Parameterization of Cloud Microphysics Based on Full Integral Moments

Abstract

his paper describes a microphysics parameterization based on integral moments of the full drop size distributions (DSDs) as opposed to a partial moments approach (sometimes referred to as Kessler-type parameterization) based on the moments integrated separately over the cloud and rain drop portion of the drop spectrum. This approach does not assume a prescribed form of a DSD but employs as model variables full moments that have clear physical meaning: drop concentration and surface area, water content, precipitation flux, and radar reflectivity. These variables can be directly measured and assimilated into the model forecast cycle without intermediate retrievals. The approach avoids division of DSDs into cloud and rain drops. This eliminates the problem of defining the threshold between these two categories and subdivision of the physical coagulation process into artificial processes of autoconversion, accretion, and self-collection. The development and testing of the parameterization was made using the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) large-eddy simulation (LES) explicit warm rain microphysical model. The conversion and sedimentation rates were parameterized in the form of a product of power functions using nonlinear regression analysis to determine exponents of the approximated expressions. The comparison of bulk and explicit microphysics models demonstrated reasonably good prediction of both thermodynamic and microphysical parameters of the stratocumulus-topped boundary layer (STBL). The weaknesses and problems of the numerical implementation of the full moment approach are also discussed.