1. 2 presented by john p. monteverdi professor of meteorology department of geosciences san...

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Presented by

John P. MonteverdiProfessor of MeteorologyDepartment of Geosciences

San Francisco State University

Research completed as part of appointments as

Visiting Scientist Spring 2000 National Severe Storms Lab

Norman, Oklahoma

National Weather Service Forecast OfficeSan Francisco Bay Area

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Great Plains, USATornado Alley

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Controls on “Wizard of Oz”

(Supercell) Tornadic Thunderstorms• Buoyancy (and forces that augment

it) • Strong shear (vertical change in wind direction and speed) which encourages rotation in updraft, strengthens updraft, and fosters “domino effect” (termed supercell cascade) to tornado

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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 dots representthe tips of the windobservations at eachlevel.

The 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

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).

Rotating thunderstorms (supercells)tend to develop in environments with large values of positive shear between the ground

and 500 mb (termed 0-6 km positive shear).

Rotating thunderstorms tend to become tornadic (tornadic supercells) when large values of positive shear are found in the inflow layer

(this tends to be the 0-1 km layer)

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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 spin (vorticity) which can be tilted into the vertical by the updraft and transformed to vertical vorticity (storm scale rotation--mesocyclone)

• In certain configurations of low level positive shear, there are forces that augment the updraft by a factor of two to three times

• In certain configurations of positive shear the storm can be forced to “deviate” from motions of other storms

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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 levelShear 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 (counterclockwise rotation) at midlevels (mesocyclone)

• Persistant mesocyclone causes precipitation hook to rear flank

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

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

Supercell Tornadic Storms: Cascade

Paradigm

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Sfc southeasterlies surmounted by mid and upper tropospheric southwesterlies creates favorable hodograph and shear favorable for tornadic supercells.

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are needed to see this picture.

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Central Valley: Great Plains West

Tornado Alley West

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Birth of a Hypothesis

Why do so many tornadoes occur in California’s

Central Valley, and, to some extent, in the coastal valleys?

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Shear and Buoyancy Associated with

70 Tornadic and Non-Tornadic

Thunderstorms in Northern and CentralCalifornia, 1990-1994John P. MonteverdiSan Francisco State University

Charles Doswell IIINational Severe Storms Laboratory

Gary LipariSan Francisco State University

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Schematic Synoptic Pattern Central Valley

Thunderstorms

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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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An Example of Very Favorable Shear forA California Tornado Event

This case shows a clockwise CURVED HODOGRAPH.It occurred at Hanford (Fresno) on the afternoon of two supercell tornadoes at nearby Lemoore.

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November 22, 1996

Upper and mid-tropospheric jet

Sfc leeside trough

Sfc southeasterlies

Curved hodograph--favorable deep layershear

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Purposes of Study

To extend previous study (LM) by determining if buoyancy and shear played a significant role in distinguishing between tornadic and non-tornadic thunderstorms in the study period.

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

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Analysis Techniques

• As in LM, 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 (from L M, suspectmost non-supercells)

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

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• 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 or there is neutral directional shear)

– 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

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Northern and Central

California Tornadoes 1990-94

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An Unusual SupercellSunnyvale, CA May 4,

1998

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An Unusual Supercell



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Buoyancy Associated With California Thunderstorms

• is typically “low” (SBCAPE ~<750 J/kg compared to >2000 J/kg in Plains) • this relatively low (when compared to warm season Great Plains values) CAPE was and is used by many forecasters as a reason to discount tornado risk in the state

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Dispensing With Outmoded Notions

How to prove that tornado occurrence is unrelated to buoyancy (strength of

convection)?

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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 were no statistically-significantdifferences between the case binbuoyancies.

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

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Shear Associated With California Thunderstorms

• deep layer shear (0-6 km) can be very large when thunderstorms occur in association with cool season patterns

• low-level shear (0-1 km) is very large in association with cool season thunderstorm patterns due to topographic channeling, particularly in the Central Valley and many coastal valleys, and to the development of a low-level barrier jet in the Sacramento Valley

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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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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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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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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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1. 2 presented by john p. monteverdi professor of meteorology department of geosciences san...

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