ON THE PARAMETERIZATION OF ICE MICROPHYSICS IN A MESOSCALE-ALPHA WEATHER FORECAST MODEL

Numerical experiments with a 3-D-dimensional mesoscale alpha weather forecast model are performed to investigate the sensitivity of the model to different parameters and the parameterized microphysics. The parameterization considers condensation and deposition of water vapor, sublimation, evaporation of both cloud water and rainwater, riming of ice crystals by cloud water, rainwater formation by autoconversion, accretion and melting as well as the sedimentation of rain and ice crystals. The results of the simulations are discussed on the basis of the analysis, estimations of skill and uncertainty, satellite data as well as observed precipitation data. These results show that the dynamics of the troposphere and the cloud microphysics can be described more realistically and that the model performance can be improved if ice processes are included. It is substantiated by all of these simulations that the relative humidity and water substance mixing ratio fields were only strongly altered by turning off the ice phase or the riming process.