Maritime precipitation distributions determined from airborne radar and multi-frequency radiometric measurements

The Tropical Rainfall Measuring Mission, TRMM, satellite will carry the first rain measuring radar in space with microwave radiometers. To support the development of the TRMM, measurements from nadir viewing airborne radars and microwave radiometers were analyzed to develop algorithms to retrieve precipitation mass distributions. The present retrieval algorithm used a three layer model. In the lower layer, the algorithm used a modification of the Hitschfeld-Bordan radar equation to find the rain distribution. A cloud physics model was used to describe the melting process of the hydrometeors in the middle layer. The refractive indices of the melting and frozen hydrometeors were derived from Maxwell-Garnet theory applied to air and ice in the upper layer and air, ice and water mixtures in the melting layer. Spherical raindrops, spheres consisting of ice and air to represent graupel and hail, disks containing ice and air to represent snow dendrites and ice cylinders to represent needles at the cloud tops were assumed in the scattering computations. A backward Monte-Carlo radiative transfer model was applied to the horizontally and vertically inhomogeneous model clouds to compute the brightness temperatures that emerged from these clouds. The distributions of computed 10 GHz radar reflectivity factors, and brightness temperatures were optimized to the corresponding measurements by applying Marquardt´s non-linear least squares minimization to precipitation model parameters. The algorithm yielded the distribution of the precipitation mass density along a flight path that passed over Typhoon Flo in 1990. The resulting model was validated by comparing the measured and the modeled distributions of the 35 GHz effective radar reflectivity factors and modeled and measured 192 GHz brightness temperatures