A melting-layer model for passive/active microwave remote sensing applications. Part I: Model formulation and comparison with observations

In this study, a 1D steady-state microphysical model that describes the ver tical distribution of melting precipitation particles is developed. The mod el is driven by the ice-phase precipitation distributions just above the fr eezing level at applicable grid points of "parent'' 3D cloud-resolving mode l (CRM) simulations. It extends these simulations by providing the number d ensity and meltwater fraction of each particle in finely separated size cat egories through the melting layer. The depth of the modeled melting layer i s primarily determined by the initial material density of the ice-phase pre cipitation. The radiative properties of melting precipitation at microwave frequencies are calculated based upon different methods for describing the dielectric properties of mixed-phase particles. Particle absorption and sca ttering efficiencies at the Tropical Rainfall Measuring Mission Microwave I mager frequencies (10.65-85.5 GHz) are enhanced greatly for relatively smal l (similar to0.1) meltwater fractions. The relatively large number of parti ally melted particles just below the freezing level in stratiform regions l eads to significant microwave absorption, well exceeding the absorption by rain at the base of the melting layer. Calculated precipitation backscatter efficiencies at the precipitation radar frequency (13.8 GHz) increase with particle meltwater fraction, leading to a "bright band'' of enhanced radar reflectivities in agreement with previous studies. The radiative propertie s of the melting layer are determined by the choice of dielectric models an d the initial water contents and material densities of the "seeding'' ice-p hase precipitation particles. Simulated melting-layer profiles based upon s now described by the Fabry-Szyrmer core-shell dielectric model and graupel described by the Maxwell-Garnett water matrix dielectric model lead to reas onable agreement with radar-derived melting-layer optical depth distributio ns. Moreover, control profiles that do not contain mixed-phase precipitatio n particles yield optical depths that are systematically lower than those o bserved. Therefore, the use of the melting-layer model to extend 3D CRM sim ulations is likely justified, at least until more-realistic spectral method s for describing melting precipitation in high-resolution, 3D CRMs are impl emented.