DPWX/Contrasting rhoHV levels in hail: 24 May 2016 vs. 30 July 2016
Author: Patrick C. Kennedy
CSU-CHILL S-band reflectivity data in a low elevation angle surveillance scan taken while a cluster of hail-producing thunderstorms was passing the radar site at 0113 UTC on 30 July 2016. The low level correlation coefficient (rhoHV) values observed in this large hail case have been compared to those recorded from a storm that produced smaller diameter hail (24 May 2016).
Overview
Hail-producing thunderstorms were observed on multiple occasions by the CSU-CHILL radar during 2016. The storms on the afternoon of 24 May and the evening of 29 July (30 July on UTC) produced significantly different hail sizes. A post-storm survey conducted after the 24 May storm affected the Windsor Colorado area yielded photographic evidence that the ground was locally covered by hail. The maximum reported hail diameter was 0.75 inches (19 mm). In contrast, a cluster of severe thunderstorms that passed over the CHILL radar site during the early evening hours of 29 July produced multiple reports of hail in the 1 - 2 inch diameter range, and two reports of 2.5 inch (tennis ball) sized hail. (An overview of the 29 July 2016 event is available here.) The radial velocity patterns in the following RHI scan imply that a well-organized updraft was present at 0130 UTC in the July case. This RHI was nearly coincident with an SPC report of 2.5 inch diameter hail at the indicated location.
S-band dual polarization patterns in the 24 May and 30 July 2016 hailstorms
The following series of plots show low elevation PPI scan data from these two hail events. Clicking on the "Fwd" screen button will advance the displayed image through a sequence of four data fields (Zh, Zdr, rhoHV x 10, and LDR). The upper frame contains the data from the 24 May storm; the 30 July (UTC date) severe storm is shown in the lower frame. For both cases, an indication of the hail region boundary is given by the solid +10 dB HDR contour (Aydin et al. JAMC 1986, p. 1509). Observed hail diameter locations are marked by letters: "W" in the 24 May storm was a 0.75 inch hail report. "C" and "S" in the 30 July storm are 2.5 inch diameter reports.
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In both storms, the hail regions are associated with the the joint existence of very high reflectivity and near 0 dB Zdr. The large hailstone diameters (in comparison to raindrops) contribute to the high reflectivity levels. The quasi-random orientations of the tumbling hailstones tends to equalize the received power at horizontal and vertical polarization; producing near 0 dB Zdr. Reductions in rhoHV to levels below ~0.90 near ground level have been associated with larger diameter hail (Ryzhkov et al. JAMC 2013 p. 2871; Fig. 1). Appreciably lower rhoHV levels occurred in 30 July 2016 storm, with values below 0.9 noted near spotter report "C". (The 2.5 inch hail report from point "S" was filed ~10 minutes after the PPI data time). The maximum hail diameters in the July storm were large enough to begin to show Mie resonance effects at the CSU-CHILL radar's 11 cm wavelength. The differential phase shift upon backscatter effects associated with Mie scattering can be an important factor in reducing rhoHV. Finally, higher depolarization levels were achieved in the July case. This may have been due to the tendency for hailstone asymmetry to increase with diameter (Knight JCAM 1986, p. 1956). It should also be noted that the presence of liquid water within or on the surface of hailstones significantly affects LDR.
RhoHV histograms in the 24 May and 30 July 2016 hail areas
The following two histograms were prepared from the rhoHV x 10 gate data that was observed inside the +10 HDR contour in these two storms. (Once again, clicking on the "Fwd" screen button will toggle between the two cases.) The rhoHV distribution in the 30 July large hail case has a significant tail extending down into the 0.8 to 0.9 range. The average rhoHV in the large hail case is 0.913 vs. 0.977 in the 24 May / sub-severe hail case.
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