Synoptic and regional-scale meteorological controls of stratus altitude in the Namib Desert

Abstract. In the Namib Desert, fog is an essential water source and occurs when and where advected marine stratus clouds intersect with the land surface. However, the meteorological controls of the cloud base height are still insufficiently understood. This study aims to develop a basic understanding of the relevant processes. We combine satellite and in situ observations with large-scale meteorological data from reanalysis data (ERA5) to compare fog events to lifted stratus at the coast (low-cloud events). In fog situations, the marine boundary layer is shallower along the entire coastline than in low-cloud situations. This is found to be related to the large-scale high-pressure systems. Fog situations exhibit a weaker Atlantic High but elevated continental pressure. The weaker Atlantic High is connected to less pronounced near-surface winds along the coastline, less cold advection, and heat fluxes upstream of the study region, leading to the shallower marine boundary layer. Increased continental pressure facilitates the development of regional mountain-plain winds that may reduce the height of the coastal inversion. These mechanisms are highlighted in a case study of an off-season fog event. To assess the predictive power of the two high-pressure systems and the regional pressure pattern, a logistic regression is trained with three corresponding features. The classification outperformed a climatological baseline by ≈ 10 %, suggesting that the features contain relevant process information. The results improve our understanding of the processes that determine the seasonal and day-to-day variability of fog versus elevated low-cloud occurrence in the Namib.