A numerical model of the cloud-topped marine boundary layer with explicit treatment of supersaturation–number concentration correlations

Abstract

A numerical model of the cloud-topped marine boundary layer is presented. The model, based on ensemble-averaged conservation equations and containing detailed aerosol microphysics, differs from previous models in one major respect: the mean supersaturation-cloud drop number concentration correlation term, generally neglected in previous numerical mean cloud models, is parametrized using a traditional boundary-layer flux-gradient relationship. The present work highlights the importance of this correlation term and shows how a simple representation of this correlation can lead to dramatically different, and improved, simulations of cloud behaviour. Simulations are presented for a well-studied marine boundary-layer stratocumulus case. Inclusion of the supersaturation-cloud drop number concentration correlation leads to more physically realistic cloud drop spectra and cloud drop concentrations throughout the height of the cloud. Although the parametrization presented must be viewed as a first approximation, the crudeness of the approach should be seen as a justification and starting point for future work.

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