Cirrus Cloud Simulation Using Explicit Microphysics and Radiation. Part I: Model Description

A mesoscale 2D/3D cloud model complex with explicit account for the water and ice cloud microphysics and radiative processes is described. The model has several versions suitable for the simulation of various cloud types, including, of particular concern in the current version, cirrus clouds. Model computations are based on the two kinetic equations for droplet and crystal size distribution functions, with division of droplet and crystal size spectra into 30 bins from 1 μm to 3.5 mm, and with account for the various mechanisms of cloud condensation and ice nuclei activation, condensation–deposition, and coalescence–accretion growth. These equations are solved along with supersaturation and radiative transfer equations for longwave and solar radiation. Simple yet accurate analytical expressions are presented for the scattering and absorption coefficients, and the single-scattering albedo. This allows detailed calculations of the optical and radiative characteristics of clouds (i.e., fluxes, divergence, and albedo) to be made at each grid point of the computational domain, thus yielding the spatial and time evolution of these properties along with the evolving cloud microstructure and phase state. Results of a simulation for a typical midlatitude cirrus cloud using the current model are presented in Part II of this paper.