Recorded presentation13B.5
Evaluation of polarimetric X-band radar rainfall estimators
Sergey Y. Matrosov, CIRES/University of Colorado and NOAA/ESRL, Boulder, CO
Different relations between rainfall rate, R, and polarimetric X-band radar parameters are evaluated using the radar, disdrometer and rain gauge measurements conducted during the four-month long field experiment in California. Compared to the power based relations, the differential phase shift (KDP) based estimators show less scatter due to variability in raindrop size distributions. These estimators, however, depend on the model assumptions about drop aspect ratio and are not applicable to lighter rainfalls. The polynomial model for the mean drop aspect ratio provides R-KDP relations that overall result in good agreement between the radar retrievals of rain accumulations and accumulation estimates from surface based rain gauges. The accumulation data obtained from power estimators that use the reflectivity (Z) and differential reflectivity (ZDR) measurements generally exhibit greater standard deviations with respect to the gauge measurements. Unlike the phase based estimators, the power based estimators have an advantage of being “point” measurements thus providing contentious QPE for the whole area of the radar coverage. The uncertainty in the raindrop shape model can result in errors in the attenuation (for Z) and differential attenuation (for ZDR) correction procedures. These errors can be on the order of measurement uncertainties and might provide biases of radar QPE for estimators that use power measurements. The phase based estimates are immune to these errors. Overall, for all estimators, the radar based rainfall accumulations showed biases that are less than 10% (relative to gauges). The standard deviations of radar retrievals were about 23% for the mean Z-R relation, 16%-22% for the KDP based estimators (depending on the drop shape model) and about 20%-32% for different Z-ZDR estimators. Comparisons of ZDR based retrievals of mean mass drop size that are grater than 1 mm with disdrometer derived values reveal an about 20-25% relative standard deviation between these two types of estimates.
Session 13B, Polarimetric Radar III
Friday, 9 October 2009, 8:30 AM-10:00 AM, Room 18
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