A Climatology of Dryline-Related Convection on the Western Plateau of Subtropical Southern Africa

Abstract

Drylines occur regularly during summer over the interior of sub-tropical southern Africa. The highest frequency of drylines is over the western interior where it is also climatologically much drier than the eastern parts. An existing drylines data set is compared with lightning data between 2010 and 2021, to determine how often drylines were associated with convective storms. A minimum stroke threshold is applied to eliminate instances of weak convection. Seventy-six percent of drylines during austral summer (October–March) were associated with significant convection with a standard deviation of 8.6. There is a peak frequency of 12 convective dryline days during December. Composite synoptic analyses were conducted for 591 convective and 186 non-convective dryline days to determine the ideal formation environment for convective drylines. Composites were also created for the days leading up to convective and non-convective drylines, showing the importance of moisture advection from the southwest Indian Ocean, through the Zambezi and Limpopo River Valleys. Early summers tend to be drier than late summers over the southern African interior and colder in the mid-levels. These differences affect the formation environment of convective drylines. Composites show that dryline-related convection during early summer tends to occur on days with a deep surface trough and a trough at 500 hPa further west. Convection during late summer is associated with increased surface moisture, higher instability, and a weaker trough at 500 hPa than during early summer.