Getting to Know Dual-Pol: A Case Study Comparison of C-Band and S-Band Dual-Polarization Radars in Northern Alabama

Recently, the National Weather Service (NWS) upgraded its Doppler radar network to include dual-polarization technology. The Hytop, AL Doppler radar (10-cm) was upgraded in early January 2012, shortly before the March 2, 2012 severe weather event. This event was a hard hitting day of severe weather for northern Alabama, producing multiple tornadoes and ample hail ranging from pea sized to baseball sized. Therefore, this event was selected as a case study to compare the upgraded Hytop radar with an established C-band (5-cm) research radar, the University of Alabama in Huntsville's ARMOR (Advanced Radar for Meteorological and Operational Research). Earlier studies have been performed comparing the two radar types in central Oklahoma, but Alabama weather patterns, terrain, and radar placement differ greatly from Oklahoma. The goal of this research project was to see if the central Oklahoma results apply to northern Alabama as well. Using the Hytop and ARMOR radars, reflectivity (Z), differential reflectivity (ZDR), correlation coefficient (CC), and specific differential phase (KDP) values were collected and recorded for six different storms from March 2nd. These six storms were chosen based on storm reports indicating hail was present in each storm. Values of each parameter were recorded for four different layers of the atmosphere often used for hail detection: lowest level, 0° C, -20° C, and -30° C. Preliminary results show that the largest difference exists in the ZDR values, with Hytop values lower in all tilts than ARMOR. These results are consistent with both the Oklahoma study, and with another local study performed with ARMOR. The results also show minor differences in reflectivity and correlation coefficient, again similar to previous studies performed with ARMOR. All of the results of this study can help forecast meteorologists better understand dual-polarization parameters and distinguish what to look for on the radar when dealing with severe hail.