Die Voorspelling van haelkorrelgroei in Suid-Afrika

Abstract

Until recently, weather forecasters had to rely on intuition and experience when forecasting the occurrence of hail. Using a tephigram, cloud development can readily be determined, but forecasting damaging hail is much more difficult. The forecaster is thus very uncertain of the accuracy of the forecast. Results of a study on the forecasting of the growth of pellets of hail in South Africa are presented. The Sky Watch system, consisting of two one-dimensional models, was used to investigate the development of hailstorms and the occurrence of hail. Different atmospheric conditions favouring the growth of large pellets of hail were studied. The study of the development of hailstorms concentrated mainly on the effect that entrainment has on the development of the storms. The advection of a dry-air mass in the upper air over an area of potential thunderstorm development, led to less vigorous storms than in the case of the advection of a cold-air mass in the upper air over such an area. This resulted in the occurrence of larger pellets of hail with the advection of a cold-air mass than in the case of the advection of a dry-air mass. Characteristics of the upper-air structure giving guidance to the forecaster on the probability of the development of damaging hail, appeared from this study. One of this is the requirement that the thunderstorm cloud top must be higher than the-40°C level for the growth of large pellets of hail. Another criterion is that low values of the cloud-base index (temperature at cloud-base multiplied with the cloud-base mixing ratio), favour the development of large pellets of hail.