Using Frontogenesis in Winter Weather Forecasting

1. 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)

2. Topics • Motivation • Frontogenesis Review • Definition • Interpretation • Diagnosing Frontogenesis • Conceptual Models • Example

3. 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

4. Motivation Frontogenesis produced Banded pcpn Mar 6, 2008 Feb 24-25, 2003

5. 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

6. 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

7. Petterssen (1936) Frontogenesis F = d/dt |Ñq| F = 1/2 |Ñq| ( E cos2b - D) q = potential temperature E = resultant deformation b = angle between the isentrope and the axis of dilatation D = divergence

9. 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

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

12. Deformation – 2/24/03 Event L

13. Deformation – 3/6/0819Z 800 - 700 mb

14. Deformation – 3/6/08 19Z 800 - 700 mb

15. Deformation – 3/6/0819Z 800 - 700 mb

16. Conceptual Models

18. Displaying Fgen Fields • WFO only : AWIPS workstation • Web: HPC Model Diagnostics pagehttp://www.hpc.ncep.noaa.gov/mdd/mddoutput/ • Web: SPC SREF page http://www.spc.ncep.noaa.gov/exper/sref/ • Web: Others?

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

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

21. 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

22. Cross section line taken perpendicular to frontal zone COLDER WARMER

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

24. Feb 24-25, 2003 Event Totals FEB 24-25, 2003

25. 24 hour Low level Fgen Forecast (Eta)STP mosaic ending at 00Z

26. 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

27. References Dr. David Schultz NSSL http://www.cimms.ou.edu/~schultz/ Pete Banacos SPC Link to his banding/Fgenconference 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