Simulating the influence of topography on cut‐off lows over Southern Africa

Abstract

AbstractCut‐off lows (COLs) over Southern Africa are known for their extreme rainfall, but the reliable prediction of COL rainfall remains a challenge because of complex interactions with topography. As there is a dearth of information on these interactions over this region, this study examines the influence of topography on the characteristics of COLs over Southern Africa. Multi‐ensemble simulations from the Weather Research Forecasting (WRF) model were run to simulate three COLs over Southern Africa, using the real topography [CONTROL] and three idealized terrains (i.e., “no topography” [NoTOPO], “only west topography” [WTOPO] and “only east topography” [ETOPO]). A COL tracking algorithm was used to track COLs in the simulation dataset. The characteristics of the examined COLs include their formation, track, rainfall and vertical structure. The results show that topography influences the characteristics of COLs, although such influence varies with individual COLs. The influence of topography on COL formation is more pronounced on COLs that form in the vicinity of the sub‐continent than the ones that form far from the sub‐continent. Topography promotes uplift of warm moist air and initiation of deep convection, which may weaken the zonal strength of the westerly jet and allow for the formation of the COL. As the COLs approach topography, their tracks become more southerly. Topography decelerates the westerly wave systems, hence the system moves faster in NoTOPO experiments than in CONTROL experiments. While topography seems not to influence the timing of the peaks in rainfall during the COL lifecycle, it does influence the spatial distribution of rainfall. The presence of western topography enhances the rainfall over the western part of Southern Africa, whereas the presence of eastern topography reduces the rainfall over the western parts of Southern Africa. These results have applications in improving the forecasting of COLs over the complex terrain of Southern Africa.