Transcript Document
Tornadogenesis within a Simulated Supercell Storm
Ming Xue School of Meteorology and Center for Analysis and Prediction of Storms University of Oklahoma
Acknowledgement: NSF, FAA and PSC 22nd Severe Local Storms Conference 6 October 2004
Why Numerical Simulations?
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Observational data lack necessary temporal and spatial resolutions and coverage
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Observed variables limit to very few
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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 (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
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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)
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All used locally refined grids
Current Simulation Study
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Single uniform resolution grid (~50x50km) covering the entire system of supercell storms
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Up to 25 m horizontal and 20 m vertical resolution
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Most intense tornado ever simulated (V>120m/s) within a realistic convective storm
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Entire life cycle of tornado captured
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Internal structure as well as indications of suction vortices obtained
25 m (LES) simulation
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Using ARPS model
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1977 Del City, OK sounding (~3300 J/kg CAPE) 2000 x 2000 x 83 grid points dx = 50m and 25m, dz min = 20m, dt=0.125s.
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Warmrain microphysics with surface friction Simulations up to 5 hours Using 2048 Alpha Processors at Pittsburgh Supercomputing Center
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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) 22 nd 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
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3km t=13250s beginning of vortex intensification View from SW
Trajectory Animations
3km Inflow from east
FFD of 2 nd cell RFD of 1 st cell 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 12750 W V h Streamwise Vort.
Cross-stream Vort .
Horizontal Vort.
Vertical Vort.
Total Vort .
13250 13450
5 Force along trajectory Buoyancy Vert. Pgrad Sum of the two -5 -76mb ~2 m s -2 +b' due to -p' Perturbation pressure 13250
Orange portion t=13250-500s – 13250+200s
14km rapid parcel rise t=13250s Beginning of low-level spinup
8km X Y Z 12750 W V h Streamwise Vort.
Cross-stream Vort .
Horizontal Vort.
Vertical Vort.
Total Vort .
13250 13450
Conclusions
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F5 intensity tornado formed behind the gust front, within the cold pool.
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Air parcels feeding the tornado all originated from the warm sector in a layer of about 2 km deep.
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The low-level parcels pass over the forward-flank gust front of 1 st or 2 nd supercell, descended to ground level and flowed along the ground inside the cold pool towards the convergence center
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The parcels gain streamwise vorticity through stretching and baroclinic vorticity generation (quantitative calculations to be completed) before turning sharply into the vertical
Conclusions
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Intensification of mid-level mesocyclone lowers mid-level pressure
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Vertical PGF draws initially negatively buoyant low-level air into the tornado vortex but the buoyancy turns positive as pressure drops
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Intense vertical stretching follows
intensification of low-level tornado vortex
genesis of a tornado
Conclusions (less certain at this time)
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Baroclinic generation of horizontal vorticity along gust front does not seem to have played a key role (in this case at least)
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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
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Exact processes for changes in vorticity components along trajectories
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Treatment and effects of surface friction and SGS turbulence near the surface
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Do many tornadoes form inside cold pool?
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Microphysics, including ice processes
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Intensification and non-intensification of low level rotation?
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Role of 1 st storm in this case
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etc etc etc.
Movie of Cloud Water Field 25 m, 7.5x7.5km domain, 30 minutes