A simulator for polarized active sensors: validation and examples of multiple scattering effects

A numerical model based on the Monte Carlo solution of the vector radiative transfer equation has been developed to simulate active sensor signals. The model accounts for general radar/lidar configurations such as airborne, space-borne and ground-based, monostatic or bistatic, and includes the polarization and the antenna pattern as particularly relevant features. The model has now been validated with analytical solutions and with results derived from other numerical methods (e.g. a second order time independent vector theory, other Monte Carlo solutions with no polarization in action) in a wide variety of meteorological conditions and spectral regions (visible and microwave). Except for contributions due to the backscattering enhancement, the model is particularly suited for evaluating multiple scattering effects both in lidar and in radar systems. Multiple scattering effects in co- and cross-polar radar returns at 35 and 94 GHz are evaluated for realistic vertically inhomogeneous scenarios involving rainfall, snow, graupel, and ice crystals extracted from cloud resolving model simulations corresponding to frontal and squall line systems. Results show that the multiple scattering enhancement (MSE) is generally connected to areas of high attenuation. MSE becomes a real issue for space-borne Ka band radars for medium to heavy precipitation, but for W band radars MSE becomes already noticeable in the presence of light precipitation. MSE can reach several tens of dB when heavy cold rain systems are considered, i.e. when the profiles include rain layers with a high density of dense ice particles aloft. When the cross-polar returns are analyzed, high LDR values also appear in the regions of multiple scattering. Thus the measurement of the cross polarized signal - if available - can enormously help to detect areas potentially affected by MSE. As a result multiple scattering effects have to be accounted for in Cloudsat applications like rainfall and snowfall retrievals and in GPM retrievals involving medium to high rain rates originating from multiphase processes. The possibility of exploiting the polarization signal in order to better understand multiple scattering effects in lidar systems is discussed as well on the basis of example calculations for optically thick water and ice clouds.