Mechanism of a Major Tornadogenesis in

a Numerically-Simulated Supercell Storm*

The Fifth International Conference on Mesoscale Convective Systems . 31 October-3 November 2006

Akira T. NODA*1,2 and Hiroshi NIINO*1

*1Ocean Research Institute, The University of Tokyo*2 Frontier Research Center for Global Change

*A part of the present content has been published (Noda and Niino, SOLA, 1, 5-8 (2005).

1.Introduction

Dynamics of a supercell is reasonably well understood (e.g., Klemp, 1987).

However, the mechanism of a tornadogenesis in a supercell, is not still well clarified.

Recent observations show:

1) Only 20% of mesocyclones spawn a tornado (Burgess, 1997).

2) Apparently similar morphologies of mesocyclones do not necessarily assure a tornadogenesis (Wakimoto and Cai, 2000).

Existence of a mesocyclone alone may not be sufficient for a tornadogenesis.

Previous numerical studies on a supercell tornado

・ Wicker (1990) One way nesting (fine horizontal grid: 70m) Vertical resolution:50m near the surface

・ Wicker & Wilhelmson(1995) Two way nesting (120m fine grids in 600m coarse grids). Vertical resolution:120m near the surface. Structure of the tornado vortex unexamined.

・ Grasso & Cotton(1995) Two-way nesting (horizontal grid: 111m, 333m,1km) 　 Vertical resolution: 25m near the surface. 　　　　　 Little analysis of the tornadogenesis process.

All studies introduced nested grids slightly before the coarse grid simulation attains a maximum circulation.

Objectives of the present study

1) To examine if a tornado spawned by a supercell storm is successfully simulated with a model having a horizontally uniform very fine mesh.

2) To clarify the mechanism of the tornadogenesis and examine the detailed structure of the tornado vortex.

3) To obtain a clue to understand why a mesocyclone alone is not sufficient for a tornadogenesis.

2.Model and experimental setting

ARPS (Advanced Regional Prediction Model) Ver. 4.5.1 (Xu et al., 1995)

・ Non-hydrostatic compressible model

・ Calculation domain 　 66.36kmx66.36kmx15.08km

・ Grid interval 　 horizontal: 70m, vertical: 10 ～ 760m (951x951x45)

・ Boundary conditions

　　 lateral: open(radiation) （ Durran and Klemp, 1983 ）

　　 vertical: free-slip (w=0, du/dz=dv/dz=0)

Rayleigh damping (e-folding time 300s) above 12km

・ Cloud physics 　 warm rain (Kessler type parameterization)

　　　　　　　　　 autoconversion, accretion(collection)

・ Turbulent mixing 　　 TKE of order 1.5

Temperature and mixing ratioDel City Storm

Wind hodograph

u

v

Composite of 1500 CST at Ft. Sill and 1620 CST at Elmore City

20 May 1977

CAPE=3218m2s-2 Ri=53

cf. Grasso & Cotton(1995)

・ Initialization　　　　　　 horizontally uniform basic state (Composite of 1500 CST

at Ft. Sill and 1620 CST at Elmore City on 20 May 1977 )

ellipsoidal thermal bubble at x=30km,y=30km,z=1.5km. (maximum anomaly of 4K;horizontal radius of

10km, vertical radius of 1.5km)

・ Time integration time-splitting

　　　 for sound waves 　　 Δt=0.03s 　 vertically implicit for w and p.

　　　 for convective motion 　 Δt=0.18s centered difference with Asselin filter(0.1)

・ Spatial finite difference scheme

horizontal advection 　 4th order, 　 vertical advection 　 2nd order

・Grid translation　 3m/s to the east and 14m/s to the north.

3.Results

Time evolution of the storm

(z=1km at t=4500s)

11km

11km

Rainwater mixing ratio Doppler velocity

tornado

mesocyclone

t=4406--4550s (dt=2.88 x 51 frames)

Evolution of tornado & funnel

gray: cloudwater >0.3g/kg

red: vertical vorticity > 0.7s-1

ground surface

(Viewed from southwest)

8.4km

Time-height cross section

Max. vertical vorticity

Max. updraft

Min. perturbation pressure

hPa

m/s

s-1

Time-height cross section

III III IVStages

Max. vertical vorticity

Max. updraft

Min. perturbation pressure

hPa

m/s

s-1

Time evolution of vertical vorticity

contour interval : 0.05s-1

shade >0.01s-1

z=1km

z=5m

(t=3900-4587s)

cool

warm

mesocyclone

gust front

Relationship between tornado and low-level updraft

(contour)vertical vorticity at z=5m

(shade) updraft at z=200m

km

km

AB

CD

E

F

cf. Bluestein et al. (2003)

z=5m z=100m

z=500m z=1000m

vertical vorticity (contour)

updraft (color shade)

The tornado vortex is located at the boundary between updraft and downdraft (e.g., Lemon & Doswell, 1979)

Structure of the tornado vortex

Vorticity budget of the tornado vortex (at z=5m)

Total

Tilting

Vertical velocity

Advection (total) Advection (horizontal)

Stretching

vert. vorticity=0.2s-1

( , , )

u v wx y z

t

w w wx y z

ω

tilting stretching

advection

vorticity vector

m/s

s-2

Vorticity budget analysis along back trajectories

dashed line: potential temp.arrows : veclocity vector

10-min backward trajectory of 30 points on the 0.2s-1 vorticity contour line at z=85m

m/s

|horizontal vorticity| (s-1)

stretching (x10-2s-2)

tilting (x10-2s-2)

vertical vorticity (s-1)

0.05

0.00

-0.05

0.10

0.15

0.20

0.25

　　 1. A supercell tornado with a funnel cloud is successfully simulated.

　　 2. Several different processes proceed in the supercell before the tornadogenesis.

　　 3. Coupling of the updraft in the low-level mesocyclone and one of the vortices along the gust front appears to cause the tornadogenesis. (This may explain why a mesocyclone alone is not sufficient for producing a tornado.)

　　 4. The direct source of the vorticity for a tornado appears to be the vertical vorticity of the gust front, which originally comes from tilting of horizontal vorticity.

5. Simulated tornado is located at the boundary between updraft and downdraft.

4.Summary

Future subjects

1.More detailed analysis of the tornadogenesis process.

2.Sensitivity study of a tornadogenesis to wind hodograph.

3.Further improvement in the horizontal resolution.

4.Introducing a frictional boundary layer.

Thank you!

Typhoon Shanshan 　（ T0613)（ http://earthobservatory.nasa.gov/NaturalHazards/natural_hazards_v2.php3?img_id=13878 ）

MODIS/AQUA 1324JST 17 SEP 2006Train derailment

3 persons died and 143 injured.

tornado

typhoon center

Bluestein et al.(2003, MWR)

Bassett, Nebraska tornado on 5 June 1999

tornado

vortices

Doppler velocityReflectivity