Cloud cover in the ECMWF reanalysis

Data from reanalyses recently carried out by several climate and numerical weather prediction centers will find a variety of applications in different branches of atmospheric science. A careful evaluation of the many aspects of these datasets is a prerequisite for their successful use. This paper describes the implementation of a fully prognostic cloud scheme into the ECMWF reanalysis system and provides a first assessment of the simulation of cloud cover by comparing it with monthly mean cloud cover derived from satellite observations in the context of the International Satellite Cloud Climatology Project for the years 1983-90. Special emphasis is put on the major cloud regimes and their intra- and interannual variation. The main deficiencies identified are an underestimation of extratropical cloud cover over the oceans by 10%-15%, an overestimation of trade wind cumulus cover by about 10%-15%, an underestimation of stratocumulus off the west coasts of the subtropical continents by 15%, and an underestimation of the summer maximum in cloud cover over the Eurasian continent. Despite these deficiencies it is shown that the reanalysis system is able to capture the main aspects of the interannual variability, especially those connected to the major El Nino events in the observation period.