Showing papers by "Robert Meneghini published in 1995"

Depolarization of radar signals due to multiple scattering in rain

Abstract: The linear depolarization ratio (LDR) of radar returns from rain is studied. Pulse intensities of copolarized and cross polarized components are calculated by using the second-order solution of the time-dependent radiative transfer equation for a finite rain layer composed of spherical raindrops. Theoretical results show significant differences in LDR values between X and Ka bands for light to moderate rainfall rates and a rapid increase in the LDR reaching a level of about -8.5 dB at radar ranges near the rear edge of the rain layer. These characteristics of the LDR are in good agreement with observations from an air-borne dual-frequency, dual-polarized radar and suggest that a part of the depolarized radar power is caused by second-order multiple scattering effects. >

Cloud Model-Based Simulation of Spaceborne Radar Observations

Abstract: Simulations of observations from potential spaceborne radars are made based on storm structure generated from the three-dimensional (3D) Goddard cumulus ensemble model simulation of an intense overland convective system. Five frequencies of 3, 10, 14, 35, and 95 GHz are discussed, but the Tropical Rainfall Measuring Mission precipitation radar sensor frequency (14 GHz) is the focus of this study. Radar reflectives and their attenuation in various atmospheric conditions are studied in this simulation. With the attenuation from cloud and precipitation in the estimation of reflectivity factor (dBZ), the reflectivities in the lower atmosphere in the convective cores are significantly reduced. With spatial resolution of 4 km X 4 km, attenuation at 14 GHz may cause as large as a 20-dBZ difference between the simulated measurements of the peak, Z(sub mp) and near-surface reflectivity, Z(sub ms) in the most intense convective region. The Z(sub mp) occurs at various altitudes depending on the hydrometeor concentrations and their vertical distribution. Despite the significant attenuation in the intense cores, the presence of the rain maximum is easily detected by using information of Z(sub mp). In the stratiform region, the attenuation is quite limited (usually less than 5 dBZ), and the reduction of reflectivity is mostly related to the actual vertical structure of cloud distribution. Since Z(sub ms) suffers severe attenuation and tends to underestimate surface rainfall intensity in convective regions. Z(sub mp) can be more representative for rainfall retrieval in the lower atmosphere in these regions. In the stratiform region where attenuation is negligible, however, Z(sub mp) tends to overestimate surface rainfall and Z(sub ms) is more appropriate for rainfall retrieval. A hybrid technique using a weight between the two rain intensities is tested and found potentially usefull for future applications. The estimated surface rain-rate map based on this hybrid approach captures many of the details of the cloud model rain field but still slightly underestimates the rain-rate maximum.