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Mechanism of a Major Tornadogenesis in a Numerically-Simulated Supercell Storm*

The Fifth International Conference on Mesoscale Convective Systems . 31 October-3 November 2006. Mechanism of a Major Tornadogenesis in a Numerically-Simulated Supercell Storm*. Akira T. NODA* 1,2 and Hiroshi NIINO* 1 * 1 Ocean Research Institute, The University of Tokyo

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Mechanism of a Major Tornadogenesis in a Numerically-Simulated Supercell Storm*

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  1. The Fifth International Conference on Mesoscale Convective Systems . 31 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.

  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.

  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.

  5. 2.Model and experimental setting ARPS (Advanced Regional Prediction Model) Ver. 4.5.1 (Xu et al., 1995) ・Non-hydrostatic compressible model ・Calculation domain66.36kmx66.36kmx15.08km ・Grid intervalhorizontal: 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 physicswarm rain (Kessler type parameterization) autoconversion, accretion(collection) ・Turbulent mixingTKE of order 1.5

  6. Del City Storm Temperature and mixing ratio cf. Grasso & Cotton(1995) Wind hodograph CAPE=3218m2s-2 Ri=53 v 20 May 1977 u Composite of 1500 CST at Ft. Sill and 1620 CST at Elmore City

  7. ・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 integrationtime-splitting for sound wavesΔt=0.03svertically implicit for w and p. for convective motionΔt=0.18s centered difference with Asselin filter(0.1) ・Spatial finite difference scheme horizontal advection4th order,vertical advection2nd order ・Grid translation3m/s to the east and 14m/s to the north.

  8. 3.Results

  9. Time evolution of the storm

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

  11. Evolution of tornado & funnel (Viewed from southwest) t=4406--4550s (dt=2.88 x 51 frames) gray: cloudwater >0.3g/kg red: vertical vorticity > 0.7s-1 ground surface 8.4km

  12. hPa Time-height cross section 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 Max. vertical vorticity Stages I II III IV

  14. Time evolution of vertical vorticity (t=3900-4587s) mesocyclone z=1km warm z=5m gust front contour interval : 0.05s-1 shade >0.01s-1 cool

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

  16. z=5m z=100m z=500m z=1000m Structure of the tornado vortex 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. 0.25 0.20 0.15 0.10 stretching (x10-2s-2) 0.05 vertical vorticity (s-1) 0.00 -0.05 |horizontal vorticity| (s-1) tilting (x10-2s-2)

  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. MODIS/AQUA 1324JST 17 SEP 2006 Train derailment tornado typhoon center Typhoon Shanshan　（T0613) （http://earthobservatory.nasa.gov/NaturalHazards/natural_hazards_v2.php3?img_id=13878） 3 persons died and 143 injured.

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

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