1. The Fifth International Conference on Mesoscale Convective Systems
The Fifth International Conference on Mesoscale Convective Systems October-3 November 2006
Mechanism of a Major Tornadogenesis in a Numerically-Simulated Supercell Storm*
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).

2. 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.
Dynamics of a supercell is now reasonably well understood.
However, the mechanism of a tornadogenesis in a supercell is not well understood yet.
Recent observational studies show that only 20% of radar-observed mesocyclones spawn a tornado.
Furthermore, Wakimoto and Cai showed that apparently similar morphologies of mesocyclones
do not assure a tornadogenesis.
These observations suggest that an existence of a mesocyclone alone may not be sufficient for a tornadogenesis.

3. 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.
One of the promising tools to study a tornadogenesis is a numerical simulation.
There have been several numerical studies on tornadogenesis from a supercell storm.
Wicker made the first successful simulation of a tornado using a one-way nesting.
Wicker & Wilhelmson simulated two tornadoes using a two-way nesting.
These studies, however did not examine the structure of the tornado vortex.
Grasso and Cotton also used two-way nested grids to simulate a tornado.
All studies, however, introduced nested grids slightly before the coarse grid simulation attains
a maximum circulation. Thus, there is some worry about how the flow field adjusted during
the introduction of the nested grid and how it affected the tornadogenesis.

4. Objectives of the present study
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.
The objectives of the present study are as follows:
First, to examine if a tornado spawned by a supercell storm is successfully simulated with a model having a horizontally uniform very fine mesh.
Second. if we succeed in simulating the vortex, clarify the mechanism of the tornadogenesis and examine the detailed structure of the tornado vortex.
Third, To obtain a clue as to why a mesocyclone alone is sufficient for a tornadogenesis.

5. 2.Model and experimental setting
ARPS (Advanced Regional Prediction Model) Ver (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
The model used for the simulation is ARPS Ver It is a non-hydrostatic compressible model. The calculation domain is about 66km by 66km horizontally and 15km vertically. The grid interval is 70m in the horizontal direction.
Vertical grid size varies from 10m near the ground to 760m near the top of the calculation domain. The boundary condition is open at the lateral boundary. Free-slip boundary condition is used at the bottom and top boundaries. A Rayleigh type damping is introduced above 12km to prevent internal gravity waves to reflect from the top boundary. We consider only warm rain process for simplicity. Turbulent mixing has been calculated by predicting turbulent kinetic energy.

6. Del City Storm v u Temperature and mixing ratio
cf. Grasso & Cotton(1995)
Wind hodograph
CAPE=3218m2s-2 Ri=53 v
20 May 1977
The case we study numerically is a storm called Del City Storm that occurred on 20 May It produced a F2 tornado in Del City, Oklahoma at 1847 CST. The upper air soundings at Fort Sill at 1500 CST and at Elmore City 1620 CST are composited to give the horizontally-uniform initial basic state of the model atmosphere.
This shows the vertical profiles of wind, temperature and mixing ratio of water vapor. The wind hodograph shows that the wind vector turns clockwise with increasing height and becomes nearly westerly at the height of 10km. The thermodynamics sounding shows that the air lifted from the surface starts condensation at the height of about 1km. Once condensation is started, the lifted air becomes warmer than the environment and continues to rise throughout the troposphere.
(del_hodo.gif: del_soud.gif) u
Composite of 1500 CST at Ft. Sill and 1620 CST at Elmore City

7. ・Time integration time-splitting
・Initialization　　　
　　　 horizontally uniform basic state (Composite of 1500 CST at Ft. Sill and 1620 CST at Elmore City on 20 May )
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.
The calculation was started from a horizontally uniform basic state by imposing an ellipsoidal thermal bubble to initiate the convection. The time integration was performed by using a time-splitting method. The time step for sound waves is 0.03s, while it is 0.18s for convective motions. The spatial advection scheme is 4th order in the horizontal and 2nd order for the vertical. The coordinate was translated at the speeds of 3m/s to the east and 14m/s to the north in order to keep the storm in the calculation domain.

8. 3.Results

9. Time evolution of the storm
This shows the time evolution of the rainwater distribution of the simulated storm. The rainwater pattern evolves in a way similar to previous storm-scale simulation studies. After 40 min a hook-shaped pattern appearas at the southwest corner. Its tip starts to roll up by 70 min. The tornado is located right here. By 80min this region is filled with rainwater and the tornado dissipates. Let us look at an enlarged view of the tornadic region given by the square.

10. Rainwater mixing ratio Doppler velocity
11km
mesocyclone
tornado
(z=1km at t=4500s)

11. Evolution of tornado & funnel
(Viewed from southwest)
t= s (dt=2.88 x 51 frames)
gray: cloudwater >0.3g/kg
red: vertical vorticity > 0.7s-1
Now I am going to present a movie that shows a development of tornado with a funnel cloud viewed from the southwest direction. The time period shown here is about 2 min and a half between 4406 and 4550 seconds.
Gray color shows the surface of cloudwater larger than 0.3g/kg and red vertical vorticity larger than 0.7s-1.
This horizontal line shows the ground surface. The horizontal width of the view is 8.4km.
ground surface
8.4km

12. Time-height cross section
hPa
Min. perturbation pressure
m/s
Max. updraft
s-1
Max. vertical vorticity

13. Time-height cross section
hPa
Min. perturbation pressure
m/s
Max. updraft
s-1
This shows similar time-height cross section except the vertical of extent is halved and the time range is between 3000 and 5000 seconds. ( maxmin-large.gif; maxmin-small.gif) The comments in the previous figure is more easily observed. Stage II begins when the pressure minimum at 2km starts to deepen. This pressure minimum causes acceleration of the updraft below 2km. After t=4000 second, another pressure minimum appears at the height of 800m, which causes rapid increase of updraft between 1000 and 2000m. This increase in the updraft at 1km appears to cause the tornadogenesis.
Max. vertical vorticity
Stages I II
III IV

14. Time evolution of vertical vorticity
(t= s)
mesocyclone
z=1km
This shows
warm
z=5m
gust front
contour interval : 0.05s-1
shade >0.01s-1
cool

15. Relationship between tornado and low-level updraftC D E F
(contour)
vertical vorticity at z=5m
(shade) updraft at z=200m km
cf. Bluestein et al. (2003) km

16. Structure of the tornado vortex
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)

17. Vorticity budget of the tornado vortex (at z=5m)
m/s
s-2
Vertical velocity
Total
vorticity vector
Stretching
Tilting
advection
vert. vorticity=0.2s-1
Advection (horizontal)
Advection (total)
tilting
stretching

18. Vorticity budget analysis along back trajectories
10-min backward trajectory of 30 points on the 0.2s-1 vorticity contour line at z=85m
dashed line: potential temp.
arrows : veclocity vector
m/s

19. |horizontal vorticity| (s-1) stretching (x10-2s-2)
0.05
0.00
-0.05
0.10
0.15
0.20
0.25
|horizontal vorticity| (s-1)
stretching (x10-2s-2)
tilting (x10-2s-2)
vertical vorticity (s-1)

20. 4.Summary
　　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.

21. 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.

22. Thank you!

23. Train derailment Typhoon Shanshan （T0613) tornado typhoon center
MODIS/AQUA 1324JST 17 SEP 2006
Train derailment
tornado
typhoon center
Typhoon Shanshan　（T0613) （
3 persons died and 143 injured.

24. Bassett, Nebraska tornado on 5 June 1999
Reflectivity
Doppler velocity
tornado
vortices
Bluestein et al.(2003, MWR)