Zdr calibration and simultaneous horizontal and vertical polarization transmit radars

For quantitative precipitation estimates it is desirable to calibrate Zdr to 0.1 dB. There are two end-to-end techniques for calibration of Zdr that can achieve such accuracy: vertical pointing (VP) data in light rain and 2) the crosspolar power (CP) technique. For a particular radar location, the opportunities to execute the VP technique may be limited. The crosspolar power technique, which relies up on the principle of reciprocity, uses a) solar measurements and b) crosspolar power measurements from any reciprocal targets. For radars that employ fast alternating horizontal (H) and vertical (V) polarized pulse transmission, precipitation measurements can be used for the required crosspolar power measurements. For dual-polarization radars that employ simultaneous H and V transmission, precipitation for the crosspolar power measurements can no longer be used because of the time lag between the transmit only H and transmit only V crosspolar powers. Thus, more time stable targets must be used, i.e., ground clutter. The CP technique is attractive since most radar sites will have suitable ground clutter targets and thus the CP technique for Zdr calibration can be executed whenever the sun is visible by the radar. This paper examines the practical, operational considerations for using the CP technique. Data from S-Pol, (NCAR's (National Center for Atmospheric Research) S-band research radar, CSU-CHIL (Colorado State University's S-band research radar), and KOUN, (NSSL's (National Severe Storm Laboratory)) S-band research radar are used to illustrate the Zdr calibration technique that may be used on the NEXRADs.