The role of drylines in thunderstorm initiation and record rainfall during an anomalously convective early summer season in South Africa

Abstract

An average of 12 drylines are associated with thunderstorm development (convective drylines) during December in the western plateau of subtropical southern Africa. During December 2021, 20 convective drylines occurred. This was the highest number since the record began in 2010. December 2021 was also unusually wet over a large part of the summer rainfall region in South Africa. Monthly rainfall records were broken in the Northern Cape, Northwest and Free State provinces. Above-average rainfall had dire consequences for the maize industry over the central interior. The large-scale circulation during December 2021 was very conducive to dryline formation over the western plateau with a deeper than usual surface trough and strong low-level northeasterly moisture flux towards the climatologically dry western plateau region. Above normal wind speeds of mid-tropospheric northwesterlies and large-scale uplift contributed to a favorable environment for strong thunderstorms, as seen during a 6-day convective dryline event. Thunderstorms which were triggered along the dryline travelled east and contributed to the above normal rainfall observed during December 2021. Dryline and rainfall frequencies show large inter-annual variability. However, a positive Pearson correlation (r = 0.45; p < 0.05) between the number of dryline days and rainfall amounts were found between 1979 and 2021. This correlation increased (r = 0.53; p < 0.05) when rainfall amounts immediately east of the western plateau domain were also included, suggesting that dryline-related convection can have a significant influence on rainfall over the western plateau and further downstream.