PREDICTION OF FRONTOGENETICALLY FORCED PRECIPITATION BANDS

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PREDICTION OF FRONTOGENETICALLY FORCED
PRECIPITATION BANDS

• PETER C. BANACOS
• NWS / Storm Prediction Center
• WDTB Winter Weather Workshop IV
• Boulder, CO 23 July 2003

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OUTLINE

• Frontogenesis
• where it fits in the forecast process
• kinematics and dynamics of frontogenesis
• synoptic pattern recognition
• Case 1 examine band formation
• Mesoscale Banding Characteristics
• modulation by local wind profile
• col point aloft
• modulation by stability
• Case 2 numerical model considerations

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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.
Frontogenesis is a lifting/forcing mechanism.
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Frontogenesis (definition)
(S. Petterssen 1936)
The 2-D scalar frontogenesis function (F )
quantifies the change in horizontal (potential)
temperature gradient following air parcel motion
F 0 frontogenesis, F nceptually, the local change in horizontal
temperature gradient near an existing front,
baroclinic zone, or feature as it moves.
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Vector Frontogenesis Function
(Keyser et al. 1988, 1992)
Change in magnitude Corresponds to vertical moti
on on the frontal scale (mesoscale bands)
Change in direction (rotation) Corresponds to
vertical motion on the scale of the baroclinic
wave itself
F is of fundamental importance
Galilean invariant full wind generalization of
the quasi-geostrophic Q-vector
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Kinematics of Frontogenesis

• The geometry of the horizontal flow has a
  first-order influence on F in most situations.
  
• Examine separate contributions of
• horizontal divergence, deformation,
• and vorticity to the field of
• frontogenesis.

Note Will focus exclusively on the Petterssen
2-D scalar frontogenesis (Fn)
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Horizontal Divergence

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

F
F0
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Horizontal Deformation
F0

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

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

• Pure vorticity 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 the most prominent role in the
  2-D frontogenesis aloft.
  
• Other processes such as diabatic heating and
  tilting effects may also contribute 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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Forecasting Applications
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Use of Frontogenesis in Forecasting

• Presence of F in 850-500mb layer can help
  diagnose and predict areas of heavy banded
  precipitation.
  
• Potential for banding can be assessed using F
  field in numerical models, with placement of
  banding refined in
• New graphic forecast tools allow location of
  banded precipitation to be conveyed to the user.

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Common Synoptic Patterns
Forecast premise for mesoscale banding
Requires a strengthening baroclinic zone in the
presence of sufficient moisture for precipitation
(AND for snow, the proper thermal
stratification). Large-scale deformation zones
are BY FAR AND AWAY the most common means of
manifesting areas of frontogenesis within the
850-500mb layer. Does NOT require a strong surf
ace cyclone, only a low-mid tropospheric
baroclinic zone

• TWO CLASSES OF BANDS
• Bands associated with surface cyclogenesis
• Bands not associated with surface cyclogenesis

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I. CYCLOGENETIC PATTERN
NW of surface cyclone --wrap around
precipitation
Mature Stage
Decaying Stage
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Northwest of Strong Cyclone 1/6/02
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Snowfall Accumulations
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II. Frontal / Weak Cyclogenesis Pattern
Confluent flow 700mb in advance of a positive
tilt trough. Weak or non-existent surface wave
cyclone along surface front. Seems to be most c
ommon in the Central and Northern Plains with
quasi-stationary arctic boundaries.
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Within 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 and
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 isotherm
orientation at lower levels.
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Sloped Continuity of F
600 mb
6hr ETA forecast valid 18z 15 OCT 01
700 mb
850 mb
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 F
The plane of the cross-section should be taken
perpendicular to the mid-level (850-500mb)
thermal wind vector or thickness lines.
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Sloped Continuity of Frontogenesis Forcing (cont.)

• The previous two slides have several important
  implications
  
• Several levels (or a x-section) 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 isentropes
/ isotherms).
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Vertical Wind Profile and banding
Idealized Hodographs
? Col point aloft
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Mesoscale Band Variations
Band movement (short and long-axis translation)
Warm season vs. cool season bands
Multiple parallel bands (stability driven)
- Non-banded (the null wind structure)
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Banded Cold Season 3-10Z 12/29/02
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Mosaic Radar 8z 12/29/02
RUC 2h frontogenesis forecast 850mb
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1.5o Base Velocity / VAD Spokane, WA0854z
12/29/02
Frontogenesis coincident with col point /
straight shear
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Banded Warm Season 12Z 6/27/01
Training thunderstorms, in gravitationally
unstable environment
VIS 1500Z
TLX 1459Z
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Banded Translation along short axis North
Dakota 0256z 1/26/03
Two problems for heavy precip
Moisture starved, and moving fast
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Non-Banded 0256z 12/25/02
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Non-Banded 0256z 12/25/02
Note strong curvature to the shear vector with
height. This tends to preclude coherent banding,
even in the presence of frontogenesis.
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Banded- Multiple 11/09/00
Montgomery Co. ?
INX 0903Z
Unlike Case 1, this case shows narrow multiple
banded precipitation. Lower stability likely
played a role.
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700-500mb Lapse Rate Comparison
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 multiple
narrow and intense (maybe 5-10 km or so in
width), bands. 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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Using EPV to Measure Stability
EPV Equivalent Potential Vorticity
A relatively simple, quick, and effective way to
evaluate CSI/MSI. Gravitational instability may
also be present. Defined by Moore and Lambert (
1993) as follows
(TERM 1)
(TERM 2)
The closer EPV is to zero, the more responsive
the atmosphere will be to a given amount of
forcing. IF EPVerlaying EPV with theta-e is an effective way to
determine if convective (gravitational)
instability also exists.
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Using EPV to Measure Stability
An example from Moore and Lambert (1993)
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Frontogenesis and Symmetric Instability
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Cloud-Layer Stratificaiton Comparison
Bismarck VAD
2-D 750mb frontogenesis
21z RUC Forecast valid at 00z
0018Z 22 Oct 02
ND
MT
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ETA 0h EPV 00z 10/22/02
700mb (thick dashed line) 600mb (thin dashed line
)
0018Z 22 Oct 02
Multiple bands exist here in negative EPV regime
over Montana.
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00z Soundings 10/22/02
Great Falls, MT
Bismarck, ND
700-500mb lapse rate 5.1 C/km
700-500mb lapse rate 6.7 C/km
850-500mb lapse rate 3.5 C/km
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Numerical Model Considerations
Date 7 February 2003 (Case 2)
Heavy snow band across southern New England QP
F/ 700mb UVV field may not tell you what you
need to know, even for a well-handled system
What you see isnt always what you get
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2/7/03 09Z RUC Forecast QPF/UVV
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2/7/03 09Z RUC Forecast 700mb Warm Advection
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2/7/03 Mosaic Radar 1215z-0022z
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2/7/03 Mosaic Radar / RUC 700mb F
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Profiler Plymouth, MA
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Boston, MA Surface Observations
BOS 13 UTC 1 1/2SM SN BOS 14 UTC 1/2 SM SN
BOS 15 UTC 1/2 SM SN SNINCR 1/ 2
BOS 16 UTC 1/2 SM SN SNINCR 1/ 3
BOS 17 UTC 1/2 SM SN SNINCR 2/ 4
BOS 18 UTC 1/4 SM SN SNINCR 2/ 6
BOS 19 UTC 1/4 SM SN SNINCR 2/ 8
BOS 20 UTC 1/4 SM SN SNINCR 2/10
BOS 21 UTC 1/4 SM SN SNINCR 1/10
BOS 22 UTC 1/4 SM -SN BOS 23 UTC 2 SM SN
BOS 00 UTC 10 SM
700 mb F, 18Z
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Snowfall Accumulations 2/7/03
Inadequate resolution likely precluded evidence
of band in UVV / QPF fields.
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Suggested Snow Band Checklist

Presence of (1/hr) limited dry air advection i
n near surface. near saturated / high low-mid l
evel RH present (east CONUS, 1000-500mb 85)
Favorable thermodynamic profile for snow (i.e.
cloud top temp Sloped region of mid-level 2-D frontogenesis
/ Deformation axis in 850-500mb range
Relative minimum in wind speed (within 850- 700mb region (col point aloft) and/or
uniform deep- layer shear profile absence of
substantial hodograph curvature
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Suggested Snow Band Checklist (cont.)

Enhancement of (1-3/hr, 5/hr in extreme
cases) Saturation through dendrite growth layer
(-12 to 16C) coincident with strong UVV (high
precipitation efficiency) Presence of negativ
e EPV, elevated potential or slantwise
instability (convective snow potential, band
multiplicity)
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SUMMARY

• When applied within the context of ingredients
  based forecasting, frontogenesis is useful for
  assessing potential for mesoscale banded
  precipitation areas.
• Doesnt require a strong cyclone, only a strong
  baroclinic zone, often developed through
  horizontal deformation and associated w/ a col
  point aloft
• Col point aloft YOUR cue to investigate F and
  banding potential
  
• Location of col point aloft approximate band
  location
  
• Banding is modulated by wind structure and
  stability
  
• Banding is not always represented by the models