Tornadogenesis
within a Simulated Supercell Storm
Ming Xue
School of Meteorology and
Center for Analysis and Prediction of Storms
University of Oklahoma
mxue@ou.edu
Acknowledgement: NSF, FAA and PSC
22nd Severe Local Storms Conference
6 October 2004
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 (?)
Theory of Mid-level Rotation
- responsible for mid-level mesocyclone
Tilting of Storm-relative Streamwise
Environmental Vorticity into Vertical
Theories of Low-level Rotation
Baroclinic Generation of Horizontal
Vorticity Along Gust Front Tilted into
Vertical and Stretched (Klemp and
Rotunno 1983)
Downward Transport of Mid-level Mesocyclone
Angular Momentum by Rainy Downdraft (DavisJones 2001, 2002)
vorticity carried
by downdraft parcel
baroclinic generation
around cold, water
loaded downdraft
cross-stream vort.
generation by sfc friction
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
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
(V>120m/s) within a realistic convective
storm
• Entire life cycle of tornado captured
• Internal structure as well as indications of
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, dt=0.125s.
• 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
Sounding for May 20, 1977 Del City, Oklahoma tornadic supercell
storm
CAPE=3300
J/kg
Storm-relative Hodograph

h
50m simulation
shown in full 50x50 km domain
Full Domain Surface Fields of 50m
simulation
t=3h 44m
Red – positive
vertical vorticity
25 m simulation
surface fields shown in subdomains
Near surface vorticity, wind, reflectivity,
and temperature perturbation
2 x 2 km
Vort ~ 2 s-1
Low-level reflectivity and streamlines of
25 m simulation
50m Movie
(30min – 4h 30min)
25m Movie
(over 20 min)
Maximum surface wind speed and
minimum perturbation pressure of 25m
simulation
120m/s
>80mb pressure drop
+50m/s
in ~1min
~120m/s max
surface winds
-80mb
time
Pressure time series in vicinity of Allison
TX F-4 Tornado on 8 June 1995 (Winn et
al 1999)
910mb
>50mb
pressure drop
850mb
Lee etc (2004)
22nd SLS Conf.
CDROM 15.3
~100mb
pressure drop
Iso-surfaces of cloud water (qc = 0.3 g kg-1, gray) and vertical vorticity (z=0.25
s-1, red), and streamlines (orange) at about 2 km level of a 50m simulation
Time-dependent Trajectories
3km
View from South
t=13250s
beginning of
vortex intensification
3km
N
t=13250s
beginning of
vortex intensification
View from SW
Trajectory Animations
3km
FFD of
2nd cell
RFD of
1st cell
Inflow
from east
Low-level jump flow
View from Northeast
Browning’s
Conceptual Model of Supercell Storm
Diagnostics along Trajectories
Orange portion t=13250-500s –
13250+200s
14km
t=13250s
Beginning of low-level
spinup
8km
X Y
Z
W
Vh
Streamwise Vort.
Cross-stream Vort.
Horizontal Vort.
Vertical Vort.
Total Vort.
12750
13250
13450
~2 m s-2
Force along trajectory
5
Buoyancy
Vert. Pgrad
Sum of the two
+b' due to -p'
-5
Perturbation pressure
-76mb
13250
Orange portion t=13250-500s –
13250+200s
14km
rapid parcel rise
t=13250s
Beginning of low-level
spinup
8km
X Y
Z
W
Vh
Streamwise Vort.
Cross-stream Vort.
Horizontal Vort.
Vertical Vort.
Total Vort.
12750
13250
13450
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
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
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)
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 lowlevel rotation?
• Role of 1st storm in this case
• etc etc etc.
Movie
of
Cloud Water Field
25 m, 7.5x7.5km domain, 30 minutes
Questions / Comments?