MESOSCALE CIRRUS CLOUD MODELING AND COMPARISONS AGAINST EXPERIMENTAL-DATA COLLECTED ON 17 APRIL 1994 DURING THE EUCREX CAMPAIGN

The influence of two initialization schemes implemented for cirrus clo ud simulation with a mesoscale model (RAMS) when run at two distinct h orizontal resolutions (40 and 13.5 km) is investigated. The first meth od combines both first guess and analyzed data issued from the Europea n Centre for Medium-Range Weather Forecasts (ECMWF). Indeed, the forme r provides a good estimation of the amplitude of relative humidity, bu t the field is phase shifted while the latter gives a correct phase bu t a bad amplitude (because of the difficulty of measuring relative hum idity at pressure levels lower than 400 mb). The second scheme uses in formation at three scale levels: ECMWF three-dimensional fields, an at mospheric vertical profile, and a cloud cover mask derived from a Nati onal Oceanic and Atmospheric Administration Advanced Very High Resolut ion Radiometer satellite image. An interpolation algorithm that conser ves the power spectrum slopes is used. Finally, several comparisons be tween the model outputs and observational data collected during the Eu ropean Cloud and Radiation Experiment campaign are made. At a horizont al scare of 13.5 km, the authors' new assimilation procedure allows a much better characterization of the whole three-dimensional cloud fiel d (i.e., both horizontally and vertically) in terms of optical thickne ss and ice water content distributions, whereas the simulations are no t improved at a larger horizontal scale (40 km).