The NCAR airborne 94-GHz cloud radar: calibration and data processing

Ulrike Romatschke , Michael Dixon, Peisang Tsai, Eric Loew, Jothiram Vivekanandan, Jonathan Emmett, Robert Rilling

The 94-GHz airborne HIAPER Cloud Radar (HCR) has now been deployed in three major field campaigns. NCAR has developed an extensive set of quality assurance and quality control procedures which are applied to all collected data. Engineering measurements performed both in the laboratory and in an antenna measurement chamber yielded calibration characteristics for the antenna, reflector, and radome. These calibration results are applied during flight, to produce the radar moments available in real-time. However, temperature changes in the instrument during flight affect the receiver gains, leading to some bias in the calibration values applied in real time. Post project, we estimate the temperature - induced gain errors and apply gain corrections to improve the quality of the final data set. In addition, the reflectivity calibration is monitored b
comparing sea surface cross section measurements against theoretically-calculated model values. These comparisons confirm that HCR is calibrated to within 1 dB of the theory. A radar echo classification algorithm was developed to identify “cloud echo” and distinguish it from artifacts such as the echo from the surface, transmitter leakage, and a number of other categories. Model reanalysis data and digital terrain elevation data were interpolated to the radar time-range grid of the radar data, to provide an environmental reference. These fields were used for the sea surface calibration and also are made available as an aid for scientific research. The data for the three major field campaigns is available at https://doi.org/10.5065/D6CJ8BV7 (NCAR/EOL Remote Sensing Facility, 2020a, CSET field campaign), https://doi.org/10.5065/D68914PH (NCAR/EOL Remote Sensing Facility, 2020b, SOCRATES campaign), and https://doi.org/10.26023/V9DJ-7T9J-PE0S (NCAR/EOL Remote Sensing Facility, 2020c, OTREC campaign).