Retrieval of microphysical properties of water, ice, and mixed-phase clouds using a triple-wavelength radar and microwave radiometer

An algorithm has been developed to retrieve the vertical profiles of cloud microphysical properties from the surface observation based on three radars operated at three microwave bands (X-band, Ka-band, and W-band) and a microwave-radiometer. This algorithm has the advantage that it can be applied to any mixing condition of liquid and ice particles in clouds (i.e., pure-water clouds, pure-ice clouds, and mixed-phase clouds). To precisely treat the scattering processes of the radar waves in clouds, we pay attention to non-sphericity of ice-particles and radar wave attenuation by scattering, which are frequently neglected in the previous studies. The hexagonal ice-particles with various aspect ratios are considered. On the other hand, the observational data used in this algorithm need two assumptions: 1) the observed cloud does not contain precipitating and melting particles, and 2) the water vapor profiles are approximated by successive steady-state representations. The accuracy of the radar calibration required for this algorithm would be within +/- 0.1 dB, which is about the order of uncertainty of radar measurements. In the retrieval procedure, the mixing condition of water-droplets and ice-particles in the target cloud sub-layer can be identified by utilizing the wavelength-dependence of the scattering and attenuation properties. Once the mixing condition of the cloud sub-layer is determined, the microphysical properties of pure-water and pure-ice clouds are estimated by the algorithm, similar to the widely-used dual-wavelength technique, except for the fact that in the present algorithm, an equivalent aspect ratio of ice-particles can be estimated, in addition to ice-water-content and ice-particle size of pure-ice clouds. For mixed-phase clouds, the microphysical profiles, which satisfy the observed radar and microwave-radiometer signals, can be estimated by iteratively changing aspect ratios of ice-particles in each sublayer. The retrieval algorithm is applied to the observational data obtained in the Vertically Pointing Measurements, Tsukuba, 2001 (VPM_TKB01). The retrieved microphysical profiles for the pure-ice clouds show reasonable performance of the present algorithm, although there are no in-situ measurements to validate the estimated values.