The Impact of Gravity Wave/Undular Bore Dissipation on the June 22, 2003 Deshler and Aurora Nebraska Tornadic Supercells AARON W. JOHNSONNOAA/NWS Weather Forecast Office, Hastings, Nebraska
RUC40 21Z Analysis Sounding between Deshler and Aurora • CAPE = 5170 J/kg • LCL Hgt = 3549 ft AGL • LFC Hgt = 4393 ft AGL
Fairbury Profiler Hodograph at 23Z • 0-1 km SRH ~ 120 m2/s2 • 0-6 km Bulk Shear = 36 KTS • 0-6 km Mean Wind ~ 220 at 27 KTS
HOWEVER…both Supercells became nearly stationary within 5-10 minutes after rapid gravity wave/undular bore dissipation was observed.
Severe Storm Reports • Large Hail: 27 reports of ¾” or larger (including the Aurora Volleyball sized hail) • Tornado: 10 reports of Tornadoes (1 Killer Tornado at Deshler) • Strong Winds: 5 reports of 60+ mph winds • Flooding: 5 reports of Flooding
Research from Event • Wakimoto (2004) – via the BAMEX project, mainly looked at Eldora Observations of the Superior Supercell. • Guyer and Ewald (2004) – mainly looked at WSR-88D characteristics of Aurora Supercell/hailstone.
Hindsight is always 20/20 however… • The environmental setup was more complex than previous literature has discussed. • Only brief mention of Storm Motion • Several inaccuracies exist in the literature including: • Incorrect labeling of the Deshler and Superior Supercells as being the same storm. • Insufficient surface boundary analysis/detection. • Assumption of Dropsonde data well south and much later than the Aurora and Deshler storms being representative of the mesoscale environment for the entire event.
Quick review on Gravity Waves/Undular Bores • Much has been written about the environmental setup needed for undular bores to exist including: Christie et al. 1978, 1979; Simpson (1987); Maxworthy (1980); Crook (1988); Smith (1988); Rottman and Simpson (1989); Haase and Smith (1989b); and Doviak and Ge (1984). • The main feature coming out of this literature is the need to trap energy in the low levels via one or multiple atmospheric characteristics
Quick review on Gravity Waves/Undular Bores • Crook (1988) defined these trapping methods into 3 main features: • a wind profile above 4 KM that opposes the motion of the waves • a low level jet that opposes the motion of the waves • temperature inversion at or below 4 KM
Relative wind speed normal to movement of gravity wave/undular bore.
Backed boundary layer winds eliminate one form of low level energy trapping.
Large updrafts and circular hodograph??? • Past studies have shown that the storm motion is located at the center of curvature of a perfectly circular hodograph. • However…Davies-Jones (2002) suggests that propagation off the hodograph occurs in the presence of a large updraft.
Impact of backed surface winds • Appears to have caused both the rapid decay of the gravity wave/undular bore field and change in storm propagation. • Observational network was slow to show these changes in wind direction due to scarcity of automated sites and slow reporting frequency. • What may have caused local backing of surface winds???
Few clues exist in Synoptic data - Mass field adjusting to Meso or smaller scale changes
Conclusions • Rapid backing of low-level winds appears to be connected to meso-low development. • Changes in wind direction/speed impacted dissipation of gravity wave/bore field and storm motion.
Conclusions • Given inherit weaknesses in the observational network to report rapid changes, it appears a dissipating portion of a gravity wave/bore field may be an indication of changes in the low-level wind field that could be observed closer to real-time. • This type of observed change may indicate rapid changes in: • Low and deep layer shear profiles • Storm motion • Storm type and duration (LL or SL supercells)