Studies of the polarimetric covariance matrix. Part I: Calibration methodology

A procedure for calibration of the radar covariance matrix for the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) radar and S-Band Dual-Polarization Doppler Radar (S-Pol) systems is described. Two relative magnitudes and three offset phases are determined that allow for the calibrated covariance matrix to be constructed. Precise calibration of Z_dr is accomplished with use of only sun calibration measurements and crosspolar power measurements from precipitation. No assumptions about the precipitation medium are made. It is also shown how to determine the co-to-cross phase offsets for the CSU-CHILL radar from precipitation data. A novel method for calculating linear depolarization ratio (LDR) that is effective in low signal-to-noise-ratio regions and that requires no knowledge of the background noise temperature is given. This technique utilizes the cross-to-cross covariances. CSU-CHILL data from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) are used to illustrate the LDR estimator and the Z_dr calibration technique.