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Shear and Buoyancy Associated with

70 Tornadic and Non-TornadicThunderstorms in Northern and

CentralCalifornia, 1990-1994

Presented by

John P. MonteverdiProfessor of MeteorologyDepartment of Geosciences

San Francisco State University

Visiting Scientist Spring 2000 National Severe Storms Lab

Norman, Oklahoma

National Weather Service Forecast OfficeSan Francisco Bay Area

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Collaborators on this research

Charles Doswell III National Severe Storms Laboratory Norman, Oklahoma

Gary Lipari MS Thesis Candidate San Francisco State University

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Organization of Talk

• Purpose of Study

• Overview of Analysis Procedures

• Results of Study

• Implications for Operations: Possible Thresholds

• Role of Shear in Tornadic Thunderstorms

• Types of Tornadic Thunderstorms

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Purposes of Study

To determine if buoyancy played a significant role in distinguishing between tornadic and non-tornadic thunderstorms in the study period

To determine if shear, particularly in the 0-1 km and 0-2 km layers, was a distinguishing characteristic between tornadic and non-tornadic thunderstorms AND between the weaker and stronger tornadic events

To determine if the data array and the statistical analyses of the results suggested possible “threshold values” to be used operationally

No!

Yes!

Possibly

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Analysis Technique

• Used soundings from OAK (mostly 00Z) (one VBG, one MFR), modified by surface conditions at site closest to event

• Considered 3 different event types for period 1990-1994, inclusive– NULL cases … all cases in which thunder observed at SAC or FAT but no observed tornadoes in California

– F0 tornado cases (suspect most non-supercells)

– F1+ tornado cases (suspect many/most supercells)

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• All cases included, not just the cool season events, although most tornado events (28 of 30) were in the cool season (November-April)

• Nearly half of null events (19/40) were warm season

• Buoyancy calculated via “SHARP” program, updated with obs from nearest surface site

• Shears calculated two ways:– Positive shear calculated by SHARP (portion of hodograph in which wind veers and speed increases with height) (Monteverdi and Lipari portion of study)

– as vector differences between top and bottom of the layers (0-1, 0-2, 0-3, and 0-6 km … all AGL), updated with surface observations (Doswell and Monteverdi portion of study)

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Current Directions of Research

• Expansion of California data set in two phases: 1995-present and 1950-1989 (with C. Doswell III)

• Comparison with low-buoyancy high-shear cases in Australia (with C. Doswell III and B. Hanstrum, Australian Meteorological Services)

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Review of Vertical Shear Concepts

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What isVertical Shear?

• Is a measure of the change in wind direction and speed with height

• Is estimated visually best from a hodographThe length of thehodograph is proportionalto the magnitude of the shear through the layer

Arrows joining windobservations at variouslevels show the shear vectorin the intervening layer.

In this case, the wind and the wind shear vectors are veeringwith height

The dots representthe tips of the windobservations at eachlevel.

This case shows a clockwise CURVED HODOGRAPH.

Shear associated with a veering wind with heightis called POSITIVE SHEAR. Positive Shear valuesare greatest in curved hodographs (in which thewind shear vectors also veer with height).

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Straight Hodogaph

Wind Veers and IncreasesIn Strength ThroughLowest Layers

However, Wind Shear Vector Does NOT VeerTo Any Great Degree

There is positive shear in this straight hodograph. But note that the wind shear vector does not veer with height.

That is why positive shear values tend to be less for straight hodographs.

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Importance of Shear

• Removes precipitation from updraft area and shunts it down wind (updraft is not suppressed and becomes more long-lived)

• Deep layer shear can create horizontal vorticity which can be tilted into the vertical by the updraft and transformed to vertical vorticity (storm scale rotation--mesocyclone)

• In certain configurations of positive shear, updraft is augmented to such a degree, that the buoyancy can be magnified by a factor of two to three times

• In certain configurations of positive shear, updraft strength can be augmented greatly on right flank of storm, causing the storm to “deviate” from motions of other storms (developing strong storm relative helicity and a greater tendency to become tornadic)

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Positive Shear

• Advantages– Is largest for veering wind shear vector profiles

(typical shear environments for right moving supercells)– Is calculated as a matter of course by programs like

SHARP (still used in many offices)

• Disadvantages– Is not displayed routinely as part of AWIPS package– Is not easily calculated by “back of envelope”

calculations, as bulk shear is (vector difference between wind at upper end and bottom ends of layer in question)

– May distract forecaster from consideration of atypical cases (e.g., Sunnyvale May 1998 F2 anticyclonic supercellular tornado)

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Review

• In short, storms growing in an environment of “rich” positive shear have a greater likelihood of being SEVERE and in some configurations of wind shear tend to “create” their own rotation. SUPERCELLS

• Storms growing in an environment without shear tend not to be severe and can only become tornadic by intercepting and ingesting pre-existing rotation. NON-SUPERCELLS

• Either may be tornadic, but the strongest tornadoes and most severe weather occur in association with supercells.

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Review of Tornadic Thunderstorm “Types”

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Types of Tornadic Thunderstorms Observed in California

Minimal Deep Layer Shear: Non-supercell Tornadic Storms (tornadic rotation associated with misocyclones) •Landspout Single Cell Storms (includes what

are called “cold core” or “high-based” funnels)• Multi-cells (Hodogaphs of small length-)

Great Deep Layer Shear With Curved Hodograph: Isolated Supercell Tornadic Storms (tornadic rotation tends to be mesocyclone-induced)• Those occurring in low buoyancy environments

tend to have relatively small dimensions: “low topped” or “mini-supercells”)Great Deep Layer Shear With “Flawed” Curved Hodograph:

Isolated Supercell Tornadic Storms (mesocyclone/misocyclone hybrid)• Supercell intercepts pre-existing low level rotation

Moderate to Strong Deep Layer Shear With Straight Hodograph: Supercell “Line” Storms ( tornadic rotation tends to develop when storm ingests misocyclone or “shear” funnels develop at intersection of bows)

•Line (Bowed Segment) Storms•Splitting isolated supercells (generally outflow dominated)

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Weak Deep Layer Shear: Single Cell Non-supercell tornadic storms: Landspout Hypothesis

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Thunderstorm does not have pre-existing rotation. Rotation exists in low level environment because of intersection of boundaries, horizontal shear along fronts or squall-lines, generation of vortices by topography. There may be a greater tendency for such low level rotation to develop and be intensified in an environment of LARGE low level (0-1 km) positive shear.

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Small Hodograph Length

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Moderate Deep Layer Shear: Straight Hodograph with Large Low Level Shear

Large Deep Layer Shear:

Straight Hodograph With Large Low Level Shear and Storm Motions Parallel To Line/Boundary

Bow Echoes: “Squall Line” With Bowed Segments

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January 9, 1995

Straight Hodograph, ButLarge Speed Shear

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Dry Layer InMid Troposphere

Moist UnstableLayer Near Ground

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Prototype Wet Microburst/Bow Echo

Sounding vs Sacramento 1/9/95

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Schematic Showing Strongest Reflectivity along Line With Bowed Segments

Sites of PossibleRotation/Tornadoes

BowSegments

7PM PST January 9, 1995

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KDAX RadarReflectivity7:00PM PST1/9/95

Initial Storm Motion On The HodographAnd Similar To Mean Wind

Interferring Outflow BoundariesProduce Bowed Segments--Bows Move SlightlyTo Right OfAnd Slower Than Mean Wind

Position of SubsynopticTrough -- Storm MotionParallel To Line

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30Evidence of rotation at tip of bow

Storm Relative Velocities6:30 PM 1/9/95

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Large Deep Layer Shear (Curved Hodograph):

Supercell Thunderstorm• A thunderstorm with a deep and persistant mesocyclone

• Deep is generally taken to mean 1/4 to 1/3 depth of precipitation echo

• Persistancy is generally taken to imply that the mesocyclone lasts at least 15 minutes

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Outmoded Notions• Supercells must be large with tops >30000

• Supercells must be associated with large buoyancy

• All supercells tend to be tornadic (<20% of supercells are associated with tornadoes)

• Supercells are rare (if buoyancy and shear are in proper ranges, both modeling and observational studies show that supercells are the dominant mode of convection).

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Supercell Tornadic Storms: Cascade Paradigm

Vertical ShearAllows PrecipitationTo Be RemovedFrom Updraft Area

Vertical ShearSufficient ToGenerate HorizontalRotation Which IsTilted Into VerticalTo Form PersistentMidlevel Mesocyclone

If low level (0-3 km)Shear Vector Veers Sufficiently (curvedhodograph), UpdraftAnd Rotation WillBe Augmented onRight Flank (withRespect to hodograph)

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Supercell Tornadic Storms: Cascade Paradigm

Hook Echo

MeanWind

StormMotion

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• Convective updraft converts 0-6 km shear into vertical vorticity at midlevels (mesocyclone)

• Persistant mesocyclone causes precipitation hook to rear flank

• Rear flank downdraft (RFD) develops in association with hook

• Dynamic Pipe Effect associated with descending TVS adjacent to RFD

• Interaction of RFD with highly sheared inflow air (shear in 0-1 km layer) under upshear (usually northwest) side of mesocyclone associated with development of tornado rotation at surface

Supercell Tornadic Storms: Cascade

Paradigm

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Supercell Tornadic Storms: Cascade Paradigm

Outmoded Notion• Cascade Process Takes Too Long…supercell storms in California have too brief a life cycle to experience “cascade” to conventional supercell tornado

Observational Studies from VORTEX 1995,

1999 show that time elapsed from mesocyclone formation to tornado is as

short as ~ 15 minutes

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November 22, 1996

Upper and mid-tropospheric jet

Sfc leeside trough

Sfc subsynoptic trough

Sfc southeasterlies

Sfc northwesterlies

Subsident westerlies

Curved hodograph--favorable deep layershear

Straight hodograph--moderate deep layershear

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QuickTime™ and aGIF decompressor

are needed to see this picture.

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Buoyancy Associated With California Thunderstorms• is typically “low” (SBCAPE ~<750 J/kg)

• this relatively low (when compared to warm season Great Plains values) CAPE was and is used by many as a reason to discount tornado risk in the state• traditionally estimated poorly anyway because of propensity of some forecasters to use 500 mb Lifted Index as ONLY indicator of instability

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Unless California tornadoesare “different animals” thanthose observed elsewhere, the shear values observedwith the “low buoyancy” California storms probably fit in this range.

What clues can be found in the research literature that might help us understand the California tornado problem?

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“…results indicate that for moderate to high vertical shears and parcel buoyancy (limited to the layer) below 500 mb, the simulated supercells generate similar mesocyclones (compared to high buoyancy Great Plains’ cases), even though the total CAPE was a factor of 2-3 times smaller for mini-supercell cases…”

Wicker and Cantrell, 1996

“…although parcel buoyancy is often small, its concentration in the strongly sheared lower troposphere promotes the development of vertical pressure forces comparable to those seen in simulated Great Plains supercells…”

McCaul and Weisman, 1996

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Analogies to California

Shear/Buoyancy Combination

Low Buoyancy Strong Low Level Shear Case

High Buoyancy, Moderate Low Level Shear Case

Both Associated With F4 Tornadoes

Davies and Johns, 1990

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Synoptic Features for Favorable Hodographs

• Strong southwesterly (to northwesterly) mid and upper tropospheric flow

• Position of mid and upper tropospheric trough axis “forces” surface southeasterly flow (either directly or by means of topography)

• Along coast frontal boundaries and ahead/along post-frontal trough lines

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Schematic Synoptic Pattern Central Valley

Thunderstorms

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Central Valley: Great Plains West

The “GreatPlains” ofCalifornia

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Combination of surface southeasterly flow and barrier-induced low level jet can yield strongly clockwise-curved hodographs in Sacramento and San Joaquin Valleys.

Topographic channeling evident in coastal valleys as well.

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Example of Favorable Shear Profile Caused

by Surface Southeasterly Flow Surmounted by Low

Level Jet

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Low Level Jet

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Topographic “Channeling” Can Contribute to Curved Hodographs Even Without

Low Level JetLemoore F1 TornadoNovember 22, 1996Supercellular

Sunnyvale F2 (F3?) TornadoSeptember 11, 1951Probable Supercellular

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In Many Cases, Channeling Effects May

Produce Straight Hodographs

Weak-Mod 0-6 km Shear -- LinesStrong 0-6 km Shear -- Splitting Supercells

December 5, 1998Richmond, CAF0 Tornado(es)(Line With Bowed Segments)

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Northern and Central

California Tornadoes 1990-94

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Average buoyancy was less than 500 J/kg for non-tornadic thunderstorms, thunderstorms with F0 tornadoes, and thunderstorms with F1/F2 tornadoes

There was no statistically-significant difference in buoyancy observed between the case sets

Buoyancy magnitude could not be used as a discriminator between non-tornadic thunderstorm, F0 and F1/F2 events.

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Results of Study

Mean shear magnitudes for F1/F2 bin are significantly larger than those observed for either the Non-tornadic (NULLS) and F0 bins

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There was a statistically significant Difference between 0-1 km shear for F1/F2 tornadoes and that for F0 tornadoes

There was a statistically significant Difference between 0-6 km shear for F1/F2 tornadoes and that for F0 tornadoes

There was no statistically significant Differences between the shear magnitudesFor the Null and F0 Bins

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Bulk Shear Values Showed

Similar Ranges

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With Much Caution Warranted Due to Small

Sample Size Some Thresholds Are

Suggested

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The data groupings suggestThat 0-1 km Positive ShearWas a discriminator for theF1/F2 events and….

….that shear thresholds canbe defined that might be ofoperational use in anticipat-ing F1/F2 Events

…and of some operationaluse in anticipating tornadoevents in general, thoughwith significant FAR

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Thresholds

POD Tornado65% (FAR=.24)

POD F1/F2100% (FAR=.40)

0-1 km Shear7.0 X10-3 s-10-6 km Shear2.8 X 10-3s-1

POD Tornado45% (FAR=0)POD F1/F2

79% (FAR=.21)

0-1 km Shear7.0 X 10-3 s-10-6 km Shear5.0 X 10-3 s-1

POD Tornado50% (FAR=.06)

POD F1/F286% (FAR=.25)

0-1 km Shear12.5 X 10-3s-10-6km Shear3.0X 10-3 s-1

"Acceptable" TornadoFAR of <.25

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Implications for Forecasting

• Buoyancy unimportant in distinguishing risk for tornadic thunderstorms from risk from general thunderstorms

• Results suggest that shear values can aid forecasters in anticipating F1/F2 events (probably supercellular )

• Results suggest that shear values alone cannot be used absolutely to distinguish between non-tornadic and F0-producing thunderstorms

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Implications for Forecasting

• For weaker (and non-supercellular) events, the presence of low level boundaries may dictate the risk for a thunderstorm to become misocyclonic (produce a non-supercell funnel cloud/tornado)

• The presence of squall lines and/or fronts in at least a moderate shear environment alsoshould trigger a risk of non-supercell tornadoes.

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Conservative Operational

Recommendations• Offices might keep thresholds in mind when synoptic patterns suggest a severe thunderstorm risk in California (field testing)

• Offices might use thresholds in alerting spotter groups for possible afternoon activation

• At this time, thresholds might be used as trigger for adding “some possibly severe” to forecast wording (first use of such wording in SF Bay Area forecasts: March 1995)

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