Tuesday, 25 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
Inter-comparison of reflectivity measurements between the precipitation radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite and ground-based validation radar should be made based on common observation volumes to reduce the variation due to the significant difference in size of the resolution volumes of the two radars. This inter-comparison needs be done carefully because the differences of calibration of the two radars may be non-ignorable. This paper focuses on the study of relative bias estimation between the two radars. The reflectivity measurements from the two types of platforms are subject to be different due to differences of frequencies of the radars, differences in viewing aspects, and probably mismatching of the common observation volumes. Accurate bias estimation can be further complicated by geometric distortion introduced into the satellite data (e.g., shift, scale, shear, rotation, etc.), which is caused by the movements and attitude perturbations of the satellite itself. Attitude motions such as roll, pitch, and yaw introduce variability into the PR measurements that are not systematic in nature creating spatial alignment error between PR and GR measurements. Changes in the satellite altitude and velocity, due to orbit eccentricity, can also produce geometric distortion. For accurate estimations of reflectivity biases between PR and ground radars, the selection and preparation of the data needs be made under consideration of the influences of these facts. This paper first examined the relationship of the reflectivities at the two frequencies of PR and GR for typical hydrometeor particles, namely ice particle and raindrops. Based on the relationships, a synthetic reflectivity data set at PR frequency (Ku band) is obtained using high-resolution S band measurements. The effects of several facts that have influence on the bias estimation were studied, namely, the PR sampling incident angles, the non-simultaneity of sampling between the two radars, the residual errors of Ku band attenuation correction, and reflectivity gradient within the PR resolution volume. The variabilities of the biases estimation from different methods are examined also. Measures are suggested to ensure either the differences caused by these facts are eliminated or effects on the bias estimation are reduced to be satisfactory.
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