Comparison of the ECMWF reanalysis with FIRE I observations: Diurnal variation of marine stratocumulus

The European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis ( ERA) results for July 1987 are compared with stratocumulus observations for the same period made during the First International Satellite Cloud Climat ology Project (ISCCP) Regional Experiment (FIRE). The FIRE I experiment, co nducted off the coast of California, provided a comprehensive observational set of data on marine stratocumulus during July 1987. The observations rep orted in this paper and used as a basis for comparison with the ERA results include the mean diurnal variation of cloud cover, liquid water path, clou d-base and cloud-top height, incoming shortwave and longwave radiation at t he surface, and monthly mean vertical profiles of (potential) temperature a nd specific humidity. It is concluded that the ERA thermal structure in the boundary layer is qui te realistic and the ERA data reveal some diurnal variability. However, in ERA the cloud thickness is overestimated and cloud cover and liquid water p ath are strongly underestimated as compared with the FIRE I observations. A s a result the ERA downwelling shortwave radiation at the surface is much l arger than observed in FIRE I. It is argued that the large difference betwe en the ERA downwelling shortwave radiation at the surface and that observed in FIRE I can lead to large biases in the sea surface temperature in a cou pled ocean-atmosphere model.