Transcript Synoptic-Scale Weather Systems of the Intermountain West

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)
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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
Frontogenesis produced
Banded pcpn
Feb 24-25, 2003
Mar 6, 2008
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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 =
F =
1/
2
d/
dt
|q|
|q| ( E cos2b - D)
q = potential temperature
E = resultant deformation
b = angle between the isentrope and the axis of dilatation
D = 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
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
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Deformation – 3/6/08
19Z 800 - 700 mb
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Deformation – 3/6/08
19Z 800 - 700 mb
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Deformation – 3/6/08
19Z 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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COLDER
Cross section line taken
perpendicular to frontal
zone
WARMER
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Eta 3 pm Monday - Cross
section taken across
front – frontal circulation
highlighted
10,000
Feet
5000
Feet
Cold Air
KSPS
Warm Air
KGLS 23
Feb 24-25, 2003
Event Totals
FEB 24-25, 2003
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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 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
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