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Using Frontogenesis in Winter Weather Forecasting

Greg PatrickWFO FWD

Nov 13, 2008

Parts of this presentation derived from presentations by Dr. David Schultz (NSSL) and Pete Banacos (formerly SPC)

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Topics

Motivation

Frontogenesis Review Definition

Interpretation

Diagnosing Frontogenesis Conceptual Models

Example

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Motivation

• Frontogenesis was a significant contribution to forcing during two of the most significant winter events across north TX in the past ~ 5 years (2/24/03 & 3/6/08)

• Winter weather events with large geographic variations in impacts can result from events where Fgen forcing is dominant

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Motivation

Mar 6, 2008

Frontogenesis producedBanded pcpn

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Frontogenesis Review

• Conceptually, F is the local change in horizontal temperature gradient near an existing front, baroclinic zone, or feature as it moves.

• When we talk about frontogenesis forcing, it’s the resulting ageostrophic circulation we are most interested in for precipitation forecasting

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Frontogenesis Review

• Frontogenesis is an intensification of a temperature gradient at the surface or aloft

• Frontolysis is a weakening of the temperature gradient at the surface or aloft

• The 2-D scalar frontogenesis function (F ) – quantifies the change in horizontal (potential) temperature gradient following air parcel motion :

F > 0 frontogenesis, F < 0 frontolysis

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Petterssen (1936) Frontogenesis

F = d/dt |Ñq|

F = 1/2 |Ñq| ( E cos2b - D)

q = potential temperatureE = resultant deformationb = angle between the isentrope and the axis of dilatationD = divergence

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Frontogenesis Review

• Diagnosis of frontogenesis results in a diagnosis of the forcing for vertical motion on the frontal scale.

• Ascent occurs on the warm side of a maximum of frontogenesis and on the cold side of a region of frontolysis

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Horizontal Deformation

Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases.

F>0

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Deformation – 2/24/03 Event

L

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Deformation – 3/6/0819Z 800 - 700 mb

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Deformation – 3/6/08 19Z 800 - 700 mb

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Deformation – 3/6/0819Z 800 - 700 mb

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Conceptual Models

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Displaying Fgen Fields

• WFO only : AWIPS workstation• Web: HPC Model Diagnostics page

http://www.hpc.ncep.noaa.gov/mdd/mddoutput/

• Web: SPC SREF page http://www.spc.ncep.noaa.gov/exper/sref/

• Web: Others?

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http://www.hpc.ncep.noaa.gov/mdd/mddoutput/Field is “fgenslope”

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http://www.spc.ncep.noaa.gov/exper/sref/Look under “Winter Weather” or “Lift”

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Example – Feb 24, 2003

• Convection developed in a zone of strong frontogenetical forcing across western and northern parts of north TX, resulting in a mixture of moderate-heavy sleet and snow in some areas.

• Models (particularly Eta) focused UVM and QPF across southern parts of the FWD CWA, closer to surface front and stronger elevated instability

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Cross section line taken perpendicular to frontal zone

COLDER

WARMER

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Eta 3 pm Monday - Cross section taken across front – frontal circulationhighlighted

Cold AirKSPS

Warm AirKGLS

5000 Feet

10,000 Feet

24FEB 24-25, 2003

Feb 24-25, 2003 Event Totals

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24 hour Low level Fgen Forecast (Eta)STP mosaic ending at 00Z

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Operational Forecasting Summary

• Frontogenesis fields should be assessed anytime a strong frontal zone affects north TX

• Look for banded QPF in numerical model output or large values of +VV in bands parallel to front as clues that Frontogenesis may be a factor

• Look for sloped continuity of Frontogenesis• Must also assess moisture and instability

parameters along with vertical temp profile

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References

Dr. David SchultzNSSL http://www.cimms.ou.edu/~schultz/

Pete BanacosSPC Link to his banding/Fgen conference paperhttp://spc.noaa.gov/publications/banacos/F_conf_030415b.pdf

Reference to dynamic explanations of F and UVM H. B. Bluestein, Vol II, Synoptic-Dynamic MeteorologyIn Midlatitudes. Pages 297-304