The Discrete Dipole Approximation for Assessing the Feasibility of Polarimetric 95 Ghz Radar Measurements

Cirrus clouds play an important role in the Earth’s radiation budget. The transfer of radiation through ice cloud layers depends on the particle phase, concentration, size distribution, and the cloud thickness and shape. Consequently, for reliable simulations of present and future climate using General Circulation Models an accurate representation of ice cloud properties is essential. This gives reason for the requirement of systematic measurements of ice clouds parameters. In recent years improvements in millimetre wave technology have allowed the development of radars with sufficient power for the detection of thin clouds and for the penetration of several cloud layers, which is a problem for lidar systems also deployed for cloud remote sensing. A dopplerized and fully polarimetric 95 GHz (3.2 mm) cloud radar is operated at GKSS since 1996. A detailed description of the system can be found in Quante et al (1998). The discrete dipole approximation (DDA), or coupled dipole method, showed its high potential for computing scattering and absorption by a target of arbitrary shape, being especially suited for investigations in the resonance region where the particle size is of the same order as the wavelength. In this paper, the benefit of applying the DDA to obtain polarimetric quantities for 3.2 mm cloud radar is assessed from a theoretical and a technical point of view. The accuracy of the method for computation of radar quantities of atmospheric ice crystals and the technical feasibility of its measurement are evaluated.