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Intense Near-Surface Wind Shear in Severe
Thunderstorm Environments:
A Closer Look at Implications for Near-Surface Stability and Tornadogenesis Potential
Intense Near-Surface Wind Shear in Severe
Thunderstorm Environments:
A Closer Look at Implications for Near-Surface Stability and Tornadogenesis Potential
Dan MillerScience and Operations Officer
National Weather Service WFO Duluth, Minnesota
Dan MillerScience and Operations Officer
National Weather Service WFO Duluth, Minnesota
NWS Duluth Minnesota
Great Lakes Operational Meteorology Workshop 14 March 2012
Greg MannScience and Operations Officer
National Weather Service WFO Detroit/White Lake,
Michigan
Greg MannScience and Operations Officer
National Weather Service WFO Detroit/White Lake,
Michigan
CAPE/Shear RelationshipsCAPE/Shear Relationships
Some Forms of CAPE/Shear in use for a long time
Some Forms of CAPE/Shear in use for a long timeSfc-6 km shearSfc-6 km shear
Sfc-3 km shear/helicity
Sfc-3 km shear/helicity
Sfc-1 km shear/helicity
Sfc-1 km shear/helicity
EHI (0-1 km, 0-2 km)
EHI (0-1 km, 0-2 km)VGP (bulk, integrated)
VGP (bulk, integrated)
Bulk Shear: 0-1 km aglBulk Shear: 0-1 km agl
Focus Increasingly on Layers Progressively Closer to the Surface
Focus Increasingly on Layers Progressively Closer to the Surface
Integration with Boundary Layer/Inflow RHIntegration with Boundary Layer/Inflow RH
Near-Surface Shear: sfc-500m aglNear-Surface Shear: sfc-500m agl
0000 UTC Norman OK: 4 May 19990000 UTC Norman OK: 4 May 1999
1000 m agl
350 m agl
350 m agl 1000 m agl
Observed StormMotion
SFC Wind160 @17kt
350 m wind165 @41kt
Near-Surface Shear: sfc-500m aglNear-Surface Shear: sfc-500m agl
All of this critical “stuff” is going on in a very shallow near-
surface layer
All of this critical “stuff” is going on in a very shallow near-
surface layerRed = SFC - 400 m aglCyan = 400 m - 1000 m aglLavender = 1000 m - 7000 m agl
Red = SFC - 400 m aglCyan = 400 m - 1000 m aglLavender = 1000 m - 7000 m agl
Near-Surface Thermodynamic ProfilesNear-Surface Thermodynamic Profiles
Big tornado outbreak daysBig tornado outbreak days
1000 m agl
400 m agl
400 m agl1000 m agl
Observed StormMotion
SFC Wind195 @15kt
400 m wind200 @32kt
0000 UTC Pittsburgh PA: 1 June 19850000 UTC Pittsburgh PA: 1 June 1985
Near-Surface Thermodynamic ProfilesNear-Surface Thermodynamic Profiles
Big tornado outbreak daysBig tornado outbreak days
Near-Surface Thermodynamic ProfilesNear-Surface Thermodynamic Profiles
What about these profiles?What about these profiles?
Near-Surface Thermodynamic ProfilesNear-Surface Thermodynamic Profiles
What about these profiles?What about these profiles?
Theory: Richardson NumberTheory: Richardson NumberRichardson 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.
𝑹𝒊=
𝒈𝜽𝝏𝜽𝝏 𝒛
|𝝏𝒗𝝏𝒛 |𝟐
Theory: Richardson NumberTheory: 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
𝑹𝒊=
𝒈𝜽𝝏𝜽𝝏 𝒛
|𝝏𝒗𝝏𝒛 |𝟐
Implications for “Effective” StabilityImplications 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.
Ri Critical ThresholdsRi Critical ThresholdsGiven a constant delta-q (4 K here), consider the relationshipbetween stability (via depth) and bulk shear through the layer. Depth not only governs the overall stability; but it is alsoimportant to consider with regard to dissipative effects. Therefore,the greater the depth the lower the Ri should be to allowcirculation extension to the surface (circulation strength dependent)
Environmental Ri valuesclose to Ric may not beconducive for lengthycirculation maintenance,because the storm (not thecirculation) inflow may force Ri < Ric causing the available surface layer tolose shear.
Laye
r Dep
th
Bulk Shear Magnitude
delta = 4 q K
Using Ri Critical ThresholdsUsing 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 agl1000 m agl
Observed StormMotion
SFC Wind195 @15kt
400 m wind200 @32kt
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)
Using Ri Critical ThresholdsUsing 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
mesocycloneRi ~ 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
Cursory Example: 5-6 June 2010Cursory Example: 5-6 June 2010
Sfc-500m Richardson Number (shaded)Sfc-500m Bulk Shear (Black Contour)
03Z 04Z
05Z 06Z
MillburyEF4
DundeeEF2
DowagiacEF2
ConstantineEF2
ColtonEF2
LincolnEF3
ClayEF3
Several More EF0-1 in the favorable zone
Cursory Example: 29 February 2012Cursory Example: 29 February 2012
Cursory Example: 2 March 2012Cursory Example: 2 March 2012
BransonEF2
HarrisburgEF4
HenryvilleEF4 West Liberty
EF3
Sfc-500m Richardson Number (shaded)Sfc-500m Bulk Shear (Black Contour)
Case Example: 17 June 2010Case Example: 17 June 201017 June 2010 Outbreak: All Tornadoes17 June 2010 Outbreak: All Tornadoes
Case Example: 17 June 2010Case Example: 17 June 2010
Saint Croix Valley Tornado: 0144 UTC - 0205 UTC 18 June 2010
Saint Croix Valley Tornado: 0144 UTC - 0205 UTC 18 June 2010
Case Example: 17 June 2010Case Example: 17 June 2010
Case Example: 17 June 2010Case Example: 17 June 2010
SPC Mesoanalysis Data: 0200 UTC 18 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 2010Case Example: 17 June 2010
RUC PFC near Rush City, MN: 0200 UTC 18 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 2010Case Example: 17 June 2010
Richardson Number: 0000 UTC 18 June 2010
Richardson Number: 0000 UTC 18 June 2010
Case Example: 17 June 2010Case Example: 17 June 2010
Richardson Number: 0100 UTC 18 June 2010
Richardson Number: 0100 UTC 18 June 2010
Case Example: 17 June 2010Case Example: 17 June 2010
Richardson Number: 0200 UTC 18 June 2010
Richardson Number: 0200 UTC 18 June 2010
Case Example: 17 June 2010Case Example: 17 June 2010
KDLH Z/SRV ~0150 UTC: 18 June 2010KDLH Z/SRV ~0150 UTC: 18 June 2010
What happens if we superimpose an updraft perturbation?
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
Extremely Important CaveatsExtremely Important Caveats
How well do the models handle near-surface layers (0-500 m)?
How well do the models handle near-surface layers (0-500 m)?
Requires some knowledge of actual storm inflow layer
Requires some knowledge of actual storm inflow layer
Thanks For Your Attention
Thanks For Your Attention
Questions/Comments/Discussion?
Questions/Comments/Discussion?dan.j.miller@noaa
.govdan.j.miller@noaa
.govgreg.mann@noaa.
govgreg.mann@noaa.
gov