Post on 14-Dec-2015
Tornadogenesis within a Simulated Supercell
Storm
Ming XueSchool of Meteorology and
Center for Analysis and Prediction of StormsUniversity of Oklahoma
mxue@ou.eduAcknowledgement: NSF, FAA and PSC
22nd Severe Local Storms Conference22nd Severe Local Storms Conference6 October 20046 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 (?)
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 (Davis-Jones 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 suction vortices obtained
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
Full Domain Surface Fields of 50m simulation
t=3h 44mt=3h 44m
Red – positive Red – positive vertical vorticityvertical vorticity
Near surface vorticity, wind, reflectivity, and temperature
perturbation
2 x 2 km2 x 2 km
Vort ~ 2 sVort ~ 2 s-1-1
Maximum surface wind speed and minimum perturbation pressure of
25m simulation
time
120m/s
-80mb
~120m/s max ~120m/s max surface windssurface winds
>80mb pressure drop>80mb pressure drop+50m/s+50m/sin ~1minin ~1min
Pressure time series in vicinity of Allison TX F-4 Tornado on 8 June
1995 (Winn et al 1999)
850mb850mb
910mb910mb
>50mb >50mb pressure droppressure 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
View from SouthView from South
t=13250st=13250sbeginning of beginning of vortex intensificationvortex intensification
3km3km
View from SWView from SW
NN
t=13250st=13250sbeginning of beginning of vortex intensificationvortex intensification
3km3km
View from NortheastView from Northeast
3km3km
RFD ofRFD of11stst cell cell
FFD ofFFD of22ndnd cell cell
InflowInflowfrom eastfrom east
Low-level jump flowLow-level jump flow
Orange portion t=13250-500s – 13250+200s
t=13250st=13250sBeginning of low-level Beginning of low-level spinupspinup
14km14km
X Y Z
8km8km
WVh
Streamwise Vort.Cross-stream Vort.Horizontal Vort.
Vertical Vort.Vertical Vort.Total VortTotal Vort..
13250132501275012750 1345013450
Force along trajectoryForce along trajectory
BuoyancyBuoyancyVert. PgradVert. PgradSum of the twoSum of the two
Perturbation pressurePerturbation pressure-76mb-76mb
55
-5-5
1325013250
~2 m s~2 m s-2-2
+b' due to -p'+b' due to -p'
Orange portion t=13250-500s – 13250+200s
t=13250st=13250sBeginning of low-level Beginning of low-level spinupspinup
14km14km
rapid parcel riserapid parcel rise
X Y Z
8km8km
WVh
Streamwise Vort.Cross-stream Vort.Horizontal Vort.
Vertical Vort.Vertical Vort.Total VortTotal Vort..
13250132501275012750 1345013450
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 low-level rotation?
• Role of 1st storm in this case
• etc etc etc.