Characteristics of warm season left-moving supercells over the Highveld of South Africa

Abstract

A supercell is a type of thunderstorm known to produce severe weather conditions that result in damage to property, injuries, and loss of life. Supercells were previously considered rare in South Africa, however since the implementation of the South African Weather Service's single polarisation S-band Doppler radar network in 2010, numerous cases have been identified. Despite this, there is a lack of research on supercells over South Africa, with very little information on their characteristics specific to the region. The aim of this study is to determine the characteristics of warm season left-moving supercells over the Highveld (Gauteng and Mpumalanga province) of South Africa. This was done using a radar derived database, consisting of 115 left-moving supercells, that occurred during the warm seasons (September to February) of 2010/11 to 2019/20. Left-moving supercells were found to be most common in October and November, peaking in the afternoon between 15:00 and 16:00 South African Standard Time (SAST). A shift in the seasonal distribution was observed, with the most active area over Gauteng in November, moving south-eastwards to the south-eastern parts of Mpumalanga by December. Supercell hotspots were identified over the south-western and eastern Highveld of Gauteng as well as the south-eastern parts of Mpumalanga, of which some areas of occurrence appeared to be influenced by topography. The average track direction was found to be from the south-west, with a southerly shift as the season progressed. On average supercells lasted approximately 1 h 12 min, travelled 49 km and at a speed of 41 kmh−1. Having a better understanding of the characteristics of supercells may provide a heightened awareness of preferred areas and times of occurrence of events, how they travel, how long they last and how fast and far they travel, thereby assisting in producing more accurate forecasts, nowcasts and warnings. More detailed studies are required to understand the influence of topography and the atmospheric conditions on these characteristics.