Documentation of cloud characteristics inferred from ground and satellite measurements within west Africa

Clouds have a major impact on the redistribution of water within the atmosphere and on the radiative fluxes at the surface. The cloud type and occurrence in West Africa has not been extensively documented and quantified. The ARM mobile facility (AMF), including vertically pointing 94GHz Doppler radar, micropulse lidar, ceilometer and flux measurements, was deployed in Niamey (Niger) from April to December 2006, documenting for eight entire months the characteristics of clouds. Independently, the satellites CloudSat and Calipso, launched in April 2006, sampled clouds from above using similar instrumentation (Cloud Profiling Radar and CALIOP) and passing over West Africa every day around 0200LT (night) and 1400 LT (day).

A survey of cloud characteristics inferred from ground and satellite measurements is presented, focusing on the seasonal evolution and the diurnal cycle of cloud occurrence. In particular, we distinguish between four cloud types: high-level clouds (cirrus or anvils), deep convective clouds, shallow convective clouds and mid-level clouds. This study highlights the frequent occurrence of these mid-level clouds located at the top of the Saharan Air Layer. High-level clouds are ubiquitous throughout the period, whereas shallow convective clouds are mainly present during the core of the West African Monsoon. The impact of each cloud category on the surface flux measurements is also quantified.

Satellite sampling provides the latitudinal variations of these characteristics and enables us to place the local AMF observations in a larger spatial and temporal framework. Combining information from ground and satellite platforms enables us to tackle the question of the temporal representativeness of the latitudinal transect sampled twice a day by the satellite. In a second step the representation of the cloud cover and associated cloud feedbacks by NWP and GCM is addressed.