Calibration Issues of Dual-Polarization Radar Measurements

Abstract

Techniques for the absolute calibration of radar reflectivity Z and differential reflectivity ZDR measured with dual-polarization weather radars are examined herein. Calibration of Z is based on the idea of self-consistency among Z, ZDR, and the specific differential phase KDP in rain. Extensive spatial and temporal averaging is used to derive the average values of ZDR and KDP for each 1 dB step in Z. Such averaging substantially reduces the standard error of the KDP estimate so the technique can be used for a wide range of rain intensities, including light rain. In this paper, the performance of different consistency relations is analyzed and a new self-consistency methodology is suggested. The proposed scheme substantially reduces the impact of variability in the drop size distribution and raindrop shape on the quality of the Z calibration. The new calibration technique was tested on a large polarimetric dataset obtained during the Joint Polarization Experiment in Oklahoma and yielded an accuracy of Z calibration within 1 dB. Absolute calibration of ZDR is performed using solar measurements at orthogonal polarizations and polarimetric properties of natural targets like light rain and dry aggregated snow that are probed at high elevation angles. Because vertical sounding is prohibited for operational Weather Surveillance Radar-1988 Doppler (WSR-88D) radars because of mechanical constraints, the existing methodology for ZDR calibration is modified for nonzenith elevation angles. It is shown that the required 0.1–0.2-dB accuracy of the ZDR calibration is potentially achievable.

The Green Ocean: Precipitation Insights from the GoAmazon2014/5 Experiment

“…As additional reference for dual-polarization radar processing and natural calibration concepts, selfconsistency relationships between radar quantities K DP , Z, and Z DR for the various wavelengths have been provided in Table 2. In comparison to continental SGP references for statistical DSD relationships (e.g., Ryzhkov et al, 2005), self-consistency coefficients in Table 2 reinforce the tropical character of the Amazon precipitation, again consistent with smaller median drop sizing (e.g., reductions in Z DR or K DP ) to achieve similar estimates of Z.…”

“…This is observed when having a smaller 'a'-coefficient than found for SGP R(Z) relations, and larger 'a'-coefficients than found for SGP R(K DP ) and R(A) relations (as in Figure 2). These changes reflect a significant change when viewed compared to Amazon dataset sampling arguments found 30 in Section 2. For Table 2, R(A) relationships are also listed for multiple temperature assumptions, highlighting one explanation for modest variability when attempting to promote these relations for practical rainfall retrievals (e.g., Ryzhkov et al, 2014;6 Atmos.…”

“…Specifically, Amazon clouds may initiate under clean (or lower) cloud 25 condensation nuclei (CCN) conditions, over a pristine forest, but also experience a range of thermodynamical and aerosol forcing influences that promote changes in cloud properties including electrification, cloud droplet size distribution and precipitation changes, or enhanced updraft intensity (e.g., Williams et al, 2002;Cecchini et al, 2016;Giangrande et al, 2016bGiangrande et al, , 2017. As described by Williams et al (2002), the prevalence of maritime convective cloud regimes over a large continent are possibly still underappreciated in the convective spectrum and its intensity, especially given the propensity to identify deeper 30 convection over the Amazon having electrification arguing continental convective characteristics.…”

State and parameter estimation

“…For operational WSR-88D radars, the reasonable range of the standard deviations of reflectivity and differential reflectivity are 1-2 dBZ and 0.1-0.3 dB, respectively (Ryzhkov et al 2005b;Doviak and Zrnic 1993). The standard error of K DP in the range of 0.24-0.48° km -1 is expected for lightly filtered estimates of K DP from differential phase φ DP for operational WSR-88Ds (Ryzhkov et al 2005b).…”

“…The standard error of K DP in the range of 0.24-0.48° km -1 is expected for lightly filtered estimates of K DP from differential phase φ DP for operational WSR-88Ds (Ryzhkov et al 2005b). The V r error can be assumed to be 1 m s -1 (Doviak and Zrnic 1993 We assume large errors in the observations to account for the worst cases.…”