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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 03 Sep 2019

Submitted as: research article | 03 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Monitoring the differential reflectivity and receiver calibration for the German polarimetric weather radar network

Michael Frech1 and John Hubbert2 Michael Frech and John Hubbert
  • 1Deutscher Wetterdienst, Observatorium Hohenpeißenberg, Albin-Schwaiger-Weg 10, 82383 Hohenpeißenberg
  • 2National Center for Atmospheric Research, Boulder, Colorado

Abstract. It is a challenge to calibrate differential reflectivity ZDR to within 0.1–0.2 dB uncertainty for dual-polarization weather radars that operate operationally 24/7 throughout the year. During operations, a temperature sensitivity of ZDR larger than 0.2 dB over a temperature range of 10°C has been noted. In order to understand the source of the observed ZDR temperature sensitivity, over 2000 dedicated solar box scans, a two dimensional scan 5° azimuth by 8° elevation that encompasses the solar disk, have been made in 2018 from which horizontal (H) and vertical (V) pseudo antenna patterns are calculated. This assessment is carried out using data from the Hohenpeißenberg research radar which is identical to the 17 operational radar systems of the German Meteorological Service (Deutscher Wetterdienst, DWD). ZDR antenna patterns are calculated from the H and V patterns which reveal that the ZDR bias is temperature dependent changing about 0.2 dB over a 12 °C temperature range. One-point calibration results, where a test signal is injected into the antenna crossguide coupler outside the receiver box or into the LNAs, reveal only a very weak temperature sensitivity (< 0.02 dB) for the receiver electronics. Thus, the observed temperature sensitivity is attributed to the antenna assembly. This is in agreement with NCAR's (National Center for Atmospheric Research) S-Pol (S-band polarimetric Radar) system, where the primary ZDR temperature sensitivity is also related to the antenna assembly (Hubbert 2017). Solar power measurements from a Canadian calibration observatory are used to compute the antenna gain, and to validate the results with the operational DWD monitoring results. The derived gain values agree very well with the gain estimate of the antenna manufacturer. The antenna gain shows a quasi-linear dependence with temperature with different slopes for the H and V channels. There is a 0.6 dB decrease in gain for a 10 °C temperature increase, which directly relates to a bias of the radar reflectivity factor Z which has not been not accounted for previously. The operational methods to monitor and calibrate ZDR used for the polarimetric DWD C-band weather radar network are discussed. The prime sources to calibrate and monitor ZDR are birdbath scans, which are executed every 5 minutes, and the analysis of solar spikes that occur during operational scanning. Using an automated ZDR calibration procedure on a diurnal timescale, we are able to keep ZDR bias within the target uncertainty of ±0.1 dB. This is demonstrated for data from the DWD radar network comprising over 87 years of cumulative dual-polarization radar operations.

Michael Frech and John Hubbert
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Michael Frech and John Hubbert
Michael Frech and John Hubbert
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Publications Copernicus
Short summary
To provide continuously well calibrated radar data from an operational dualpolarization weather radar network is a challenge. In this work we investigate the observed temperature sensitivity of radar data of the radar network of the German Meteorological Service. We employ dedicated scans with the sun as a target to investigate this. Based on those data, part of the temperature sensitivity can be attributed to the antenna assembly.
To provide continuously well calibrated radar data from an operational dualpolarization weather...