Climatologies of ultra‐low clouds over the southern West African monsoon region and their representation in the Weather Research and Forecasting model WRF

Two recent publications (Knippertz et al. 2011, Geophys. Res. Let., Schrage and Fink, Mon. Wea. Rev, revised version submitted to Mon. Wea. Rev.) have highlighted that summertime southern West Africa is frequently affected by an extended cover of shallow, non-precipitating clouds only few hundred meters above the ground. These clouds are associated with nocturnal low-level wind speed maxima and frequently persist into the day, considerably reducing surface solar radiation. Both publications highlighted the challenge to observe the nocturnal stratus with the existing satellite and ground-based networks in southern West Africa. Knippertz et al. (2011) demonstrated that while the involved phenomena are well represented in re-analysis data, climate models show large errors in low-level wind, cloudiness, and solar radiation. The authors conclude that such model errors in low-level daytime cloudiness could strongly affect the regional energy and moisture budgets, which might help to explain the notorious difficulties of many models to simulate the West African climate. They suggest more efforts to improve the monitoring, modeling, and physical understanding of these ultra-low clouds and their importance for the West African monsoon system. In the context, we will first compare stratus information from several satellite sensors or products (e.g., METEOSAT SEVIRI infra-red and visible channels, CALIPSO, Cloudsat, MISR, MODIS, and ISCCP) among each other and against surface eye observations of stratus cloudiness for the 2006 summer monsoon season. Seasonal and diurnal climatologies of the stratus will be shown and discussed. One focus will be put on the typical time of dissipation for these cloud decks for various geographic regions within the study domain and how the disappearance relates to orographic features such as terrain height and slope. In the second part, it will be demonstrated for simulations during the 2006 summer monsoon that WRF reasonably represents stratus formation at night with almost all available boundary layer parameterization schemes. First results indicate that the disappearance of the low stratus cloud deck is more sensitive to the used boundary layer parameterization as well as to the vertical resolution and the surface physics. Most configurations show a too early disappearance. Ongoing research is also focused on the influence of radiation and microphysics schemes.