Retrieval of hydrometeor distributions over the ocean from airborne single-frequency radar and multi-frequency radiometric measurements

An algorithm was developed to retrieve vertical profiles of hydrometeor mass, and hydrometeor particle density and volumetric weighted diameters, derived from simultaneous airborne nadir directed 10 GHz radar measurements and 10, 19, 34 and 92 GHz radiometric measurements. The retrieval algorithm used a three layer model, i.e. frozen hydrometeors in the upper layer, melting hydrometeors below the freezing level in the middle layer, called the melting layer, and rain in the lower layer. In the lower layer, the algorithm used a modification of the Hitschfeld-Bordan radar equation. A cloud physics model was used to describe the melting process of the hydrometeors in the melting layer. The dielectric constant of the frozen and melting hydrometeors was represented by a Maxwell-Garnet mixture of ice, air and water. In the upper layer, the frozen hydrometeors were assumed to have a particle size distribution (PSD) with the same small-size parameter, μ, as the rain drops in the lower layer. The PSD of the hydrometeors in the middle layer was determined by assuming continuity of vertical mass flux of the melting particles. Mie scattering computations and a finite cloud radiative transfer model were used in an optimizing procedure, based on Marquardt's non-linear least squares minimization, to match the model derived profile of the radar reflectivity factor, and brightness temperatures with measurements obtained on September 17, 1990 near the eye of typhoon Flo in the western Pacific. The algorithm yielded profiles of the mass density of the precipitation, the particle density and the volumetrically weighted diameter (VWD) of the PSD. The model was validated by comparing the measured profile of the 35 GHz effective radar reflectivity factor and the 192 GHz brightness temperature with their values computed using the hydrometeor parameters obtained from the retrieval algorithm.