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ABSTRACT
A microphysical parameterization for shallow cumulus and boundary layer stratocumulus clouds has been developed. Similar to the Khairoutdinov and Kogan parameterization for stratocumulus clouds, the new parameterization is based on an explicit microphysical large-eddy simulation (LES) model as a data source and benchmark for comparison. The predictions of the bulk model using the new parameterization were tested in simulations of shallow cumulus and boundary layer stratocumulus clouds; in both cases the new parameterization matched the predictions of the explicit microphysics LES quite accurately. These results show the importance of the choice of the dataset in parameterization development and the need for it to be balanced by realistic dynamic conditions. The strong sensitivity to representation of rain evaporation is also demonstrated. Accurate formulation of this process, tuned for the case of cumulus convection, has substantially improved precision of rain production.
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1. Introduction
The parameterization of clouds in numerical models depends among other things on the model's grid size. The most accurate parameterization is possible in large-eddy simulation (LES) models that employ high spatial resolution and, therefore, are capable of accurate description of turbulent dynamics. In particular, finescale resolution of individual updrafts/downdrafts allows accurate calculation of local supersaturation and, therefore, a physically grounded representation of nucleation, drop condensational growth, and evaporation, in addition to coagulation and gravitational fallout (sedimentation). These microphysical processes can be formulated in LES models in two ways. In the first approach, referred to as explicit microphysics, cloud drop size distributions (DSD) are described by many size categories and evolve in unconstrained manner according to dynamic and microphysical processes. The computationally less expensive approach is to predict several moments of DSD rather than DSD itself. These moments represent bulk cloud characteristics-for example, liquid water content or cloud drop concentration. Bulk microphysical parameterizations, being relatively simple and computationally efficient, are extensively used in LESmodels, as well as inmesoscale and even some GCM models. Currently many bulk parameterizations of warm-rain cloud microphysics (Tripoli and Cotton 1980; Beheng 1994; Khairoutdinov and Kogan 2000; Cohard and Pinty 2000; Seifert and Beheng 2001; Liu and Daum 2004), as well as more general liquid- and ice-phase microphysics (Lin et al. 1983; Ferrier 1994; Ferrier et al. 1995; Morrison et al. 2005; Milbrandt and...