A mixed-phase cloud scheme based on a single prognostic equation

A mixed-phase cloud scheme is proposed. It is derived by considering simplifications from cloud microphysics parameterizations currently used in cloud models. The scheme considers only one prognostic variable (the total water content) partitioned diagnostically into solid and liquid 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 since it is computationally fast and easy to implement. This allows simulations 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-phase 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 substantially less than the amount obtained from other schemes using temperature as the only indicator for the appearance of mixed-phase clouds.