An investigation of the vertical structure of clouds over West Africa in the Met Office Unified Model using the CloudSat simulator and CloudSat observations

The Cascade project, a NERC-funded collaborative effort among several universities and research organizations in the UK, seeks to understand the organization and interaction of tropical convection at many scales, and its potential to be captured by numerical models. High-resolution (up to 1-km grid length) simulations by the non-hydrostatic UK Met Office Unified Model (UM) are used over domains several thousand km across. One area of focus is African easterly waves in combination with the African Monsoon Multidisciplinary Analyses (AMMA) project. For this geographical region, we study the mean vertical cloud occurrence in the UM at 40km, 12km, and 4km horizontal resolution for two weeks of model data from 25th July 2006 to 8th August 2006.

CloudSat was launched in April 2006 to join the A-Train constellation of satellites, carrying a 94GHz radar. Since then it has provided us with vertical cloud profiles around the globe on a daily basis. We use the CloudSat simulator to calculate radar reflectivities from the model variables to apply the same reflectivity-based analysis of cloud occurrence. The modeled cloud occurrences are averaged over the two-week period for each hour of the day to study the diurnal variation of the vertical cloud structure. By applying existing methods to determine cloud and precipitation top, we analyse the daily development of low cloud over the Gulf of Guinea, a region of deep convection in the African monsoon, and cumulus clouds at the top of the Saharan boundary layer.

This work gives new insight into the evolution and vertical structure of clouds in the UM over West Africa and the changes in modeled clouds with increasing horizontal resolution. The observed and modeled clouds are linked to known features in the dynamic state over West Africa, in particular in terms of their location in relation to the African Easterly Jet.