Tornadogenesis within a Simulated Supercell

Storm

Ming XueSchool of Meteorology and

Center for Analysis and Prediction of StormsUniversity of Oklahoma

mxue@ou.eduAcknowledgement: NSF, FAA and PSC

22nd Severe Local Storms Conference22nd Severe Local Storms Conference6 October 20046 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, dzmin = 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=3300CAPE=3300J/kgJ/kg

Storm-relative Hodograph

h

50m simulation shown in full 50x50 km

domain

Full Domain Surface Fields of 50m simulation

t=3h 44mt=3h 44m

Red – positive Red – positive vertical vorticityvertical vorticity

25 m simulation surface fields shown in

subdomains

Near surface vorticity, wind, reflectivity, and temperature

perturbation

2 x 2 km2 x 2 km

Vort ~ 2 sVort ~ 2 s-1-1

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 ~120m/s max surface windssurface winds

>80mb pressure drop>80mb pressure drop+50m/s+50m/sin ~1minin ~1min

Pressure time series in vicinity of Allison TX F-4 Tornado on 8 June

1995 (Winn et al 1999)

850mb850mb

910mb910mb

>50mb >50mb pressure droppressure drop

Lee etc (2004)22nd 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 SouthView from South

t=13250st=13250sbeginning of beginning of vortex intensificationvortex intensification

3km3km

View from SWView from SW

NN

t=13250st=13250sbeginning of beginning of vortex intensificationvortex intensification

3km3km

Trajectory Animations

View from NortheastView from Northeast

3km3km

RFD ofRFD of11stst cell cell

FFD ofFFD of22ndnd cell cell

InflowInflowfrom eastfrom east

Low-level jump flowLow-level jump flow

Browning’s Conceptual Model of Supercell

Storm

Diagnostics along Trajectories

Orange portion t=13250-500s – 13250+200s

t=13250st=13250sBeginning of low-level Beginning of low-level spinupspinup

14km14km

X Y Z

8km8km

WVh

Streamwise Vort.Cross-stream Vort.Horizontal Vort.

Vertical Vort.Vertical Vort.Total VortTotal Vort..

13250132501275012750 1345013450

Force along trajectoryForce along trajectory

BuoyancyBuoyancyVert. PgradVert. PgradSum of the twoSum of the two

Perturbation pressurePerturbation pressure-76mb-76mb

55

-5-5

1325013250

~2 m s~2 m s-2-2

+b' due to -p'+b' due to -p'

Orange portion t=13250-500s – 13250+200s

t=13250st=13250sBeginning of low-level Beginning of low-level spinupspinup

14km14km

rapid parcel riserapid parcel rise

X Y Z

8km8km

WVh

Streamwise Vort.Cross-stream Vort.Horizontal Vort.

Vertical Vort.Vertical Vort.Total VortTotal Vort..

13250132501275012750 1345013450

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 1st or 2nd 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 1st storm in this case

• etc etc etc.

Movie of

Cloud Water Field25 m, 7.5x7.5km domain, 30

minutes

Questions / Comments?