A climatology of drylines and dryline related convection in sub-tropical southern Africa

Surface moisture boundaries or drylines are considered important role players on severe thunderstorm days by South African weather forecasters. On 11 December 2017 one of several thunderstorms which occurred along a dryline, developed into a supercell thunderstorm that resulted in a destructive tornado over the Vaal Marina south of Johannesburg. While the importance of drylines for convection initiation in the region is relatively well understood, the climatological characteristics of drylines and dryline related convection were unknown until recently. Making use of an objective dryline identification algorithm and ERA5 reanalysis data, a climatology of synoptic drylines for the subtropical southern African interior is developed for 1979-2021. The Southern African Plateau (SAP) domain falls within 16-32°S; 18-29°E. Subsequently cloud-to-ground lightning stroke data from the South African Weather Service’s lightning detection network are used to identify days where drylines result in convection between 2010-2021 for a smaller domain within 24-30ºS; 18-24.5ºE. This region is characterized by its high rainfall variability and has some of the highest dryline frequencies in southern Africa. Drylines are found to occur regularly during spring and summer (September - February), almost daily during December, but rarely in winter. Dryline frequencies show large interannual variability with as few as 47 drylines in the summer of 2015/16 of which 38 were convective dryline days, and as many as 73 drylines in the summer of 2016/17 of which 61 were convective. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November, but over the central and western SAP during December. Synoptic composite maps are used to gain insight into the different formation environments of drylines that result in convection and those that do not.