Mesoscale Cirrus Cloud Modeling and Comparisons against Experimental Data Collected on 17 April 1994 during the EUCREX Campaign

Abstract

The influence of two initialization schemes implemented for cirrus cloud simulation with a mesoscale model (RAMS) when run at two distinct horizontal resolutions (40 and 13.5 km) is investigated. The first method combines both first guess and analyzed data issued from the European Centre for Medium-Range Weather Forecasts (ECMWF). Indeed, the former provides a good estimation of the amplitude of relative humidity, but the field is phase shifted while the latter gives a correct phase but a bad amplitude (because of the difficulty of measuring relative humidity at pressure levels lower than 400 mb). The second scheme uses information at three scale levels: ECMWF three-dimensional fields, an atmospheric vertical profile, and a cloud cover mask derived from a National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer satellite image. An interpolation algorithm that conserves the power spectrum slopes is used. Finally, several comparisons between the model outputs and observational data collected during the European Cloud and Radiation Experiment campaign are made. At a horizontal scale of 13.5 km, the authors? new assimilation procedure allows a much better characterization of the whole three-dimensional cloud field (i.e., both horizontally and vertically) in terms of optical thickness and ice water content distributions, whereas the simulations are not improved at a larger horizontal scale (40 km).