The Impact of Gravity Wave/Undular Bore Dissipation on the June 22, 2003 Deshler and Aurora Nebraska Tornadic Supercells AARON W. JOHNSON NOAA/NWS Weather Forecast Office, Hastings, Nebraska

Brief Review of June 22, 2003

06/23 00Z 250 MB Chart

06/22 21Z HPC Surface Analysis

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

RUC40 21Z Analysis of ID Method for Left/Right-Moving Supercells

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.

2145Z Visible Satellite Imagery

KUEX Base Reflectivity at 2145 Z

KUEX Base Velocity at 2243 Z

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.

Inversion below 4 km

LLJ and winds above 4 km opposing motion

KUEX Base Reflectivity at 2258 Z

KUEX Base Reflectivity at 2345 Z

KUEX Base Velocity at 2345 Z

KUEX Base Reflectivity at 0028 Z

KUEX Base Reflectivity at 0046 Z

Backed boundary layer winds eliminate one form of low level energy trapping.

Impact of backing winds on hodograph curvature.

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???

Regional Surface plot 22Z.

Regional Surface plot 00Z.

Regional Surface plot 01Z.

Few clues exist in Synoptic data -Mass field adjusting to Meso or smaller scale changes

Meso-low development along converging boundaries?

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)

Questions???