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 Dan Miller Science and Operations Officer National Weather Service WFO Duluth, Minnesota NWS Duluth Minnesota Great Lakes Operational Meteorology Workshop14 March 2012 Greg Mann Science and Operations Officer National Weather Service WFO Detroit/White Lake, Michigan Greg Mann Science and Operations Officer National Weather Service WFO Detroit/White Lake, Michigan

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)

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

Near-Surface Shear: sfc-500m agl 0000 UTC Norman OK: 4 May m agl 350 m agl 1000 m agl Observed Storm Motion SFC Wind 350 m wind

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

Near-Surface Thermodynamic Profiles Big tornado outbreak days 1000 m agl 400 m agl 1000 m agl Observed Storm Motion SFC Wind 400 m wind 0000 UTC Pittsburgh PA: 1 June 1985

Near-Surface Thermodynamic Profiles Big tornado outbreak days

Near-Surface Thermodynamic Profiles What about these profiles?

Near-Surface Thermodynamic Profiles What about these profiles?

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. 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.

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 Ri c = 0.25 Theoretical critical threshold for forced Turbulence Ri c < Ri < 1 Graduated mechanical mixing 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 Ri c = 0.25 Theoretical critical threshold for forced Turbulence Ri c < Ri < 1 Graduated mechanical mixing

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 < Ri c ) establishing an inflow within the intense shear layer reservoir. 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 < Ri c ) establishing an inflow within the intense shear layer reservoir.

Ri Critical Thresholds Given a constant delta-  (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) Given a constant delta-  (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 Ri c may not be conducive for lengthy circulation maintenance, because the storm (not the circulation) inflow may force Ri < Ri c causing the available surface layer to lose shear. Environmental Ri values close to Ri c may not be conducive for lengthy circulation maintenance, because the storm (not the circulation) inflow may force Ri < Ri c causing the available surface layer to lose shear. Layer Depth Bulk Shear Magnitude delta 

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.) 400 m agl 1000 m agl Observed Storm Motion SFC Wind 400 m wind Identify regions of appreciable 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)

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

Cursory Example: 5-6 June 2010 Sfc-500m Richardson Number (shaded) Sfc-500m Bulk Shear (Black Contour) Sfc-500m Richardson Number (shaded) Sfc-500m Bulk Shear (Black Contour) 03Z04Z 05Z 06Z Millbury EF4 Millbury EF4 Dundee EF2 Dundee EF2 Dowagiac EF2 Dowagiac EF2 Constantine EF2 Constantine EF2 Colton EF2 Colton EF2 Lincoln EF3 Lincoln EF3 Clay EF3 Clay EF3 Several More EF0-1 in the favorable zone Several More EF0-1 in the favorable zone

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

Case Example: 17 June June 2010 Outbreak: All Tornadoes

Case Example: 17 June 2010 Saint Croix Valley Tornado: 0144 UTC UTC 18 June 2010

Case Example: 17 June 2010

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

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

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

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

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

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

DiscussionDiscussion 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 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

Thanks For Your Attention Questions/Comments/Discussion?