Understanding the Role of Low- and Mid-level Cloudiness for the West African Monsoon: the DACCIWA Project

Recent research by the authors revealed that the southern parts of West Africa, from the coast to about 10°N, are frequently covered by an extensive deck of shallow, low (200 – 400 m above ground) stratus or stratocumulus clouds during the summer monsoon season. Climatologically, the ultra-low stratus deck forms after sunset along the Guinea coast, spreads inland and reaches its maximum northward extent of 10-11°N between 0900 and 1000 UTC of the following day. The maximum affected area is approximately 800,000 km2. After about 1000 UTC, the northern boundary gets fragmented due to the breakup of stratus decks into fair-weather cumuli. It has been shown that the night-time cloud formation is related to a subtle balance between ‘‘stratogenic'' upward (downward) fluxes of latent (sensible) heat caused by shear-driven turbulence below the night-time low-level jet, cold advection, orographic lifting, and radiative cooling on one hand, and ‘‘stratolytic'' dry advection and latent heating on the other hand. Up to one third of the nights unaffected by rainfall systems remained cloud free in central Benin (~9°N) during the 2006 AMMA campaign, the reason of which are not fully understood. A consequence of this subtle balance is that Coupled Model Intercomparison Project 3 (CMIP3) global climate models often failed to represent the ultralow clouds, leading to errors in surface solar radiation of up to 90 W m-2. New results using CMIP5 and YOTC (Years of Tropical Convection) GASS (GEWEX Atmosphere System Study) global climate models show that the representation of low-level cloudiness has not improved in the latest generation of global models. This may be related to problems in simulating the shallow meridional circulation. In general, validation of weather and climate models in terms of the representation of ultra-low clouds is problematic due to the misrepresentation of ultra-low cloud decks in most satellite products. These findings were one major motivation to create the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project (2013-2018) that is funded by the European Union with 16 partners in Africa and Europe. DACCIWA will carry out a large surface, upper-air and multi-aircraft campaign in southern West Africa in June-July 2016. The campaign data set will help to improve our understanding of the dynamics of the formation/dissipation of low-level stratus and mid-level altostratus in this region and beyond. From the beginning, Pete Lamb was very much interested in this research, got involved in the creation of the DACCIWA project and agreed to chair its scientific advisory board. Sadly, the DACCIWA project team lost a friend and an invaluable source of help and support. Not only have we dedicated a BAMS Inbox article to Pete Lamb, but his legacy is also a motivation to us to further our understanding of West African Meteorology and Climate and to train young, early-career African researchers.