Modelling Fog and Low Stratiform Clouds in the Namib Desert with COSMO-FOG

The Namib Desert – one of the driest regions on earth – is a typical coastal desert with very scarce rainfall and regularly occurring fog. Fog water input often exceeds rainfall and thus fog water deposition is a major source of water for the ecosystem and could be a source of water for human settlements. Therefore, a better understanding of the processes that control fog formation and its spatio-temporal patterns is important.<br> To investigate the spatio-temporal evolution of fog events and the processes controlling fog occurrence, the three-dimensional fog model COSMO-FOG is developed. For this purpose, the microphysical parametrisation of the one-dimensional fog and boundary layer model PAFOG is implemented into the three-dimensional numerical weather prediction model COSMO. To have access to additional diagnostic tools, COSMO/MESSy (COSMO including the Modular Earth Submodel System) is used instead of the pure COSMO model. The processes controlling fog occurrence and its spatial variability in the Namib region are analysed with COSMO-FOG using the MESSy submodel TENDENCY.<br> At first the model behaviour and consistency of COSMO-FOG is investigated in an idealised environment for a horizontally homogeneous marine stratus. A sensitivity study for the COSMO model and COSMO-FOG with different vertical model grid spacings is performed. The simulations with COSMO-FOG yield a more realistic cloud water content for the marine stratus compared to the COSMO model. The simulations with different vertical grids show that a fine vertical grid is necessary to resolve sharp gradients at cloud top.<br> In order to investigate the spatio-temporal patterns, diurnal life cycle phases and contributing atmospheric processes of fog and low stratiform clouds in the Namib Desert, individual case studies in the austral spring season 2017 are simulated with COSMO-FOG. In the afternoon, stratiform clouds are often present above the Atlantic Ocean and close to the coastline. In the evening, these clouds proceed onshore and intercept with the terrain of the ascending "Great Escarpment" thus forming fog. The extension of fog and stratiform clouds is caused by the advection of cold and moist air masses interacting with turbulent mixing. The advection of warm continental air masses with an easterly wind causes lower cloud heights and a smaller spatial extent of fog and stratiform clouds. Large-scale subsidence enhances this effect.<br> The results obtained with COSMO-FOG are evaluated with satellite retrievals and ground-based measurements. COSMO-FOG largely captures the spatial distribution of fog and low stratiform clouds and the meteorological situation. Especially for coastal stations the diurnal cycle and onset of the fog events are in agreement with the observations. At inland stations COSMO-FOG sometimes underestimates the extension of fog dependent on the synoptic situation.