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

A new bulk microphysical parameterization for large-eddy simulation (LES) m odels of the stratocumulus-topped boundary layer has been developed using a n explicit (drop spectrum resolving) microphysical model as a data source a nd benchmark for comparison. The liquid water is divided into two categorie s, nonprecipitable cloud water and drizzle, similar to traditional Kessler- type parameterizations. The cloud condensation nucleus (CCN) count, cloud/d rizzle water mixing ratios, cloud/drizzle drop concentrations, and the clou d drop integral radius are predicted in the new scheme. The source/sink ter ms such as autoconversion/accretion of cloud water into/by drizzle are regr essed using the cloud drop size spectra predicted by an explicit microphysi cal model. The results from the explicit and the new bulk microphysics sche mes are compared for two cases: nondrizzling and heavily drizzling stratocu mulus-topped boundary layers (STBLs). The evolution of the STBL (characteri zed by such parameters as turbulence intensity, drizzle rates, CCN depletio n rates, fractional cloud cover, and drizzle effects on internal stratifica tion) simulated by the bulk microphysical model was in good agreement with the explicit microphysical model.