A MIXED-PHASE CLOUD SCHEME BASED ON A SINGLE PROGNOSTIC EQUATION

A mixed-phase cloud scheme is proposed. It is derived by considering s implifications from cloud microphysics parameterizations currently use d in cloud models. The scheme considers only one prognostic variable ( the total water content) partitioned diagnostically into solid and liq uid phases at subfreezing temperatures. The scheme preserves the basic microphysical processes characterizing natural mixed-phase clouds and does not include any adjustable (nonphysical) parameter. The proposed approach can be used in mesoscale or large-scale atmospheric models s ince it is computationally fast and easy to implement. This allows sim ulations that would have been otherwise impossible with more elaborate cloud microphysics schemes, because of excessive computational load. Numerical experiments with a mesoscale model have shown that mixed-pha se clouds generated with the scheme are confined to localized regions and are strongly correlated with the intensity of the vertical lifting . The amount of supercooled liquid water (SLW) thus obtained is substa ntially less than the amount obtained from other schemes using tempera ture as the only indicator for the appearance of mixed-phase clouds.