Mid-level clouds over West Africa : origin, properties and impacts

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Description

Clouds have a major impact on the distribution of water within the atmosphere and on radiative fluxes, both at the surface and at the top of the atmosphere. However, in West Africa, the cloud type, occurrence and radiative effects have not been extensively documented. The Sahel region is characterized by a strong seasonality with precipitation occurring from June to September (monsoon season). During this period, most rainfall comes from convective systems, also leading to high cloud covering. Taking advantage of the one-year ARM Mobile Facility (AMF) deployment in 2006 in Niamey (Niger), Bouniol et al (2012) documented the distinct cloud types observed over this Sahelian location, and quantified their occurrence, seasonal cycles and radiative impact at the surface. They found that deep convective cloud is not the only cloud type that can be observed in this region. Mid-level clouds (with a base around 6 km height) also frequently occur and have a substantial impact on surface short-wave and long-wave radiative fluxes. Furthermore, in a process-oriented evaluation of climate models that were included in the last Cloud Feedback Intercomparison Project, Roehrig et al (2013) showed that these mid-level clouds are not well represented in numerical models. The aim of this work is to document the environment in which such clouds occur and the physical processes explaining their life cycle as well as their radiative properties over the West African region. Firstly, using the data set collected at Niamey, the annual cycle is documented, and the results show the ubiquitous presence of mid-level clouds. The macrophysical and microphysical properties of each mid-level cloud are further documented. A clustering method is applied to this data set in order to group mid-level clouds sharing the same morphological properties in an attempt to relate them to thermodynamical regimes. Three main mid-level cloud families are identified and their main characteristics are described. Secondly, we used the FENNEC ground-based data set to examine the mid-level clouds in the Sahara and more precisely in Bordj Badji Mokhtar (BBM). The clustering method is also generalised larger scale over West Africa combining complementary satellite datasets (CloudSat, CALIPSO, MSG, AIRS, Megha-Tropiques, ...).