Cloud Survey over West Africa Reveals Climate Impact of Mid-Level Clouds

Bhattacharya, A., Pacific Northwest National Laboratory

Cloud Processes

Cloud Life Cycle

Bouniol D, F Couvreux, PH Kamsu-Tamo, M Leplay, F Guichard, F Favot, and EJ O'Connor. 2012. "Diurnal and seasonal cycles of cloud occurrences, types, and radiative impact over West Africa." Journal of Applied Meteorology and Climatology, 51(3), 10.1175/jamc-d-11-051.1.

Clouds occurring at different levels in the sky have varying impacts on Earth’s energy budget.

Clouds occurring at different levels in the sky have varying impacts on Earth’s energy budget.

Clouds with bases between five and seven kilometers above the Earth’s surface, also known as mid-level clouds, occur over West Africa all year-round and may have major impacts on the Earth’s energy budget, scientists report using a first-ever, detailed survey of cloud types over the region.

Taking advantage of a rich set of ground-based observations collected by the Atmospheric Radiation Measurement (ARM) Climate Research Facility's Mobile Facility deployed in Niamey, Niger, in 2006, scientists from Europe published a comprehensive survey of different cloud types over West Africa and estimated their impact on the region’s climate.

Led by Dominique Bouniol, 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 (bases within 5 kilometers), and deep convective clouds. The latter two produce rain in the region.

Out of these four cloud types, the mid-level clouds appear to have the strongest impact on the Earth’s energy budget. They scatter incoming sunlight but trap outgoing energy from the Earth. Being the only ones to do so all year round, they exert a major impact on West African climate.

The only other cloud type that exerts comparable influence on radiation is thunderstorm-causing 'anvil' clouds. These have flat bottoms but spread laterally, sometimes for hundreds of kilometers, but they only occur in the region 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. Even so, it is difficult to measure the impact of clouds on the energy budget of the Earth, more so in places like West Africa, where setting up instrumentation is a logistical challenge. The authors hope that their research will provide much-needed information to calibrate weather prediction and climate models with the observed characteristics of clouds over West African arid regions.