An Inter-comparison of S-band and C-band Polarimetric Radar Signatures of Large Hail and Tornado Debris on 2 March 2012

On the 2^nd of March 2012 a powerful storm system produced two significant tornadoes (EF2 and EF3) in the morning hours over northern Alabama. A couple reports of large (1.75 inch diameter) hail were also associated with these tornadic storms. In the afternoon and evening hours of the same day, severe storms produced numerous reports of hail one inch in diameter or larger over northern Alabama and southern Tennessee. Reported hail sizes in these storms ranged from pea size (0.25 inch diameter) all the way up to baseball size (2.75 inch diameter) and softball size (4.25 inch diameter). Four additional EF-0 and EF-1 tornadoes occurred during the afternoon storms.

These severe storms were observed simultaneously by the UAHuntsville Advanced Radar for Meteorological and Operational Research (ARMOR) and the NOAA NWS Hytop (KHTX) Weather Surveillance Radar – 1988 Doppler (WSR-88D). ARMOR is a C-band polarimetric radar that has been in near continuous operation in its current research-quality configuration since 2006. ARMOR data has been available in real-time within the Huntsville (HUN) Weather Forecast Office (WFO) since 2008 and is often used in operations. KHTX, which is an S-band operational weather radar, was recently upgraded to dual-polarization capability in January 2012. The two radars are separated by about 70 km and observed all the tornadoes and most of the hail reports within 125 km range, thus affording an excellent opportunity to document and inter-compare the S- and C-band polarimetric radar signatures of a severe storm outbreak over the southeastern United States. The objectives of this research are to improve our understanding of polarimetric radar signatures of severe weather and their potential uses in operational storm monitoring.

In particular, we investigate similarities and differences in S-band and C-band signatures of large hail and tornado debris and how this information can best be used in the nowcasting process. Prior research, largely conducted at S-band, has shown that large hail is typically characterized by large horizontal radar reflectivity (Z_H) (> 50 dBZ) and near zero differential reflectivity (Z_DR), or the so-called Z_DR hole. More recent research has shown that C-band signatures of melting hail are often characterized by both large Z_H and elevated Z_DR in the range of 3-8 dB, likely due to resonance at C-band. However, it is unclear as to if and under what conditions (e.g., hail and rain particle size distributions including maximum particle size, melting depth, and rain vs. hail fraction) C-band and S-band hail signatures are ever similar. Even less is known about the relative similarities and differences in polarimetric radar tornado debris signatures (TDS) at C-band and S-band. Finally, given two simultaneous views of severe weather at different ranges, we also explore the potential effect of range (and hence height and beam width) on the nature and utility of polarimetric radar signatures in the operational storm monitoring process.