Tornado Tornadogenesis and Veryhighresolution Numerical Simulations Ming Xue

Tornado, Tornadogenesis and Very-high-resolution Numerical Simulations Ming Xue (薛明) School of Meteorology and Center for Analysis and Prediction of Storms University of Oklahoma, USA mxue@ou. edu

What is a tornado? • tornado is not typhoon or hurricane • tornado: A rotating column of air usually accompanied by a funnel-shaped downward extension of a cumulonimbus cloud and having, winds whirling destructively at speeds of up to 500 km/h or > 100 m/s.

A supercell tornado

Tornado Climatology in USA for over 70 years max frequency: 35 (≥F 2)/100 years within 40 km radius Tornado Alley (龙卷胡同)

Favorable Environment for Tornadogenesis • The elevated terrain to the west and the Gulf of Mexico play important roles • All intense tornadoes occur within supercell storms • Supercell storms require large CAPE and strong vertical wind shear • Late spring and late afternoon and early evening provide the most favorable conditions

Favorable Environmental for Severe Convection – A dryline example q

A conceptual model of tornadic supercell

Schematic plan view of a tornadic thunderstorm near the surface

Typical life cycle of tornado

Formation Stage

Mature Stage

Roping Stage

Multi-vortex tornado – suction vortices

3 May 1999 Oklahoma Tornado Outbreak Copyright Daily Oklahoman 1999 The Daily Oklahoman

May 3, Oklahoma Tornado Damage

More Damage Pictures

supercell with hook echo

Processes that can change component vorticity • Redistribution n Advection Tilting Stretching – angular momentum conservation • Generation of new vorticity n by horizontal buoyancy gradient – baroclinic generation – 力管效应

Theory of Mid-level Rotation - responsible for mid-level mesocyclone

Tilting of Streamwise Environmental Vorticity into Vertical – source of mid-level rotation

Theories of Low-level Rotation

Baroclinic Generation of Horizontal Vorticity Along Gust Front (Klemp and Rotunno 1983)

Downward Transport of Mid-level Angular Momentum by Rainy Downdraft (Davis-Jones 2002)

Observation and Simulation Studies • In recent years, much more observations of tornadoes have been collected (e. g. , VORTEX-95), mobile Doppler radar has become the most effective observational tool (Z and Vr only, limited spatial coverage and resolution) • Still, much of our current understandings of tornado dynamics were gained from numerical simulations • Model output is more complete in both time and space as well as in variables

Earlier Simulation Studies • Klemp and Rotunno (1983) A study of the tornadic region within a supercell thunderstorm. JAS (dx~250 m) • Rotunno, R. and J. B. Klemp, 1985: On the rotation and propagation of simulated supercell thunderstorms. JAS • Wicker and Wilhelmson (1995) Simulation and analysis of tornado development and decay within a three-dimensional supercell thunderstorm. JAS • Grasso and Cotton (1995) Numerical Simulation of a tornado vortex. JAS • Adlerman, Droegemeier, and Davies-Jones (1999) A Numerical Simulation of Cyclic Mesocyclogenesis. JAS • Others

Limitations of Existing Simulation Studies • All use local refinement, usually nested grid, techniques. • The natural tornadogenesis process affected by the time and location of refinement • None had sufficient resolution to truly resolve tornado itself • None of the nested grids had large enough domain to gain a complete picture • Coarse time resolution of output limited detailed analysis

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 • Data output every second • Most intense tornado ever simulated (V>120 m/s) • Entire life cycle of tornado simulated • Internal structure as well as indications of suction vortices obtained • Detailed analyses are being performed

Simulation of tornado within a supercell storm • Using MPI version of ARPS (Advanced Regional Prediction System, Xue et al 2000, 2002, 2003) • 1977 Del City, OK sounding (3300 J/kg CAPE) • 2000 x 83 model grid • dx = 25 m, dzmin = 20 m, dt=0. 125 s. • Up to 5 h simulations • Using 2048 Alpha Processors at Pittsburgh Supercomputer Center • 15, 000 GB of 16 -bit compressed data generated over 30 minutes of simulation, output at 1 s intervals

Sounding for May 20, 1977 Del City, Oklahoma tornadic supercell storm

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

Full Domain Surface Fields of 50 m simulation See Movie

Maximum surface wind speed and minimum pressure of 25 m simulation 120 m/s -80 mb time

Near surface vorticity, wind, reflectivity, and temperature perturbation 2 x 2 km

The model tornado as seen by a radar 8 x 8 km

25 m simulation over 30 minutes See movie

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

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

Flow-dependent Trajectories

Trajectory Animations

Preliminary Findings • 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 parcels pass over the forward-flank gust front of 1 st or 2 nd supercell, descended to ground level and flowed along the ground towards the convergence center • The parcels gain streamwise vorticity through stretching and baroclinic vorticity generation before it turns sharply into the vertical • Intensification of mid-level mesocyclone lowers mid-level pressure • Vertical PGF thus created is responsible for the lifting of low-level negatively buoyant air into the tornado vortex • Intense vertical stretching follows intensification of low-level tornado vortex genesis of a tornado

Prelimary Findings • Baroclinic vorticity generation of horizontal vorticity along gust front does not seem to have played a key role • Downward transport of vertical vorticity associated with mid-level mesoscale cyclone does not seem to be a key process either • Their relative effect to be more quantitatively determined via detailed trajectory analyses.

x y z w Vh

To do list • Finish calculation of sources terms and time evolution of vorticity components along trajectories • Calculate forces (PGF, Buoyancy) terms along trajectory • Analysis of the internal structure of simulated tornado • Understand the cause of tornado demise • Study the role and effects of surface friction and SGS turbulence • Investigate when a tornado will and will not form