A satellite-based analysis of semi-direct effects of biomass burning aerosols on fog and low cloud dissipation in the Namib Desert

Abstract. In the Namib Desert fog is the only regular water input and thus a crucial water source for its fauna and flora. Each year between June and October, in some synoptic settings, absorbing biomass burning aerosols (BBA) are overlying the stratocumulus clouds in the adjacent Southeast Atlantic, and sometimes are reaching the coastal fog and low clouds (FLCs) in Namibia. In this study, a novel 15-year data set of geostationary satellite observations of FLC dissipation time in the Namib Desert is used together with reanalysis data in order to better understand possible semi-direct effects of BBA on the dissipation of FLCs in the Namib. This is done by investigating both FLC dissipation time and synoptics depending on BBA loading. It is found that FLC dissipation time is significantly later on high BBA loading days. BBA are transported to the Namib along moist free-tropospheric air by a large-scale anticyclonic recirculation pattern. At the surface, the associated longwave heating strengthens a continental heat low, which modifies the circulation and boundary layer moisture along the coastline, complicating the attribution of BBA effects. During high BBA days, the vertical profiles of the temporal development of air temperatures highlight contrasting day and nighttime processes modifying the local inversion. These processes are thought to be driven by greenhouse warming by the moisture in the BBA plumes and BBA absorption (only during daytime). A statistical learning framework is used to quantify meteorological and BBA influences on FLC dissipation time. The statistical model is able to reproduce the observed differences in FLC dissipation time between high and low BBA days and attributes these differences mainly to differences in circulation, boundary layer moisture and near-surface air temperature along the coastline. However, the model is underfitting and is not able to reproduce the majority of the FLC dissipation variability. While the model does not suggest that BBA patterns are important for FLC dissipation, the findings show how the moist BBA plumes modify local thermodynamics to which FLC dissipation is shown to be sensitive. The findings highlight the difficulties of disentangling meteorological and aerosol effects on cloud development using observations.