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 processes that influence the stratus altitude and, by extension, the spatial occurrence of fog. 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 by up to 130 m 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. In parallel, higher continental pressures on fog days facilitate the formation of stronger nocturnal katabatic winds which can mix with, undercut or block the advected cloudy marine air. An alternative mechanism is highlighted in a case study of an off-season fog event, where a ridging high produces hot and dry subsiding easterly air masses which lower and intensify the inversion from above. To assess the predictive power of the continental and Atlantic 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.