Relationships between dryline intensity, deep moist convection and regional circulation over interior southern Africa

Abstract Drylines are key mesoscale boundaries that influence the spatial distribution of convection in many semi‐arid regions, yet their intensity and variability remain poorly quantified outside well‐studied areas such as the US Great Plains. This study investigates the role of moisture gradient intensity across drylines in driving thunderstorm development over the semi‐arid western plateau of southern Africa. Dryline moisture gradients and their association with deep moist convection are quantified using European Centre for Medium‐Range Weather Forecast Reanalysis v5 and lightning observations from the South African Lightning Detection Network for the period 2010–2022. Moisture gradients are, on average, 31% stronger on convective than non‐convective dryline days during October–December (OND) and 25% stronger during January–March (JFM) at 1500 UTC, the most convective time of day. Moisture gradients are highest in the early morning and evening, when the medians of local gradient maxima range between 3.5 and 4.2 g·kg −1 per 0.25° grid cell depending on the sample time within the diurnal cycle. However, these maxima are out of phase with peak afternoon convection, indicating that dryline intensity alone is less important than its interaction with daytime heating and associated low‐level circulations. Convective dryline days are further classified into strong and weak gradient cases to examine their formation environments. Strong cases are associated with deeper low‐pressure structures between 850 and 500 hPa and lower thickness values between 850 and 700 hPa over the western plateau. They also exhibit enhanced low‐level moisture flux, higher specific humidity, and more concentrated uplift along the dryline. Correspondingly, afternoon lightning frequency within 1.5° east of the dryline is 3.0–3.6 times higher on strong days than on weak days in OND, and 1.4–1.5 times higher in JFM. These results highlight the importance of strong drylines, particularly during OND, in supporting favorable environments for enhanced convective activity.