Modeling of the melting layer. Part I : Dynamics and microphysics

To obtain the full description of the dynamical and microphysical finescale structures required for the computation of the radar-derived brightband parameters, a numerical model has been developed. A bulk microphysics module was introduced into a nonhydrostatic, fully compressible dynamic framework. A microphysical parameterization scheme, with five water categories (vapor, cloud water, snow, melting snow, and rain), describes the interactions related to the evolution of the melting layer (melting and diffusional exchanges of mass of each hydrometeor category). Dynamic, thermodynamic, and microphysical processes are fully coupled. The main characteristics of the bulk parameterization scheme for melting of snow are the following: 1) wet snow is described by its water content and by an additional prognostic variable, namely, the diameter of the smallest snowflake not yet completely melted; 2) the fall velocity of the melting snowflakes is based on the laboratory observations; and 3) a size-dependent ventilation coefficient of the melting particles is used. With this new formulation of the melting process some approximate analytical relations between variables that characterize the melting layer are obtained. The results of simulations show that the nonuniformity of the snow content causes horizontal variability of various atmospheric properties within the melting layer, which leads to the generation of convective cells. The impact of the induced finescale dynamics on the microphysical structure within the melting zone is analyzed.