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A Dual-Wavelength Polarimetric Analysis of the 16 May 2010 Oklahoma City Extreme Hailstorm J University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Publications, Agencies and Staff of the .SU . U.S. Department of Commerce Department of Commerce 4-2012 A Dual-Wavelength Polarimetric Analysis of the 16 May 2010 Oklahoma City Extreme Hailstorm J. PICCA University of Oklahoma Norman Campus, [email protected] A. RYZHKOV University of Oklahoma Norman Campus Follow this and additional works at: http://digitalcommons.unl.edu/usdeptcommercepub PICCA, J. and RYZHKOV, A., "A Dual-Wavelength Polarimetric Analysis of the 16 May 2010 Oklahoma City Extreme Hailstorm" (2012). Publications, Agencies and Staff of ht e U.S. Department of Commerce. 513. http://digitalcommons.unl.edu/usdeptcommercepub/513 This Article is brought to you for free and open access by the U.S. Department of Commerce at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Publications, Agencies and Staff of the .SU . Department of Commerce by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. APRIL 2012 P I C C A A N D R Y Z H K O V 1385 A Dual-Wavelength Polarimetric Analysis of the 16 May 2010 Oklahoma City Extreme Hailstorm J. PICCA AND A. RYZHKOV Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, and NOAA/OAR National Severe Storms Laboratory, Norman, Oklahoma (Manuscript received 3 May 2011, in final form 18 October 2011) ABSTRACT A comparative analysis of a supercell hailstorm using simultaneous observations with S-band and C-band polarimetric radars supported by abundant ground-truth reports is presented in this study. The storm oc- curred on 16 May 2010 and produced a swath of extremely damaging hail across a large portion of the Oklahoma City, Oklahoma, metro area. Hail sizes over 10 cm in diameter and hail drifts upward of 1.5 m in height were reported. Both S-band (KOUN) and C-band [University of Oklahoma Polarimetric Radar for Innovations in Meteorology and Engineering (OU-PRIME)] polarimetric radars in Norman, Oklahoma, sampled the storm at ranges less than 60 km, so that high-resolution dual-wavelength polarimetric data were obtained. At C band, this analysis mostly presents raw Z and ZDR (due to problems with differential phase resulting from an incorrect censoring threshold in the examined case) while taking into account the possibility of attenuation in the interpretation of these data. Among the issues investigated in the study are the relation of hail size measured at the surface to the polarimetric signatures at both wavelengths, the difference between polarimetric signatures at the two wavelengths of hail aloft and near the surface (where melting hail is mixed with rain), and the three-body scatter spike (TBSS) signature associated with large hail. 1. Introduction significant modification for applications at C band pri- marily because Z of small- and medium-size melting There is growing interest in hail studies, stimulated by DR hail at C band is greater than at S band (Ryzhkov et al. the pending introduction of operational dual-polarization 2007; Tabary et al. 2010; Anderson et al. 2011). Typi- radars that measure differential reflectivity Z ,dif- DR cally, wet hail is mixed with rain, and anomalously high ferential phase V , and cross-correlation coefficient DP Z due to resonance scattering associated with large r between the signals at orthogonal polarizations, in DR hv raindrops and small melting hail at C band offsets the addition to the conventional radar reflectivity factor Z low intrinsic Z of moderate–large hail. (Bringi and Chandrasekar 2001). The Z of hailstones DR DR The modeling studies of Ryzhkov et al. (2009, 2011) is lower than that of raindrops with similar reflectivity and Kumjian et al. (2010a) support this interpretation and because of their tumbling during descent and their gen- show that Z of melting hail is very sensitive to radar erally lower dielectric constant. The same factors are DR wavelength. The first direct comparisons of polarimetric responsible for lower specific differential phase K and DP hail signatures observed by closely located S- and C-band r in hail compared to values in rain. hv radars (Borowska et al. 2011; Gu et al. 2011) are generally Polarimetric algorithms for hydrometeor classification, consistent with results of these theoretical simulations. which utilize a combination of Z, Z , K , and r , DR DP hv Another important feature that must be considered for demonstrate good skill for hail detection at S band as hail detection using shorter wavelengths is that the radar limited validation studies show (e.g., Heinselman and reflectivity of hail at these wavelengths may be signifi- Ryzhkov 2006; Depue et al. 2007). Polarimetric hail de- cantly lower than at S band. The corresponding differ- tection algorithms originally developed for S band need ence has been termed the ‘‘hail signal’’ in previous studies (Atlas and Ludlam 1961; Eccles and Atlas 1973; Bringi Corresponding author address: J. Picca, National Weather Cen- et al. 1986; Feral et al. 2003). ter, Suite 2100, 120 David L. Boren Blvd., Norman, OK 73072. Determination of hail size remains challenging. The E-mail: [email protected] most recent version of the hydrometeor classification DOI: 10.1175/MWR-D-11-00112.1 Ó 2012 American Meteorological Society 1386 MONTHLY WEATHER REVIEW VOLUME 140 algorithm (HCA) developed at the National Severe model simulations of melting hail. They computed radar Storms Laboratory for polarimetrically upgraded Weather scattering characteristics of a rain–hail mixture for out- Surveillance Radar-1988 Doppler (WSR-88D) radars puts of 1D and 2D cloud models with spectral (bin) detects ‘‘rain mixed with hail’’ (Park et al. 2009) and microphysics. Indeed, these studies indicate that large- does not distinguish between large and small hail. The hail determination rules must account for the height of Colorado State University HCA distinguishes between the radar resolution volume with respect to the storm ‘‘graupel–small hail’’ and ‘‘hail’’ without specifying the freezing level. borderline size between small hail and hail (Lim et al. Moreover, there are indications that very large dry 2005). According to the criterion of the U.S. National hail or hail undergoing wet growth above the freezing Weather Service (NWS), hail is considered large and level have anomalously low rhv (Balakrishnan and Zrnic´ capable of inflicting substantial damage to property if its 1990; Rowe et al. 2007; Picca and Ryzhkov 2010) and diameter exceeds 2.5 cm. Depue et al. (2007) recom- a significantly high linear depolarization ratio (LDR) mended using the hail differential reflectivity parameter (Kennedy et al. 2001). Balakrishnan and Zrnic´ (1990) HDR, defined as (Aydin et al. 1986) demonstrate that in the presence of large, resonance- size scatterers, whose backscatter differential phase d is high, r is significantly reduced. In addition, d increases HDR 5 Z 2 g(ZDR), (1) hv and rhv decreases if hailstones acquire a water film, where which occurs in the wet growth regime. The study of Rowe et al. (2007) demonstrates that rhv proved even more important than ZDR in discriminating between large g(ZDR) 5 27 if ZDR # 0dB and small hailstones. However, their study examined g(Z ) 5 19Z 1 27 if 0 # Z # 1. 74 dB, DR DR DR hailstorms much farther from the radar site ($100 km g(ZDR) 5 60 if ZDR . 1. 74 dB (2) versus approximately 50 km) and over more rural areas than the 16 May 2010 case, likely creating greater un- to identify large hail with a diameter exceeding 1.9 cm certainty in the hail reporting. Additionally, whereas (considered large according to the past NWS definition) Rowe et al. (2007) investigate S-band data only, this using an HDR threshold of 21 dB. study takes advantage of both S- and C-band data for The main limitation of the previous methods, in- a comparative analysis. Thus, the potential of using the cluding the approach by Depue et al. (2007), is that they rhv signature for identifying very large hail is further do not account for the hail melting process, which has explored. a very strong impact on the vertical profile of ZDR. In- Another interesting radar signature associated with deed, if large melting hail is mixed with rain originating large hail is the ‘‘three-body scattering signature’’ (TBSS), from the complete melting of smaller graupel–hail, the which was studied by Zrnic´ (1987), Wilson and Reum resulting ZDR can easily exceed 1.74 dB and the condi- (1988), Lemon (1998), Hubbert and Bringi (2000), tion HDR . 21 dB suggested by Depue et al. (2007) is Lindley and Lemon (2007), Zrnic´ et al. (2010), and equivalent to Z . 81 dBZ, making little sense. Although Kumjian et al. (2010b). The TBSS is observed as a radial the occurrence of high ZDR with large hail is much more spike in Z. Moreover, Hubbert and Bringi (2000) noted likely at C band, it can happen at S band as well, as the appearance of very high ZDR and low rhv in the shown in the in situ reports section below. Furthermore, TBSS observed with polarimetric radar. Therefore, po- whereas the cases of Depue et al. (2007) were located in tential links between the TBSS and hail size should be the Colorado high plains, the 16 May 2010 case occurred investigated. in an environment characterized by relatively higher Validation studies of polarimetric hail detection al- moisture, which enhances melting and in turn ZDR. gorithms are rare. Notable exceptions include the works Therefore, methods for estimating hail size should be of Heinselman and Ryzhkov (2006) and Depue et al. substantiated by retrievals from cloud models that ex- (2007) at S band, and Boodoo et al. (2009) and Tabary plicitly treat the microphysics of melting hail, as regional et al.
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