275 Calibration of System Bias in ZH and ZDR of S-band Dual-Polarization Radar

Thursday, 31 August 2017
Zurich DEFG (Swissotel Chicago)
Hae Lim Kim, Weather Radar Center, KMA, Seoul, Korea, Republic of (South); and S. H. Jung, S. A. Jung, and S. K. Lee

Handout (2.0 MB)

Radar measurements suffered from the mis-calibration of radar system, this error leads to significant uncertainty in radar-based quantitative precipitation estimation (QPE) as well as hydrometeor classification. Thus, absolute calibration of reflectivity (ZH) and differential reflectivity (ZDR) is essential for producing high quality weather radar data, particularly for rainfall measurement.
In this study, we analyzed long-term variability of calibration bias in ZH and ZDR measurements from S-band dual-polarization radar at Yong-In Testbed (YIT) to product stable and accurate system bias of radar. Two ZH biases were derived based on the self-consistency principle between ZH and specific differential phase (KDP) and the direct comparison with simulated ZH from drop size distribution (DSD) measurement of two-dimensional video disdrometer (2DVD), respectively. ZDR biases were calculated by using three approaches based on empirical relationship between ZH and ZDR, vertical pointing measurements, direct comparison with simulated ZDR from DSD.
We examined calibration bias of ZH and ZDR according to different calibration methods during the period from May 2015 to October 2016. As a result, mean values of ZH biases based on self-consistency principle and 2DVD measurements were -3.28 and -2.72 dB, respectively. Although, ZH-KDP self-consistency calibration method is stable, the number of data used for calculation differed according to rainfall cases. In addition, ZH calibration bias calculated from the 2DVD is affected by the precipitation system that passes over the 2DVD and shows more variability compared to ZH-KDP self-consistency method.
The mean values of ZDR biases by ZH-ZDR relationship, vertical pointing measurement, and simulated ZDR from DSD were -0.03 dB, -0.05, and -0.11 dB, respectively. ZH-ZDR relationship method shows the smallest bias value compared to other methods. For the use of vertical pointing measurements, ZDR bias can be calculated only when the rainfall system passes over the radar site and depends on the selection of rain regime under the melting layer. However, this method yields a stable bias during the precipitation passes over the radar site.
 Temporal trend of ZH and ZDR biases are well matched with each other. However, the standard deviation of both ZH and ZDR biases obtained from 2DVD measurements were relatively larger than other methods (not shown), due to DSD variability in vertical, drop sorting, under-sampling problem of 2DVD measurements. 

The research is supported by “Development and application of cross governmental dual-pol radar harmonization (WRC-2013-A-1)” project of the Weather Radar Center, Korea Meteorological Administration.

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