On modeling air/spaceborne Radar returns in the melting layer

The bright band is the enhanced radar echo associated with the melting of hydrometeors in stratiform rain. To simulate this radar signature, a scattering model of melting snow is proposed in which the fractional water content is prescribed as a function of the radius of a spherical mixed-phase particle consisting of air, ice, and water. The model is based on the observation that melting starts at the surface of the particle and then gradually develops toward the center. To compute the scattering parameters of a nonuniform melting particle, the particle is modeled as a sphere represented by a collection of 64³ cubic cells of identical size where the probability of water at any cell is prescribed as a function of the radius. The internal field of the particle, used for deriving the effective dielectric constant, is computed by the conjugate gradient and fast Fourier transform (CGFFT) numerical methods. To make computations of the scattering parameters more efficient, a multilayer stratified-sphere scattering model is introduced after demonstrating that the scattering parameters of the nonuniformly melting particle can be accurately reproduced by the stratified sphere. In conjunction with a melting layer model that describes the melting fractions and fall velocities of hydrometeors as a function of the distance from the 0°C isotherm, the stratified-sphere model is used to simulate the radar bright-band profiles. These simulated profiles are shown to compare well with measurements from the Precipitation Radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite and a dual-wavelength airborne radar. The results suggest that the proposed model of a melting snow particle may be useful in studying the characteristics of the bright-band in particular and mixed-phase hydrometeors in general.