Simulated Dual-Frequency Ratio Measurements in Heavy Rain

Information on the rain and snow size distributions (DSD) is one of the objectives of National Aeronautics and Space Administration's Global Precipitation Measurement (GPM) Mission. The dual-frequency precipitation radar (DPR) on board GPM core satellite is the primary resource for the retrieval of DSD. The DPR operates at Ku- and Ka-band and these frequencies have different sensitivities to the precipitation. Both frequencies are subject to attenuation but to different degrees. The three ‘channels' of the DPR operate in three somewhat different regimes. The high sensitivity Ka-band channel is used to detect solid and/or light precipitation, while the standard Ka-band channel, with better range resolution but poorer sensitivity, is spatially aligned with the Ku-band channel to measure relatively higher intensity precipitation. From the simultaneous measurements of Ku- and Ka-band in the inner swath, the objective is to measure rain and snow parameters more accurately, in part, by estimating parameters of the DSD. The DSD retrieval algorithm uses a three-parameter gamma distribution where the mass weighted diameter (Dmass), normalized intercept parameter with respect to the liquid water content, and the shape parameter are the unknown parameters. A key problem is whether an adequate representation of the DSD can be obtained with only two independent measurements. This study seeks a relationship between the dual frequency ratio (DFR) and Dmass in heavy rain. The disdrometer observations from recent GPM ground validation field campaigns are used to simulate DFR and to determine the uncertainty in the derived DSD parameters.