On May 17th, 2012, the National Weather Service (NWS) office at Greenville-Spartanburg (GSP)
monitored both the Greenville-Spartanburg (KGSP) and Columbia (KCAE) Weather Surveillance
Radar (WSR-88D) radars (Fig. 1) to issue a Severe Thunderstorm Warning and Severe Weather
Statements for a cluster of storms in Laurens County, South Carolina. KCAE recently
converted to dual-polarization while KGSP was still a single-polarization radar. During
the event, spotters under the storm cores were actively called. A few reports also were
received through the Public Observation Program (POP) and from media and NWS GSP Facebook
pages. The multiple verification sources and reports provided a good opportunity to compare
the legacy and new dual-polarization products to storm reports. During this short event,
comparisons were made between cells that produced pea size or no hail, hail less than one
inch diameter, and hail greater than or equal to one inch diameter (severe hail).
Fig.1. Map showing the location of the KGSP and KCAE radars
Based on the freezing level and environmental conditions, the 55 dBz core height expected
for severe hail was around 23,000 ft above mean sea level (MSL). The core height was
observed on KGSP at 1717 UTC and a Severe Thunderstorm Warning was issued at 1720 UTC valid
until 1815 UTC. Figure 2 shows the Warning polygon and reports of hail.
Fig. 2. NWSChat Severe Thunderstorm Warning polygon (solid yellow line) and hail reports (symbols and table)
2. PEA SIZE OR NO HAIL PRODUCING CELL
At 1724 UTC, the 3.1 degree reflectivity (Z) slice from KGSP (Fig. 3) displayed a Three Body Scatter Spike (TBSS) over
southern Laurens County near Waterloo.
Fig. 3. 1724 UTC KGSP 3.1 degree Z, TBSS highlighted by the red arrow
However, the 1721 UTC 0.5 degree KCAE dual-polarization products (Differential Reflectivity
(ZDR)around 1 dB, Correlation Coefficient (CC) around 1, and Specific Differential Phase (KDP)
greater than 1 deg km-1) did not indicate that the large hail was as low as 5,000 ft above
ground level (AGL; Fig. 4). Dual-polarization products higher in the storm indicated the
presence of hail, but the hail apparently melted as it reached the surface. Hail larger than
golf balls generally have ZDR values around 0 dB, CC 0.80-0.95, and areas where KDP is not
computed (due to low values of CC) or is less than 1 deg km-1. No reports of hail greater
than pea size were associated with this storm.
Fig. 4. 1721 UTC four-panel display of KCAE dual-polarization products (clockwise from upper left: reflectivity,
differential reflectivity, correlation coefficient, and specific differential phase)
3. HAIL LESS THAN ONE INCH DIAMETER
Two storms of similar strength were south of Laurens at 1736 UTC (Fig. 5). Both storms had 60 to 65 dBZ cores with 50 dBZ around 25,000 ft MSL.
Fig. 5. Four panel display of KGSP reflectivity. 0.5 degree (top left), 0.9 degree (top right), 3.1 degree (bottom left), 8.0 degree (bottom right)
Simultaneous KGSP and KCAE scans of Laurens County occurred at 1736 UTC. The 0.5 degree slice from KGSP sampled the two storms around 2,400 ft AGL and KCAE sampled the same area around 4,700 ft AGL. Despite the difference in altitude, the 0.5 degree slices from the two radars had very similar values of reflectivity. The eastern storm actually appeared to have slightly greater values of dBZ and a well developed vertical storm structure. However, the KCAE dual-polarization products (Fig. 6) indicated that the western storm had ZDR values a little less than 2 dB, KDP over 3 deg km-1, and a fairly large area of CC values around 0.95. The eastern storms had CC values around 1 with higher ZDR values. A post from the public on WYFF's (NBC Channel 4 Greenville) Facebook page indicated that dime size hail fell six miles southwest of Laurens at 1735 UTC. A picture of mainly pea size hail also was posted. No hail reports were associated with the eastern storm.
Fig. 6. As in Figure 4, except for 1736 UTC
4. HAIL GREATER THAN OR EQUAL TO ONE INCH DIAMETER
The final event occurred near the town of Clinton between 1753 UTC and 1805 UTC. The first returns on the KCAE 0.5 degree slice were observed east of Clinton at 1749 UTC. The storm continued to develop as it drifted west across the town. At 1753 UTC, a POP report of nickel size hail was received from Clinton. At 1753 UTC, KCAE 0.9 degree slice dual-polarization products (not shown) indicated hail as low as 5,800 ft AGL just east of Clinton, but not as low as 3,800 ft on the 0.5 degree slice. Dual-polarization products at 1757 UTC indicated that hail was present at a lower altitude than detected in the 1753 UTC scan, with 60 dBZ, ZDR around 1.6 dB, CC less than 0.95, and a CC TBSS (Fig. 7).
Fig. 7. As in Figure 4, except for 1757 UTC, TBSS within blue oval
At 1806 UTC, the storm had a core greater than 60 dBZ extending from 3,900 ft up to 10,000 ft AGL. In addition, the lower four slices contained a TBSS. The dual-polarization products from KCAE at 1806 UTC indicated the presence of large hail as low as 3,900 ft AGL. In Fig. 8, the radar returns just west of Clinton had ZDR values <0 dB, CC values between 0.85 to 0.90, reflectivity values between 65 dBZ and 69 dBZ , and areas of no KDP due to low CC. In addition, a TBSS and a small side lobe were evident in dBZ, ZDR, and CC (Fig. 8).
Fig. 8. As in Figure 4, except for 1806 UTC, TBSS within the blue ovals and side lobe indicated by red arrows
At 1805 UTC, a trained spotter reported ping-pong ball sized hail three miles west of Clinton. The spotter called in the report and posted a picture of the hail on the NWS GSP Facebook page (Fig. 9).
Fig. 9. Zach Llyod, Skywarn Spotter, picture of ping-pong ball size hail near Clinton, South Carolina
A cluster of thunderstorms over Laurens County on May 17, 2012, was observed with both the KGSP single polarization and KCAE dual-polarization WSR-88D radars. The new dual-polarization products and storm reports were in very good agreement. The combination of meeting the 55 dBz core height criterion of one inch or greater diameter hail, WSR-88D Probability of Severe Hail (POSH) at 50 to 60%, and the occurrence of a TBSS, gave the warning forecaster confidence that large hail would occur with the thunderstorm west of Laurens, prompting the Severe Thunderstorm Warning issuance at 1720 UTC. However, using the dual-polarization products, the warning forecaster observed increasing values of CC lower in the storm, indicating that the hail melted as it fell. The second cell, at 1736 UTC, satisfied the 55 dBz core height criterion and again the warning forecaster was confident that a severe hail event would occur south of Laurens. Dual-polarization products indicated the storm had hail aloft, but the lowest slices had CC values around 1 with high ZDR, favoring heavy rain. The Clinton storm developed very quickly and by the radar's second scan the dual-polarization products indicated that storm possessed hail aloft. As the hail core descended, lower radar slices revealed decreasing values of CC, TBSS, and a small side lobe. By 1806 UTC, both the KCAE and KGSP radars detected a very reflective storm, a tall hail core, and POSH at 100%. In this case, the KCAE dual-polarization products, especially CC, provided a precise location and altitude of the hail.
Acknowledgements. The guidelines for determining the 55 dBz core height warning criteria from the freezing level and environmental conditions were developed by Justin Lane (NWS Greenville-Spartanburg). The Severe Thunderstorm Warning polygon and storm report graphics and table were supplied by NWSChat. Information on the detection of hail using dual-polarization radar was obtained from the Warning Decision Training Branch "Dual-Pol Radar Applications: Hail Detection" module. The photo of hail stones near Clinton was provided by Zach Llyod, Skywarn Spotter. Larry Lee and Patrick Moore (NWS Greenville-Spartanburg) provided a helpful review and quality control of this document.
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