Simulations of Radar Bright Band at Multiple Frequencies and Its Comparisons with Airborne Radar Measurements

The melting layer, often observed by the radar as a layer of enhanced radar reflectivity (the so-called radar bright band), is an important meteorological process. An understanding of the microphysical properties of the melting hydrometeors and their electric scattering and propagation effects is of great importance in accurately estimating parameters of the precipitation from spaceborne radar and radiometers, such as TRMM PR and TMI and future GPM DPR and GMI. However, one of the most difficult problems in the study of the radar signature of the melting layer is the determination of the effective dielectric constants of melting hydrometeors. Although a number of mixing formulas are available to compute the effective dielectric constants of dry and melting snow, their results vary to a great extent when the particles are partially melted. Furthermore, it is physically unclear as to how to select among these various formulas. In this study, we first derive the effective dielectric constants of uniformly mixed snow and water particles at X-, Ku-, Ka- and W-bands from their internal electric fields by using a high-resolution computational model in which the particles are precisely described not only by shape but also by particle composition. The stratified-sphere scattering model, a sphere composed of multiple layers, is then employed to compute scattering parameters for non-uniformly melting hydrometeors whose fractional water content is prescribed as a function of the radius of the sphere. In conjunction with a melting layer model that describes the melting fractions and fall velocities of hydrometeors as a function of the distance below the 0C isotherm, the radar bright-band profiles are simulated for air- or space-borne radars operating at X-, Ku-, Ka- and W-bands. These simulated profiles will then be compared with the simultaneous measurements of the bright band made by the NICT (then the Communications Research Lab. of Japan) X- and Ka-band airborne radar during CaPE in 1991, the NASA EDOP and CRS (X- and W-band) airborne radars during the CRYSTAL-FACE in 2002, the NASA JPL dual-frequency airborne Ku and Ka band radar (APR-2) during the Wakasa Bay AMSR-E validation campaign over the Sea of Japan in 2003, and the TRMM Ku-band Precipitation Radar (PR). Thus, the validity of the bright-band models used in the simulations can be effectively checked through these multi-frequency radar measurements.