Comparison of ERA40 cloud top phase with POLDER-1 observations

A detailed grid-point-based comparison of the cloud top phase derived from the 40-year reanalyses (ERA40) of the European Centre for Medium-Range Weather Forecasts (ECMWF) with satellite measurements is presented. For this purpose an algorithm is implemented to extract a two-dimensional "satellite-like'' field of the cloud top phase from ERA40 data. This field is compared with cloud top phase data from the Polarization and Directionality of the Earth Reflectances (POLDER-1) instrument which was in orbit from November 1996 to June 1997. The thermodynamic cloud phase in ERA40 data is parameterized as a function of temperature with pure liquid clouds above 0 degrees C, pure ice clouds below -23 degrees C and mixed clouds in between. The matching performance of clouds derived from ERA40 with clouds observed by POLDER-1 is best in the extratropical storm track regions and over the tropical continents. Detailed comparison of the cloud top phase for grid points where both data sets indicate a cloud shows a relatively good agreement for warm (T > 0 degrees C) and cold (T < -30 degrees C) clouds. In the intermediate temperature range, ERA40 contains too many ice clouds compared to POLDER-1. The comparison reveals only small seasonal variations. A slightly revised parameterization for the phase partitioning of condensed cloud water is suggested (shifting the thresholds by -10 degrees C), which leads to a generally improved agreement between cloud top phases derived from ERA40 and POLDER-1.