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Dan Miller Science and Operations Officer National Weather Service WFO Duluth, Minnesota

Intense Near-Surface Wind Shear in Severe Thunderstorm Environments: A Closer Look at Implications for Near-Surface Stability and Tornadogenesis Potential. Dan Miller Science and Operations Officer National Weather Service WFO Duluth, Minnesota. Greg Mann Science and Operations Officer

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Dan Miller Science and Operations Officer National Weather Service WFO Duluth, Minnesota

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  1. Intense Near-Surface Wind Shear in Severe Thunderstorm Environments:A Closer Look at Implications for Near-Surface Stability and Tornadogenesis Potential Dan Miller Science and Operations Officer National Weather Service WFO Duluth, Minnesota Greg Mann Science and Operations Officer National Weather Service WFO Detroit/White Lake, Michigan Great Lakes Operational Meteorology Workshop 14 March 2012 NWS Duluth Minnesota

  2. CAPE/Shear Relationships Some Forms of CAPE/Shear in use for a long time Sfc-6 km shear Sfc-3 km shear/helicity Sfc-1 km shear/helicity EHI (0-1 km, 0-2 km) VGP (bulk, integrated)

  3. Bulk Shear: 0-1 km agl Focus Increasingly on Layers Progressively Closer to the Surface Integration with Boundary Layer/Inflow RH

  4. Near-Surface Shear: sfc-500m agl 350 m agl 1000 m agl 350 m wind 165 @41kt SFC Wind 160 @17kt 1000 m agl Observed Storm Motion 350 m agl 0000 UTC Norman OK: 4 May 1999

  5. Near-Surface Shear: sfc-500m agl All of this critical “stuff” is going on in a very shallow near-surface layer Red = SFC - 400 m agl Cyan = 400 m - 1000 m agl Lavender = 1000 m -7000 m agl

  6. Near-Surface Thermodynamic Profiles Big tornado outbreak days 400 m agl 1000 m agl 400 m wind 200 @32kt SFC Wind 195 @15kt Observed Storm Motion 1000 m agl 400 m agl 0000 UTC Pittsburgh PA: 1 June 1985

  7. Near-Surface Thermodynamic Profiles Big tornado outbreak days

  8. Near-Surface Thermodynamic Profiles What about these profiles?

  9. Near-Surface Thermodynamic Profiles What about these profiles?

  10. Theory: Richardson Number Richardson Number in general describes the ability of a fluid to mix and the modality of the mixing process. Mathematically - it is the ratio of Thermal Stratification to Shearing Potential. In practice - it is useful for identifying regions of free convection, forced turbulence, forced mechanical mixing, and laminar/stratified flow.

  11. Theory: Richardson Number Ri < 0 indicates convective instability (only the numerator can be negative) Ri ~ 1 indicates thermal stratification is balancing mechanical mixing Ri > 1 indicates laminar flow Ric = 0.25 Theoretical critical threshold for forced Turbulence Ric <Ri < 1 Graduated mechanical mixing

  12. Implications for “Effective” Stability • Stratification is necessary for the preservation of strong near surface shear - minimizes momentum mixing. • Stratified regions are not available in whole, rather in laminated layers - so depth considerations are important when assessing the progressive availability of the entire depth. • Availability of shear for a rotating updraft increases as rotational velocity increases. • localized speed maxima associated with the circulation bore into the stratified region via localized shear instability (Ri < Ric) establishing an inflow within the intense shear layer reservoir.

  13. Ri Critical Thresholds Given a constant delta-q (4 K here), consider the relationship between stability (via depth) and bulk shear through the layer. Depth not only governs the overall stability; but it is also important to consider with regard to dissipative effects. Therefore, the greater the depth the lower the Ri should be to allow circulation extension to the surface (circulation strength dependent) Environmental Ri values close to Ric may not be conducive for lengthy circulation maintenance, because the storm (not the circulation) inflow may force Ri < Riccausing the available surface layer to lose shear. delta q = 4 K Layer Depth Bulk Shear Magnitude

  14. Using Ri Critical Thresholds First things first, diagnose regions favorable for deep organized convection (including elevated) via parameter space evaluation (SPC meso page/LAPS/etc.) Identify regions of appreciable 0-500 m agl bulk shear (0-1 km often too deep) Especially coincident with relatively high CINH (> 50 J/kg) (i.e. nocturnal/pre warm front) 400 m agl 1000 m agl 400 m wind 200 @32kt SFC Wind 195 @15kt Observed Storm Motion

  15. Using Ri Critical Thresholds Assess availability of accessing shear given a superimposed circulation using 0-500 m agl Ri: Ri > 1 - generally unavailable Ri ~ 1 - only accessible to a very strong parent mesocyclone Ri ~ 0.5 ± 0.25 - shear layer available to localized perturbation Ri < 0.25 - turbulence disrupts ambient shear (i.e. shear transitions to flow) additional storm scale modulation necessary Ri < 0 - free convective turbulence encourages large eddies

  16. Cursory Example: 5-6 June 2010 Dowagiac EF2 Millbury EF4 Clay EF3 Colton EF2 03Z 04Z Sfc-500m Richardson Number (shaded) Sfc-500m Bulk Shear (Black Contour) Constantine EF2 Dundee EF2 Several More EF0-1 in the favorable zone Lincoln EF3 05Z 06Z

  17. Cursory Example: 29 February 2012 Harrisburg EF4 Branson EF2 Cursory Example: 2 March 2012 Sfc-500m Richardson Number (shaded) Sfc-500m Bulk Shear (Black Contour) Henryville EF4 West Liberty EF3

  18. Case Example: 17 June 2010 17 June 2010 Outbreak: All Tornadoes

  19. Case Example: 17 June 2010 Saint Croix Valley Tornado: 0144 UTC - 0205 UTC 18 June 2010

  20. Case Example: 17 June 2010

  21. Case Example: 17 June 2010 SBCAPE/CIN 0-6 km Bulk Shear 0-1 km Bulk Shear SPC Mesoanalysis Data: 0200 UTC 18 June 2010 0-1 km SRH 100 mb LCL Height LCL-LFC Mean RH

  22. Case Example: 17 June 2010 RUC PFC near Rush City, MN: 0200 UTC 18 June 2010 Very Strong sfc-500 m agl bulk shear Sig SBCIN, but not “capped”

  23. Case Example: 17 June 2010 Richardson Number: 0000 UTC 18 June 2010

  24. Case Example: 17 June 2010 Richardson Number: 0100 UTC 18 June 2010

  25. Case Example: 17 June 2010 Richardson Number: 0200 UTC 18 June 2010

  26. Case Example: 17 June 2010 What happens if we superimpose an updraft perturbation? KDLH Z/SRV ~0150 UTC: 18 June 2010

  27. Discussion • Significant/Violent long-track tornadoes are typically coincident with environments containing extreme near surface shear • Near surface stratification is necessary for the production of significant surface layer shear • Richardson Number is very useful in accessing the “effective” stability and accessibility of the near surface shear layer to a superimposed circulation Extremely Important Caveats How well do the models handle near-surface layers (0-500 m)? Requires some knowledge of actual storm inflow layer

  28. Thanks For Your Attention Questions/Comments/Discussion? dan.j.miller@noaa.gov greg.mann@noaa.gov

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