Extended Abstract (652K)J1.6
High-resolution rainfall rate and DSD estimation from X-band polarimetric radar measurements
Marios Anagnostou, University of Connecticut, Storrs, CT; and E. N. Anagnostou and W. F. Krajewski
The paper presents a rainfall estimation technique based on algorithms that couple, along-a-radar-ray, profiles of horizontal polarization reflectivity (ZH), differential reflectivity (ZDR), and differential propagation phase shift (FDP) from X-band polarimetric radar measurements. Based on in-situ raindrop size distribution (DSD) data and using a three-parameter “normalized” gamma DSD model, relationships are derived that correct X-band reflectivity profiles for specific and differential attenuation, while simultaneously retrieving variations of the “normalized” intercept DSD parameter (Nw). The algorithm employs an iterative scheme to intrinsically account for raindrop oblateness variations from equilibrium condition. The study is facilitated from a field experiment conducted in the period October to November 2001 in Iowa City, Iowa, where we collected observations from X-band dual-polarization mobile radar (XPOL) simultaneously with high-resolution in-situ disdrometer and rain gauge rainfall measurements. The observed rainfall events ranged in intensity from moderate stratiform precipitation to high intensity (>50 mm/h) convective rain cells. The XPOL measurements were tested for calibration, noise, and physical consistency using corresponding radar parameters derived from coincidentally measured raindrop spectra. Retrievals of Nw from the attenuation correction scheme are shown to be unbiased and consistent with Nw values calculated from independent raindrop spectra. The attenuation correction based only on profiles of reflectivity measurements is shown to diverge significantly from the corresponding polarimetric-based corrections. Several rain retrieval algorithms were investigated using matched pairs of instantaneous high-resolution XPOL observations with rain rates from three-minute averaged raindrop spectra at close range (~5 km), and minute rain rate gauge measurements from further ranges (~10 km). It is shown that combining along-a-ray, reflectivity attenuation and corresponding Nw retrievals with corrected ZDR values we get the best performance in rainfall estimation with about 40% (62%) relative standard deviation in the radar-disdrometer (radar-gauge) differences. The case tuned reflectivity-rainfall rate (Z-R) relationship gives about 65% (88%) relative standard deviation for the same differences. The systematic error is shown to be low (~3% overestimation) and nearly independent of rainfall intensity for the multi-parameter algorithm, while for the standard Z-R it varied from 10% underestimation to 3% overestimation.
Joint Session 1, Flash Flood Forecasting in the Urban Environment (Joint with the Symposium on Planning, Nowcasting and Forecasting in the Urban Zone and 18th Conf on Hydrology; Room 6E)
Monday, 12 January 2004, 1:30 PM-5:30 PM, Room 6E
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