Using Frontogenesis in Winter Weather Forecasting

Greg Patrick WFO FWD Nov 13, 2008 Parts of this presentation derived from presentations by Dr. David Schultz (NSSL) and Pete Banacos (formerly SPC) 1

Topics

 Motivation  Frontogenesis Review   Definition Interpretation  Diagnosing Frontogenesis   Conceptual Models Example 2

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 3

Motivation

Frontogenesis produced Banded pcpn

Feb 24-25, 2003 Mar 6, 2008 4

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 5

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

= 1/ 2

F

= d/ dt |  q | (

E

|  q | cos2 b -

D

) q = potential temperature

E

b = resultant deformation = angle between the isentrope and the axis of dilatation

D

= divergence 7

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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 9

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Horizontal Deformation

F

>0 Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases.

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Deformation – 2/24/03 Event

12

Deformation – 3/6/08

19Z 800 - 700 mb 13

Deformation – 3/6/08

19Z 800 - 700 mb 14

Deformation – 3/6/08

19Z 800 - 700 mb 15

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” 19

http://www.spc.ncep.noaa.gov/exper/sref/ Look under “Winter Weather” or “Lift” 20

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 21

COLDER

Cross section line taken perpendicular to frontal zone

WARMER 22

10,000 Feet 5000 Feet Cold Air KSPS Eta 3 pm Monday - Cross section taken across front – frontal circulation highlighted Warm Air KGLS

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Feb 24-25, 2003 Event Totals

FEB 24-25, 2003 24

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 Schultz NSSL http://www.cimms.ou.edu/~schultz/ Pete Banacos SPC Link to his banding/Fgen conference paper http://spc.noaa.gov/publications/banacos/F_conf_030415b.pdf

Reference to dynamic explanations of F and UVM H. B. Bluestein, Vol II, Synoptic-Dynamic Meteorology In Midlatitudes. Pages 297-304 27