Transcript The Effect of Gravity Wave/Undular Bore Dissipation on the

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?
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???