A New Cloud Physics Parameterization in a Large-Eddy Simulation Model of Marine Stratocumulus

TLDR

Liquid water in stratocumulus‑topped boundary layers is typically divided into nonprecipitable cloud water and drizzle, following Kessler‑type parameterizations. The study develops a new bulk microphysical parameterization for LES of stratocumulus‑topped boundary layers, using an explicit microphysical model as a data source and benchmark. The new scheme predicts CCN counts, cloud and drizzle mixing ratios, drop concentrations, and integral radius, regresses autoconversion/accretion terms from explicit spectra, and is evaluated against explicit microphysics for nondrizzling and heavily drizzling STBL cases. The bulk microphysical model reproduces the evolution of turbulence, drizzle, CCN depletion, cloud cover, and stratification in STBLs, matching the explicit microphysical model.

Abstract

A new bulk microphysical parameterization for large-eddy simulation (LES) models of the stratocumulus-topped 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.