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

NWS Duluth MinnesotaGreat Lakes Operational Meteorology Workshop 14 March 2012

Greg MannScience and Operations Officer

National Weather Service WFO Detroit/White Lake,

Michigan

CAPE/Shear RelationshipsSome Forms of

CAPE/Shear in use for a long timeSfc-6 km shearSfc-3 km

shear/helicitySfc-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 SurfaceIntegration with Boundary Layer/Inflow RH

Near-Surface Shear: sfc-500m agl

0000 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 agl

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

Near-Surface Thermodynamic Profiles

Big tornado outbreak days

1000 m agl400 m agl

400 m agl1000 m agl

Observed StormMotion

SFC Wind195 @15kt

400 m wind200 @32kt

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 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 NumberRi < 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” 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 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 q = 4 K

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 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 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 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 2012

Cursory 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 201017 June 2010 Outbreak: All Tornadoes

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

UTC 18 June 2010

Case Example: 17 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 2010RUC 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?

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 CaveatsHow 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?dan.j.miller@noaa

.govgreg.mann@noaa.

gov