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? 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 (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, dz min = 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=3300J/kg

Storm-relative Hodograph

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 time 120m/s -80mb ~120m/s max surface winds >80mb pressure drop +50m/s in ~1min

Pressure time series in vicinity of Allison TX F-4 Tornado on 8 June 1995 (Winn et al 1999) 850mb 910mb >50mb pressure drop

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

View from South t=13250s beginning of vortex intensification 3km

View from SW N t=13250s beginning of vortex intensification 3km

Trajectory Animations

View from Northeast 3km RFD of 1 st cell FFD of 2 nd cell Inflow from east Low-level jump flow

Browning’s Conceptual Model of Supercell Storm

Diagnostics along Trajectories

Orange portion t= s – s t=13250s Beginning of low-level spinup 14km

X Y Z 8km WVhWVh Streamwise Vort. Cross-stream Vort. Horizontal Vort. Vertical Vort. Total Vort

Force along trajectory Buoyancy Vert. Pgrad Sum of the two Perturbation pressure -76mb ~2 m s -2 +b' due to -p'

Orange portion t= s – s t=13250s Beginning of low-level spinup 14km rapid parcel rise

X Y Z 8km WVhWVh Streamwise Vort. Cross-stream Vort. Horizontal Vort. Vertical Vort. Total Vort

Force along trajectory Buoyancy Vert. Pgrad Sum of the two Perturbation pressure -76mb ~3 m s -2

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 1 st or 2 nd 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 1 st storm in this case etc etc etc.

Movie of Cloud Water Field 25 m, 7.5x7.5km domain, 30 minutes

Questions / Comments?