One of the challenges of the Global Precipitation Measuring Mission is the need to develop robust algorithms to analyze data from the spaceborne dual-wavelength precipitation radar (DPR). Dual-wavelength radar data offer the potential for improved estimates of rain rate as well as the retrieval of parameters of the drop size distribution in rain and snow. Aside from instrument-related errors such as calibration accuracy and beam alignment, the problem of attenuation correction is perhaps the most critical. Many of the dual-wavelength retrieval methods that have been proposed can be classified according to the way in which the attenuation is corrected. A basic distinction among methods is forward- versus backward-going retrievals. In the backward techniques, a path-integrated attenuation is required and the solution proceeds from the surface toward the radar. In the forward approach, the solution proceeds from the storm top toward the surface and dispenses with the need for path attenuation. Among the backward methods is the integral-equation approach in which the attenuation is expressed in terms of the drop size distribution parameters obtained at prior range gates. Numerically, this procedure takes the form of a pair of recursion relations for two parameters of the size distribution. An alternative to the integral-equation formulation is the use of the specific attenuation - radar reflectivity factor relationship or k-Z relation. This approach can be interpreted as the dual-wavelength analogue to the rain rate retrieval algorithm for the TRMM Precipitation Radar. Because of the similarity between the spaceborne dual-wavelength and ground-based dual-polarimetric formulations, the study may lead to a better understanding of methods used for both types of radars. The objective of the proposal is to study the nature of the different approaches to solving the dual-wavelength equations in the presence of error sources such as mixed phase hydrometeors, cloud liquid water, and water vapor as well as errors in path attenuation, calibration and beam misalignment. Cloud resolving models and airborne multi-parameter data sets can be used to develop and test spaceborne algorithms and help assess the capabilities and deficiencies of the GPM dual-wavelength weather radar.
