Cloud Survey over West Africa Reveals Climate Impact of Mid-Level Clouds
Clouds with bases between five and seven kilometers of Earth’s surface, also known as mid-level clouds, that occur year round over West Africa may have major impacts on Earth’s energy budget. Using observations collected by the Atmospheric Radiation Measurement (ARM) Mobile Facility deployed in Niamey, Niger, in 2006, scientists from Europe published a comprehensive, first-ever survey of different cloud types over West Africa and estimated their impact on the region’s climate. The team identified four types of clouds in the region: cirrus or high-level clouds with bases above 8 kilometers, mid-level clouds with bases between 5-7 kilometers, low-level clouds (bases within 5 kilometers), and deep convective clouds. The latter two produce rain in the region. Of these four cloud types, mid-level clouds appear to have the strongest impact on Earth’s energy budget. They scatter incoming sunlight but trap outgoing energy. As the only clouds to do so year round, mid-level clouds exert a major impact on West African climate. The only other cloud type that exerts comparable influence on radiation is the thunderstorm-causing ‘anvil’ cloud. These clouds have flat bottoms that spread laterally, sometimes for hundreds of kilometers, but occur in the region only during the monsoon season. Their impact on radiation is thus limited. Climatologists agree that clouds produce by far the largest source of uncertainty in climate models. It is difficult to measure the impact of clouds on Earth’s energy budget, and more so in places like West Africa, where setting up instrumentation is a logistical challenge. The authors hope their research will provide much-needed information to calibrate weather prediction and climate models with the observed characteristics of clouds over African arid regions.
Bouniol, D., F. Couvreux, P.-H. Kamsu-Tamo, M. Leplay, F. Guichard, F. Favot, and E. J. O’Connor. 2012. “Diurnal and Seasonal Cycles of Cloud Occurrences, Types, and Radiative Impact over West Africa,” Journal of Applied Meteorology and Climatology 51, 534-53. DOI: 10.1175/JAMC-D-11-051.1.