The use of TRMM precipitation radar observations in determining ground radar calibration biases

Since the successful launch of the Tropical Rainfall Measuring Mission (TRM M) satellite, measurements of a wide variety of precipitating systems have been obtained with unprecedented detail from the first space-based radar [p recipitation radar (PR)]. In this research, a methodology is developed that matches coincident PR and ground-based volume scanning weather radar obser vations in a common earth parallel three-dimensional Cartesian grid. The da ta matching is performed in a way that minimizes uncertainties associated w ith the type of weather seen by the radars, grid resolution, and difference s in radar sensitivities, sampling volumes, viewing angles, and radar frequ encies. The authors present comparisons of reflectivity observations from t he PR and several U.S. weather surveillance Doppler radars (WSR-88D) as wel l as research radars from the TRMM field campaigns in Kwajalein Atoll and t he Large Biosphere Atmospheric (LBA) Experiment. Correlation values above 0 .8 are determined between PR and ground radar matched data for levels above the zero isotherm. The reflectivity difference statistics derived from the matched data reveal radar systems with systematic differences ranging from +2 to -7 dB. The authors argue that the main candidate for systematic diff erences exceeding 1 to 1.5 dB is the ground radar system calibration bias. To verify this argument, the authors used PR comparisons against well-calib rated ground-based systems, which showed systematic differences consistentl y less than 1.5 dB. Temporal analysis of the PR versus ground radar systema tic differences reveals radar sites with up to 4.5-dB bias changes within p eriods of two to six months. Similar evaluation of the PR systematic differ ence against stable ground radar systems shows bias fluctuations of less th an 0.8 dB. It is also shown that bias adjustment derived from the methodolo gy can have significant impact on the hydrologic applications of ground-bas ed radar measurements. The proposed scheme can be a useful tool for the sys tematic monitoring of ground radar biases and the studying of its effect.