Parameterization of drop effective radius for drizzling marine stratocumulus

Abstract

We describe a parameterization of cloud drop effective radius Re developed for drizzling marine stratocumulus clouds. The parameterization was derived using data from the CIMMS LES model with explicit formulation of drop size resolving microphysics. We showed that in the case of moderate drizzle an accurate parameterization of Re in the 8 ÷ 20 μm radius range can be obtained on the basis of two microphysical variables: liquid water content and drop concentration. For heavy drizzle, only a three‐variable parameterization proves to be sufficiently accurate. We developed two versions of the parameterization. The first one is based on traditional cloud microphysical prognostic variables representing partial moments of the drop distribution function: cloud water content, drizzle water content, and cloud drop concentration. The second one is based on the full moments of the drop size distribution function: drop concentration, total liquid water content, and radar reflectivity. The first version of the parameterization is rather sensitive to the value of the threshold radius that divides cloud and drizzle water. As a result, the errors are larger than in the second full moment version. It is concluded that a three‐variable full moment parameterization provides the most accurate representation of effective radius in precipitating stratocumulus cloud layers.