Low cloud errors over the southeastern Atlantic in the NCEP CFS and their association with lower-tropospheric stability and air-sea interaction

Abstract

The low cloud errors in the tropical Atlantic and their connection with lower-tropospheric stability and air-sea interaction in a simulation of the Climate Forecast System (CFS) are investigated. The CFS is the coupled ocean-atmosphere circulation model for operational climate prediction at the National Centers for Environmental Prediction. Compared with observations, the CFS produces too few low clouds over the southeastern Atlantic and too many high clouds over the whole tropical Atlantic. The center of the low clouds is also shifted westward in the model away from the cold tongue region to the central ocean. The underestimation of low clouds in the southeastern Atlantic may be one of the potential sources of the warm biases of sea surface temperature (SST) in this region. The low clouds are linked to a stable or inversion layer between 850 hPa and 925 hPa in observations, but are associated with a stable layer between 700 hPa and 850 hPa in the CFS, which may suggest that low clouds are generated in a higher layer in the CFS than in the real world. On average, the lower troposphere is less stable in the CFS than in the observations over the tropical Atlantic region, which is more favorable for the development of deep convection. Moreover, the low cloud variability in the South Atlantic is mainly associated with SST and wind stress anomalies in the subtropical South Atlantic in the CFS and in the southeastern Atlantic and along the African coast in the observations. Copyright 2008 by the American Geophysical Union.