Tornadogenesis within a Simulated Supercell Storm Ming Xue

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 22 nd 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 n n 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 (~50 x 50 km) covering the entire system of supercell storms • Up to 25 m horizontal and 20 m vertical resolution • Most intense tornado ever simulated (V>120 m/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 83 grid points • dx = 50 m and 25 m, dzmin = 20 m, dt=0. 125 s. • Warmrain microphysics with surface friction • Simulations up to 5 hours • Using 2048 Alpha Processors at Pittsburgh Supercomputing Center • 15 TB of 16 -bit compressed data generated by one 25 m 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

50 m simulation shown in full 50 x 50 km domain

Full Domain Surface Fields of 50 m simulation t=3 h 44 m 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

50 m Movie (30 min – 4 h 30 min)

25 m Movie (over 20 min)

Maximum surface wind speed and minimum perturbation pressure of 25 m simulation 120 m/s >80 mb pressure drop +50 m/s in ~1 min ~120 m/s max surface winds -80 mb time

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

Lee etc (2004) 22 nd SLS Conf. CDROM 15. 3 ~100 mb 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 50 m simulation

Time-dependent Trajectories

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

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

Trajectory Animations

3 km FFD of 2 nd cell RFD of 1 st 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 -500 s – 13250+200 s 14 km t=13250 s Beginning of low-level spinup

8 km 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 -76 mb 13250

Orange portion t=13250 -500 s – 13250+200 s 14 km rapid parcel rise t=13250 s Beginning of low-level spinup

8 km X Y Z W Vh Streamwise Vort. Cross-stream Vort. Horizontal Vort. Vertical Vort. Total Vort. 12750 13250 13450

Conclusions • F 5 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.

Movie of Cloud Water Field 25 m, 7. 5 x 7. 5 km domain, 30 minutes

Questions / Comments?