Long-term Climatology of Vertical Profiles of Polarimetric Variables and Ice-microphysical Retrievals at X-band. Part I: Radar Calibration

Abstract. In a two-part series of papers, a climatology of quasi-vertical profiles (QVPs) of polarimetric variables and ice-microphysical retrievals such as ice water content, total number concentration and mean volume diameter is presented. QVPs are generated from plan position indicator scans at 18° elevation angle measured with the X-band radar located in the city of Bonn in western Germany between 2013 and 2023. They have been statistically analysed including error analysis with special emphasis on the characteristics of the melting layer and the dendritic growth layer. This long-term climatology improves the understanding of microphysical processes in stratiform cloud regimes and provides a reference for numerical weather prediction modellers to e.g. advance existing microphysical bulk paramerisation schemes.

While part two analyses and discusses the climatology, this first part of the series describes the prior thorough calibration of the radar reflectivity factor (Z_H) and the differential reflectivity (Z_DR). One method uses the relation between Z_H and Z_DR in light rain to calibrate Z_H. Best fits are determined from simulated Z_H and Z_DR values obtained with T-matrix calculations for various temperatures and values of the width of the canting angle distribution using a large disdrometer dataset of drop size distributions measured over Germany (mostly Bonn). Since this Z_H calibration technique strongly depends on the accuracy of Z_DR and encountered deficiencies in the birdbath scan, QVPs in light rain have been used to calibrate Z_DR. Obtained Z_H offset values are validated and compared using both satellite information and self-consistency relationships including specific differential phase (K_DP). The successful calibration of Z_H is confirmed by the root-mean-square error (0.70 dBZ), the mean- absolute error (0.60 dBZ), and the mean-bias (-0.50 dBZ) compared to the offsets obtained from satellite information. Offsets calculated by applying self-consistency relations show larger discrepancies, which favours the suitability of the novel method.