Tornadogenesis within a Simulated Supercell Storm

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Tornadogenesis within a Simulated Supercell
Storm

• Ming Xue
• School of Meteorology and
• Center for Analysis and Prediction of Storms
• University of Oklahoma
• mxue_at_ou.edu
• Acknowledgement NSF, FAA and PSC

22nd Severe Local Storms Conference 6 October 2004
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Why Numerical Simulations?

• Observational data lack necessary temporal and
  spatial resolutions and coverage
• Observed variables limit to very few
• VORTEX II trying to change all these (?)

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Theory of Mid-level Rotation- responsible for
mid-level mesocyclone
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Tilting of Storm-relative Streamwise
Environmental Vorticity into Vertical
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Theories of Low-level Rotation
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Baroclinic Generation of Horizontal Vorticity
Along Gust Front Tilted into Vertical and
Stretched (Klemp and Rotunno 1983)
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Downward Transport of Mid-level Mesocyclone
Angular Momentum by Rainy Downdraft (Davis-Jones
2001, 2002)
vorticity carried by downdraft parcel
baroclinic generation around cold, water loaded
downdraft
cross-stream vort. generation by sfc friction
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Past Simulation Studies

• Representative work by several groups
• Klemp and Rotunno (1983), Rotunno and Klemp
  (1985)
• Wicker and Wilhelmson (1995)
• Grasso and Cotton (1995)
• Adlerman, Droegemeier, and Davies-Jones (1999)
• All used locally refined grids

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Current Simulation Study

• Single uniform resolution grid (50x50km)
  covering the entire system of supercell storms
• Up to 25 m horizontal and 20 m vertical
  resolution
• Most intense tornado ever simulated (Vgt120m/s)
  within a realistic convective storm
• Entire life cycle of tornado captured
• Internal structure as well as indications of
  suction vortices obtained

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25 m (LES) simulation

• Using ARPS model
• 1977 Del City, OK sounding (3300 J/kg CAPE)
• 2000 x 2000 x 83 grid points
• dx 50m and 25m, dzmin 20m, dt0.125s.
• Warmrain microphysics with surface friction
• Simulations up to 5 hours
• Using 2048 Alpha Processors at Pittsburgh
  Supercomputing Center
• 15TB of 16-bit compressed data generated by one
  25m simulation over 30 minutes, output at 1 s
  intervals

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Sounding for May 20, 1977 Del City, Oklahoma
tornadic supercell storm
CAPE3300 J/kg
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Storm-relative Hodograph
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50m simulation shown in full 50x50 km domain
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Full Domain Surface Fields of 50m simulation
t3h 44m Red positive vertical vorticity
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25 m simulation surface fields shown in
subdomains
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Near surface vorticity, wind, reflectivity, and
temperature perturbation
2 x 2 km
Vort 2 s-1
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Low-level reflectivity and streamlines of 25 m
simulation
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50m Movie(30min 4h 30min)
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25m Movie(over 20 min)
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Maximum surface wind speed and minimum
perturbation pressure of 25m simulation
120m/s
gt80mb pressure drop
50m/s in 1min
120m/s max surface winds
-80mb
time
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Pressure time series in vicinity of Allison TX
F-4 Tornado on 8 June 1995 (Winn et al 1999)
910mb
gt50mb pressure drop
850mb
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Lee etc (2004) 22nd SLS Conf. CDROM 15.3 100mb
pressure drop
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Iso-surfaces of cloud water (qc 0.3 g kg-1,
gray) and vertical vorticity (z0.25 s-1, red),
and streamlines (orange) at about 2 km level of a
50m simulation
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Time-dependent Trajectories
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3km
t13250s beginning of vortex intensification
View from South
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3km
t13250s beginning of vortex intensification
N
View from SW
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Trajectory Animations
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3km
View from Northeast
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(No Transcript)
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Brownings Conceptual Model of Supercell Storm
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Diagnostics along Trajectories
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Orange portion t13250-500s 13250200s
14km
t13250s Beginning of low-level spinup
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8km
X Y Z
W Vh
Streamwise Vort. Cross-stream Vort. Horizontal
Vort.
Vertical Vort. Total Vort.
13450
13250
12750
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2 m s-2
Force along trajectory
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Buoyancy Vert. Pgrad Sum of the two
b' due to -p'
-5
Perturbation pressure
-76mb
13250
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Orange portion t13250-500s 13250200s
14km
rapid parcel rise
t13250s Beginning of low-level spinup
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8km
X Y Z
W Vh
Streamwise Vort. Cross-stream Vort. Horizontal
Vort.
Vertical Vort. Total Vort.
13450
13250
12750
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3 m s-2
Force along trajectory
Buoyancy Vert. Pgrad Sum of the two
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-5
Perturbation pressure
-76mb
13250
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Conclusions

• F5 intensity tornado formed behind the gust
  front, within the cold pool.
• Air parcels feeding the tornado all originated
  from the warm sector in a layer of about 2 km
  deep.
• The low-level parcels pass over the forward-flank
  gust front of 1st or 2nd supercell, descended to
  ground level and flowed along the ground inside
  the cold pool towards the convergence center
• The parcels gain streamwise vorticity through
  stretching and baroclinic vorticity generation
  (quantitative calculations to be completed)
  before turning sharply into the vertical

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Conclusions

• Intensification of mid-level mesocyclone lowers
  mid-level pressure
• Vertical PGF draws initially negatively buoyant
  low-level air into the tornado vortex but the
  buoyancy turns positive as pressure drops
• Intense vertical stretching follows ?
  intensification of low-level tornado vortex ?
  genesis of a tornado

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Conclusions (less certain at this time)

• Baroclinic generation of horizontal vorticity
  along gust front does not seem to have played a
  key role (in this case at least)
• Downward transport of vertical vorticity
  associated with mid-level mesocyclone does not
  seem to be a key process either (need
  confirmation by e.g., vorticity budget
  calculations)

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Many Issues Remain

• Exact processes for changes in vorticity
  components along trajectories
• Treatment and effects of surface friction and SGS
  turbulence near the surface
• Do many tornadoes form inside cold pool?
• Microphysics, including ice processes
• Intensification and non-intensification of
  low-level rotation?
• Role of 1st storm in this case
• etc etc etc.

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Movie of Cloud Water Field25 m, 7.5x7.5km
domain, 30 minutes
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Questions / Comments?