Ashley Wolf NWS Green Bay Mesovortex Evolution and Dual Polarization Debris Signatures Associated with the 7 August 2013 Tornadic QLCS
Introduction • Brief Radar Overview • Focus will be on the tornadic MV phase • Compare Tornadic Mesovortex and TDS Evolution • Application and utility of TDS criteria for tornadic QLCS events
Event Overview • Two phases of mesovortex development. Focus will be on the second phase which produced 6 known tornados (5 EF1 / 1EF2) within 45 minute period. • Little or no damage was observed between the tornadic MVs. • Tornadic MV development close to GRB radar (<40 nm). Allowed for good data set for further analysis post-event.
Two Episodes of Mesovortex Formation Focus on Episode 2 Episode 1 0400-0515 Episode 2 0515-0700
Radar Overview This segment of the QLCS most likely to be balanced. Tornadic MVs form generally in 30 mile corridor bounded by bow apex to south and thunderstorm outflow boundary to north. apex Mesovorticesdeveloped as line segment surges and pivots to N-S orientation. Becomes favorably aligned to 0-3 km shear vectors(Schaumann & Przybylinski (2012). MVs intensify rapidly & become tornadic. Line segment accelerates to 65 KTS.
TDS criteria… TDS Criteria Used In This Study • Reflectivity > 30dBZ • CC ≤ 0.80 • ZDR ≈ 0 • Associated with “Strong” Velocity Signature • WDTB recommended criteria CC < 0.80 • Schultz et al criteria CC < 0.70
MV1 SRM 0.9 SRM 0.9 MV2 MV2 Height (km) MV1 MV1 0522 0527 CC 0.9 CC 0.9 * Max TDS Height MV2 MV1 0527 0522 Height (km) Time from MV genesis to TDS ~ 2 Vol Scans MV1 – Max initial TDS Height ~ 2 KM MV1 – Max TDS Height ~ 3.5 KM MV2 – Max initial TDS Height ~ 0.5 KM MV2 – Max TDS Height ~ 2.25 KM MV1 & MV2 eventually merge. * Elapsed time following MV Genesis >>>>> *Time of initial MV Genesis
MV4 MV4 SRM 0.9 0538 UTC MV4 Elapsed time following MV Genesis >>>>> More rapid evolution associated with MV4 Time from MV genesis to TDS ~ 1 Vol Scan Max initial TDS Height = Max TDS Height ~ 1 KM CC 0.5
Z 0.5 0542 MV5 BV 0.9 0542 CC 0.5 0542 Elapsed time following MV Genesis >>>>> MV5 appears to develop on or just south of boundary. Time from MV genesis to TDS ~ 1 Vol Scan. Very rapid evolution!!! Classic ND characteristics. Appeared to be the strongest MV. Max initial TDS height = Max TDS Height ~ 2.25 KM
MV6 Z 0.9 0604 Vr (m/s) SRM 0.9 0604 0604 Time from MV genesis to TDS ~ 1 Vol Scan. MV6 forms on boundary. Classic ND Vr characteristics once again. Max initial TDS Height ~ 0.5 KM Max TDS Height ~ 1.5 KM ?? CC 0.5
Summary of Observed MV Evolution and TDS Characteristics • Max rotational velocity (Vr) generally AOB 1.5 km • Non-descending MV evolution • Mean maximum Vr ~ 26 ms-1 • Mean maximum Vr at initial TDS detection ~ 19 ms-1 • Average TDS depth ~ 2 km. Greatest TDS depth ~ 3.5 km (MV5) • Max TDS depth observed in same volume scan in which TDS first identified in half the cases. • Time from MV genesis to first observed TDS ~ 1 volume scan • Initial TDS observed before maximum Vr
TDS Distance vs MAX TDS Height vs EF Rating (Various Convective Modes) TDS Distance vs MAX TDS Height vs EF Rating Includes Various Convective Modes (Alabama) August 7, 2013 Max TDS heights observed in this event were comparable or slightly higher than EF1/EF2 tornadoes examined by Schultz et al (2012). Still lots to learn about characteristics of QLCS debris signatures! From Schultz et al (2012) – Their Figure 10.
Applying TDS Criteria for Tornadic QLCS Events • Use caution when applying TDS criteria in tornadic QLCS events! • Tornadic circulations are smaller-scale, develop very rapidly, are short-lived and typically shallow compared to classic supercell-type storms. • Associated TDS will be more difficult to identify • Range dependent • Transient • May be embedded in clutter/noise near the radar • Smaller in size (diameter) and shallower in depth • May require less stringent dual-pol TDS criteria
Utility of the Dual-Pol Data • During an Event • Identifying TDS signatures aids in warning decision process. Confirms existence of short-lived QLCS tornadoes. Very difficult to identify visually as typically shrouded in rain. • Post Event: Damage Survey • Despite apparent MV merger based on the SRM data, two TDS signatures were identified in close proximity. • Two distinct damage paths able to be identified during damage survey. • No damage reports received for last tornado (MV6). After examining dual-pol data, decided to investigate for damage. • Discovered EF-1 tornado damage path just east of Green Bay. • This event stresses the importance of a thorough damage survey following suspected tornadic QLCS events. Prior to dual-pol and TDS applications, straight line wind damage may have been assumed (with perhaps no survey conducted at all). Helps the science!
Further Research • Compare supercell TDS to QLCS TDS Characteristics • Better understand relationship between QLCS MV evolution and associated TDS signatures
Acknowledgements • Thank You! • Gene Brusky • Ed Townsend • Ron Przybylinski • Jason Schaumann • Questions???