 40 dBZ contour of cells producing hailstones ≤1 cm and >1 cm in diameter.

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