A new cloud physics parameterization in a large-eddy simulation model of marine stratocumulus

A new bulk microphysical parameterization for large-eddy simulation (LES) models of the stratocumulustopped boundary layer has been developed using an explicit (drop spectrum resolving) microphysical model as a data source and benchmark for comparison. The liquid water is divided into two categories, nonprecipitable cloud water and drizzle, similar to traditional Kessler-type parameterizations. The cloud condensation nucleus (CCN) count, cloud/drizzle water mixing ratios, cloud/drizzle drop concentrations, and the cloud drop integral radius are predicted in the new scheme. The source/sink terms such as autoconversion/accretion of cloud water into/by drizzle are regressed using the cloud drop size spectra predicted by an explicit microphysical model. The results from the explicit and the new bulk microphysics schemes are compared for two cases: nondrizzling and heavily drizzling stratocumulus-topped boundary layers (STBLs). The evolution of the STBL (characterized by such parameters as turbulence intensity, drizzle rates, CCN depletion rates, fractional cloud cover, and drizzle effects on internal stratification) simulated by the bulk microphysical model was in good agreement with the explicit microphysical model.