Calibration transfer methodology for cloud radars based on ice cloud observations

This article presents a calibration transfer methodology between radars of the same and different frequency bands. This method enables the absolute calibration of a meteorological radar by transferring it from another co-located instrument with known calibration, by simultaneously measuring vertical cloud reflectivity profiles. The advantage is that the added uncertainty in the newly calibrated instrument can reach the magnitude of the reference instrument calibration. This is achieved by carefully selecting comparable data, including the identification of the reflectivity range that avoids the disparities introduced by differences in sensitivity or scattering regime. The result is a correction coefficient used to compensate measurement bias in the uncalibrated instrument. Calibration transfer uncertainty can be reduced by increasing the number of sampling periods. The methodology was applied between co-located W-band radars deployed during the ICE-GENESIS campaign (Switzerland 2020-2021). A difference of 2.2 dB was found in their reflectivity measurements, with an uncertainty of 0.7 dB. The calibration transfer was also applied to radars of different frequency, an X-band radar with unknown calibration and aW-band radar with manufacturer calibration, the difference found was -16.7 dB with an uncertainty of 1.2 dB. The method was validated through closure, by transferring calibration between three different radars in two different case studies. For the first case, involving three W-band radars, the bias found was of 0.2 dB. In the second case, involving two W-band and one X-band radar, the bias found was of 0.3 dB. These results imply that the biases introduced by performing the calibration transfer with this method are negligible.