Diagnosis of cirrus cloud occurrence using large-scale analysis data and a cloud-scale model

摘要

The development of cirrus clouds is governedby large-scale synoptic movements such as updraft regions in convergence zones,but also by smaller scale features, for instance microphysical phenomena,entrainment, small-scale turbulence and radiative field, fall-out of the icephase or wind shear. For this reason, the proper handling of cirrus life cyclesis not an easy task using a large-scale model alone. We present some resultsfrom a small-scale cirrus cloud model initialized by ECMWF first-guess data,which prove more convenient for this task than the analyzed ones. This model isStarr's 2-D cirrus cloud model, where the rate of ice production/destruction isparametrized from environmental data. Comparison with satellite and localobservations during the ICE89 experiment (North Sea) shows that such anefficient model using large-scale data as input provides a reasonable diagnosisof cirrus occurrence in a given meteorological field. The main driving featuresare the updraft provided by the large-scale model, which enhances or inhibitsthe cloud development according to its sign, and the water vapour availability.The cloud fields retrieved are compared to satellite imagery. Finally, the useof a small-scale model in large-scale numerical studies is examined.