This proposal addresses development of physical validation techniques for the TRMM PR 2A25 algorithm using an advanced dual-wavelength and dual-polarized radar in a sub-tropical coastal location. This radar system is the CP-2 radar which will be operational near Brisbane in November 2006. A long-term engineering and scientific collaboration between the National Center of Atmospheric Research (NCAR; Dr. James Wilson) and the Bureau of Meteorology Research Center (BMRC; Dr. Thomas Keenan) has been established. The combined use of dual-wavelength radars (S and X-bands) with matched 1 degree beams, together with the S-band dual-polarization capability permits for the first time, estimation of X-band path attenuation due to wet ice separately from attenuation due to rain (including supercooled rain). This would enable k-Z relations to be derived in the mixed phase region. Mixed phase hydrometeors pose a challenge for TRMM-based retrievals; there is also uncertainty as to how well the future GPM dual precipitation radar (DPR) will be able to identify mixed phase regions in convection. The vertical structure (and k-Z) assumptions used for convective storms by the 2A25 algorithm can be evaluated using the dual-polarized/dual-wavelength capability of the CP-2. Especially important to validate are strong convective events over land where the vertical structure can depart significantly from that assumed in the 2A25 algorithm. In rainfall, S-band polarimetric (Zdr, Kdp) measurements with an additional X-band path attenuation constraint would enable estimation of the three parameters (No, Do and m) of a gamma drop size distribution (dsd); these can be compared with an inferred set of parameters based on the 2A25 output variables during TRMM overpass events. In particular, the accuracy of the alpha-adjustment procedure in 2A25 which alters the initial No values along each PR beam can be validated, especially for precipitation events over land and ocean. In light stratiform and convective rain where the path integrated attenuation (PIA) values are low, the initial No values inferred from the 2A25 algorithm, if not appropriate for the regime in question, can lead to biases in the rain rate retrieval. The polarimetric radar data can also be used to verify if these initial dsd arameter selection values are appropriate for the regime in question. This component of the CP-2 radar comparison with TRMM overpass data will also assist evaluation (via simulations) of GPM DPR algorithms which are expected to provide more accurate estimates of No and Do parameters of the dsd.
Because the CP-2 radar is an operational radar we will have considerable opportunities to compare with TRMM overpass events over the planned three year period of this project. The combined CP-2 radar and TRMM PR data are also expected to be useful for evaluation of future radar configurations for GPM ground validation sites. The advantage of the CP-2 radar location is that it experiences a wide variety of precipitation events, including oceanic and continental storm types of interest to TRMM. Our principal collaborators will be Dr. Thomas Keenan of the BMRC, and Dr. James Wilson of NCAR. We also plan to collaborate with Prof. Christian Kummerow's team at CSU using their objective rain-type classification methodology in order to place local dsd and vertical attenuation structure information into a global framework.
