Characteristics of warm season supercell thunderstorms over the Gauteng and Mpumalanga Provinces of South Africa

Abstract

A supercell thunderstorm, notorious for producing large hail, damaging winds, tornadoes, and flooding, often results in damage to property, injuries, and loss of life. Supercells were previously considered rare in South Africa, but in more recent years, evidence has shown this not to be the case. With the installation of the South African Weather Service Doppler radar network in 2010, several cases have since been detected. However, no radar-derived database exists and research on supercells, including their occurrence and characteristics, is limited over South Africa. The aim of this research is to investigate the characteristics of supercells over the Gauteng and Mpumalanga provinces of South Africa during the warm seasons (September to February) of 2010 to 2020. To achieve this a radar-derived database was created, using all available radar data from the Irene and Ermelo radar, with as many supercells identified as possible. These supercells were analysed to investigate their temporal, spatial, track and lifespan characteristics. A database comprising 115 left-moving and 6 right-moving events over 67 event days was established. On a given event day an average of 2 supercells were found to occur with their activity peaking in the afternoon between 13:00 and 14:00 UTC. On average 13 supercell events and 7 event days (containing at least 1 supercell) occurred per season and their frequency peaked between October and November. The peak in the monthly distribution occurred earlier over Gauteng in October and November, but over Mpumalanga the peak was later during December. Supercell activity occurred over the southern Highveld in September, spreading northwards by October and then to the south-east, while decreasing in the west, in December. The spatial distribution (initiation, track, and demise) showed hotspots in activity in places over southern Gauteng as well as the south-eastern parts of Mpumalanga. Supercells were predominantly short-lived (lasting ≤ 2 hours), with only 12 moderate-lived left-moving events (lasting > 2 hours but < 4 hours). On average the left-moving events lasted 1 hour 12 minutes, tracking an average distance of 49 km, while on average right-moving events lasted 49 minutes and tracked 27 km. The left-moving supercells had an average track from the south-west to north-east, however, their track shifted from west south-westerly earlier in the season to southerly in January. All right-moving supercell events had a track from the north-west to south-east. The short-term radar-based climatology of supercell characteristics that were identified in this research, will allow for a better understanding and thus potentially improving forecasting, nowcasting and warning for these thunderstorms over the Gauteng and Mpumalanga provinces of South Africa. A further investigation into the environment and topography over the area is required, to fully understand why these characteristics were observed. The radar-derived supercell database is a first for South Africa and thus provides a foundation on which numerous other studies can be conducted.