Development of a Cirrus Parameterization Scheme: Performance Studies in HIRLAM

Abstract

A parameterization scheme for the formation and maintenance of cirrus is presented. The technique combines the description of the microphysical processes specific to ice with the observational experience of cirrus behavior. The scheme is a further development of the cloud parameterization approach presented in Sundqvist et al The performance of this cirrus scheme is tested with the High-Resolution Limited Area Model (HIRLAM). Ice crystals are assumed to form through homogeneous freezing of solution droplets, at temperatures below ?38°C, provided that the relative humidity exceeds a temperature-dependent threshold for the relative humidity with respect to ice. The onset of ice crystal nucleation is followed by the relaxation of the cloud water and relative humidity toward the quasi-steady state, forced by the large-scale convergence of water vapor. Ice crystals, horizontally advected from the neighboring grid boxes, enable spreading of cirrus whenever the relative humidity exceeds saturation. Performance studies with HIRLAM show that there is a large structural agreement between the satellite images and the forecasted cirrus in the extent and the pattern of the cover. The predicted mean ice water content decreases with the decreasing temperature, reflecting the temperature-dependent behavior of the measured data. In a series of sensitivity tests the most sensitive scheme parameters are revealed and analyzed.