Frontogenesis Forcing and Banded Precipitation

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Frontogenesis Forcingand Banded Precipitation

• Peter Banacos
• NOAA/NWS/NCEP/SPC
• WDTB Winter Weather Workshop
• 09 October 2002

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Presentation Outline

• Ingredients based methodology for winter
  precipitation
• Elements of frontogenesis
• Favorable synoptic patterns
• Case study examples / inferring frontogenesis
• Summary

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Ingredients Based Forecasting
Purpose to focus the forecaster on the necessary
conditions (ingredients) needed for a specific
meteorological event to take place.
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Making Forecast Assessments

• Diagnose three ingredients first
• Moisture sufficiency, potential for evaporation
  in boundary layer, dry slots.
• Thermal Stratification precipitation type,
  melting/evaporative cooling/other diabatic
  effects.
• Lift examine forcing for assent.
• Lastly, assess microphysical aspects and
  instability for possible enhancement to
  precipitation rate

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5 Lift Mechanisms Commonly Assessed for Winter
Precip.

• Traditional,large-scale QG forcing (temperature
  advection / isentropic lift, differential
  vorticity advection) precipitation trends often
  mesoscale
• Convective mechanisms lake-effect or snow
  squalls in cold advection regimes, elevated
  instability
• Orographic lift requires knowledge and
  familiarity of local terrain influences
• Jet Streaks/coupled jets cellular mesoscale
  areas of assent
• Frontogenesis forcing linear/mesoscale areas of
  assent
• ? Frontogenesis Forcing is the focus of this
  talk.

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Frontogenesis (definition)

• The 2-D frontogenesis function (F) quantifies
  the change in (potential) horizontal temperature
  gradient following air parcel motion
• Fgt0 frontogenesis, Flt0 frontolysis
• Conceptually, the local change in horizontal
  temperature gradient near an existing front,
  baroclinic zone, or feature as it moves.

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Kinematics of Frontogenesis

• Examine separate contributions of
• horizontal divergence, deformation,
• and vorticity to the field of
• frontogenesis.

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Horizontal Divergence

• Divergence (Convergence) acts frontolytically
  (frontogenetically), always, irrespective of
  isotherm orientation.

Flt0
Fgt0
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Horizontal Deformation
Fgt0

• Flow fields involving deformation acting
  frontogenetically are prominent in many cases.

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Horizontal Deformation (cont.)
Flt0
Need to consider orientation of isotherms
relative to axis of dilatation.
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Horizontal Vorticity
F0

• Acts to rotate isotherms, cannot tighten or
• weaken them.

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Other Contributing Factors to Frontogenesis

• The kinematic field, and deformation in
  particular, plays an important role in
  frontogenesis.
• Other processes such as diabatic heating and
  tilting effects may also contribute in an
  important way to frontogenesis.
• Examples differential solar heating
• Latent heating with convective motions
  (documented in coastal frontogenesis process).

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Dynamics of Frontogenesis (vertical circulation)
Flow field dominated by deformation.
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Dynamics of Frontogenesis (cont.)
Ageostrophic circulation develops as a response
to increasing temperature gradient.
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Dynamics of Frontogenesis (cont.)
When we talk about frontogenesis forcing, its
the resulting ageostrophic circulation we are
most interested in for precipitation forecasting.
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Use of Frontogenesis in Forecasting

• Presence of F in 800-600mb layer can help
  diagnose and predict areas of heavy banded
  precipitation.
• Fgt0 contributes toward symmetric instability
  (SI), but heavy precipitation can occur without
  the presence of SI.
• F DOES NOT require a strong surface cyclone, only
  a baroclinic zone (this fact can lull the
  forecaster into neglecting heavy snow potential
  (see Moore and Blakley, 1988). This is also why F
  rarely plays an important role as a forcing
  mechanism in the warm season.

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Frontogenesis and Symmetric Instability
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Common Synoptic Patterns

• Generally, look for situations leading to a
  strong baroclinic zone in the low to middle
  troposphere.
• Although F can manifest itself in many ways,
  there are a few common synoptic patterns that
  occur relatively frequently.

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NE of Weak Surface Cyclonewith Southward Moving
Cold Front

• This synoptic pattern is examined in detail in
  case 1.

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NW of Strong Cyclonewrap around precipitation
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NW of Strong Cyclone 1/6/02
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North of Strong E-W Frontal Zone
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North of Strong E-W Frontal Zone3/13/02
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Example Case of Frontogenesis and Banded
Precipitation

• Date 15 October 2001 (Case 1)
• Narrow band (1-2 counties wide) of moderate to
  heavy rainfall from eastern KS to central IL.
• Associated with weak surface features but a
  moderately strong baroclinic zone and
  frontogenesis forcing.

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700mb 00z 15 OCT 01
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Surface 15 OCT 2001
12z
00z
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925mb 12z 15 OCT 01
Large-scale deformation field - eastern
KS/western MO
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18z 15 OCT 01
18z mosaic base reflectivity and surface
observations
18z 600mb Frontogenesis
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• Rainfall rates between 0.10 and 0.25 occurred
  for a 6 hour period from 15-20z.
• Moderate to heavy precipitation can persist
  longer (12 hours) with slower moving systems or
  mature extratropical cyclones.

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Topeka, KS 12z 15 OCT 01
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700mb Frontogenesis / Base Reflectivity
0 hr ETA 12z
6 hr ETA 18z
1150z
1805z

• Organization of precipitation increases as F
  orientation becomes aligned with lower levels.

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Sloped Continuity of F
6hr ETA forecast valid 18z 15 OCT 01
600mb
700mb
850mb

• Presence of parallel axes of positive
  frontogenesis sloping upward toward colder air is
  a common aspect of heavy banded precipitation
  areas.

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Sloped Continuity of Frontogenesis Forcing (cont.)

• The previous two slides have several important
  implications
• Several levels should be assessed for spatial
  continuity and orientation of F, to see if
  banding is likely to occur at a given time.
• Vertical averaging should probably be avoided.
• The sloped continuity tells us something about
  the structure of the wind field we can use to
  infer frontogenesis from single sounding
  (observed or model derived), VAD, or wind
  profiler data, and large-scale flow fields.

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Role of Deep-Layer Shear Profile
Nature of environmental wind profile may be
conducive to seeder-feeder mechanism and rapid
precipitation generation / elongation of bands
during initial development phase.
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Role of Deep-Layer Shear (cont.)
Martin (1998)

• Note banding orientation (parallel to isotherms).

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Case 2 9 NOV 00
Montgomery Co. ?
INX 0903Z
Unlike Case 1, this case shows narrow multiple
banded precipitation. Lower stability likely
played a role.
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9 NOV 00 CASE

• A stripe of snow in se KS on Nov 9th with
  accumulations of 6-12" in
• 3 hours. The stripe was oriented n-s and was
  about 4 miles wide between
• the two 6" contours. We had one place that got 2
  inches and about 2 to 3
• miles away they got 9-12 inches. We didnt even
  know it occurred until I
• placed about 30 phone calls into that county from
  the rural directory
• telephone book. The 00z ETA had the 20C 500 temp
  bullseye to the
• south over se Oklahoma. But the 12z raobs
  verified it much further north
• at SGF. Looks like the inversion was around 700mb
  with neutral lapse
• rate at SGF above 700. I have a feeling the snow
  was convective. The
• snow fell from 8-11z right as the 700-500mb
  trough was moving through
• that area. One guy told me it was the biggest
  snow in over 20 years. The
• area only covered about 10 of Montgomery
  county!
• - Forecaster, WFO ICT

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00z 9 NOV 2000 Intense Baroclinic Zone
Surface
925mb
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00z 9 NOV 2000
700mb
850mb
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700-500mb Lapse Rates
SGF 12z 11/09/00
TOP 12z 10/15/01
7.8 C/km
4.5 C/km
Near neutral or unstable lapse rates (with
respect to a moist adiabat) implies sharp/narrow,
stronger, and multiple banded precipitation
organization. Resulted in 2-3/hr snowfall rates
on Nov 9, 2000.
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Modulation of Band Intensity by Instability for a
constant value of F
As gravitational or symmetric stability
decreases, the horizontal scale of the band
decreases while the intensity of the band
increases. Multiple bands become established in
an unstable regime.
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Case 3 00z 25 Feb 01
Deformation zones associated with strong cyclones
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Unidirectional Shear Profile
ABR 00z
BIS 00z
H7-H5 lapse rate 6.4 C/km
H7-H5 lapse rate 4.5 C/km
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ABR VAD WIND PROFILE
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Upper-Air 00z 25 FEB 2001
500 mb
700 mb
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Upper-Air 12z 25 FEB 2001
700 mb
500 mb
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INL 12z 25 FEB 01
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Anticipation of Banded Precip
ALB 1/7/02 0242Z
leads to more accurate prediction of heavy
precipitation.
KALB 070451Z 35007KT 1/4SM SN FZFG OVC008
M01/M01 A2952 RMK AO2 TWR VIS 1/2 SLP998 SNINCR
4/011 P0020 T10061006 400391011 RVRNO
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SUMMARY

• When applied within the context of ingredients
  based forecasting, frontogenesis is useful for
  assessing potential for banded winter
  precipitation, which is generally a good
  candidate for SPC mesoscale discussions.
• Doesnt require a strong cyclone, only a strong
  baroclinic zone, often developed through
  horizontal deformation and associated largely w/
  unidirectional vertical shear in the low to mid
  levels.
• Sloped continuity in time and space of F leads
  most directly to strong banded precipitation.
• Examine lapse rates in precipitation generating
  layer to assess modulating role of instability
  (upright/symmetric).

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References
Bosart, L. F., 1981 The Presidents Day
snowstorm of 18-19 February 1979 A
subsynoptic-scale event. Mon. Wea. Rev., 109,
1542-1566. Keyser D., M. J. Reeder, and R. J.
Reed, 1988 A generalization of Petterssens
frontogenesis function and its relation to the
forcing of vertical motion. Mon. Wea. Rev., 116,
762-780. Martin, J. E., 1998 The structure and
evolution of a continental winter cyclone. Mon.
Wea. Rev., 126, 329-348. Moore, J. T., and P. D.
Blakley, 1988 The role of frontogenesis forcing
and conditional symmetric instability in the
Midwest snowstorm of 30-31 January 1982. Mon.
Wea. Rev., 116, 2155- 2171. Sanders, 1986
Frontogenesis and symmetric stability in a major
New England snowstorm. Mon. Wea. Rev.,
114,1847-1862. Schultz D. M., 2001 Reexamining
the cold conveyor belt. Mon. Wea. Rev., 129,
2205-2225. Schultz D. M., and P. N. Schumacher,
1999 The use and misuse of conditional symmetric
instability. Mon. Wea. Rev., 127,
2709-2732. Steigerwaldt, H., 1986 Deformation
zones and heavy precipitation. NOAA Technical
Memorandum, NWS CR-83. 15pp.