龙卷, 龙卷生成机制和特高分辨率数值模拟 薛明 美国俄克拉荷马大学 气象系与风暴分析预报中心

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

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 > 100m/s.

A supercell tornado

Tornado Climatology in USA for over 70 years max frequency: 35 (≥F2)/100 years within 40km 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 

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 Advection Tilting Stretching – angular momentum conservation Generation of new vorticity 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)

Tornadogenesis – rapid intensification of low-level rotation Earlier simulation studies (Klemp and Rotunno 1983, Rotunno and Klemp 1985) suggest that the baroclinically generated strong horizontal vorticity along the low-level cold pool boundary is the most important source of low-level rotation, as this horizontal vorticity is tilted into vertical direction and intensifies through stretching This process generates horizontal vorticity that is several times the magnitude of the mean shear vorticity and that is more favorably oriented to be tilted into vertical cyclonic vorticity.

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

Tornadogenesis – rapid intensification of low-level rotation Davis-Jones et al (2001) points out that the tilting of horizontal vorticity into the vertical and the subsequent intensification of rotation due to stretching cannot explain the intensification of rotation near ground, because the tilted vortex tubes cannot intercept the ground and the vertical stretching is strongest above the ground. Davis-Jones points to the importance of the presence of rainy downdraft. Three roles are identified of the downdraft: A negatively buoyancy downdraft can impact the ground with considerable force and spring out rapidly. The downdraft may transport high-momentum air down to the surface. Cool downdraft enhances baroclinic boundaries therefore cause more generation of horizontal voriticity that can be tilted into vertical direction. The strong low-level rotation induces low-pressure and downward PGF  weakening of tornado – vortex volve effect.

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~250m) 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 (~50x50km) 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>120m/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 2000 x 83 model grid dx = 25m, dz min = 20m, dt=0.125s. 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 50m simulation t=3:44

Full Domain Surface Fields of 50m simulation See Movie

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

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

The model tornado as seen by a radar 8x8km

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 50m simulation

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

Flow-dependent Trajectories

Trajectory Animations

Preliminary Findings 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 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.

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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