Prediction of Frontogenetically Forced Precipitation Bands

PREDICTION OF FRONTOGENETICALLY FORCED
PRECIPITATION BANDS
PETER C. BANACOS
NWS / Storm Prediction Center
WDTB Winter Weather Workshop IV
Boulder, CO ~ 23 July 2003

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

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.

Frontogenesis (definition)
F
=
D
Dt
∇pθ
(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 < 0 frontolysis
‣ Conceptually, the local change in horizontal temperature gradient near
an existing front, baroclinic zone, or feature as it moves.

Vector Frontogenesis Function
F
=
F n +
n
F
s
s
(Keyser et al. 1988, 1992)
D
F n = − ∇pθ
Dt
D
F s = n ⋅( k × ∇pθ
)
Dt
F is of fundamental importance…
Change in magnitude
‣ Corresponds to vertical motion on the frontal scale
(mesoscale bands)
Change in direction (rotation)
‣ Corresponds to vertical motion on the scale of the
baroclinic wave itself
‣ Galilean invariant
‣ full wind generalization of the quasi-geostrophic Q-vector

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
(F n )
frontogenesis

Horizontal Divergence
Divergence (Convergence) acts frontolytically (frontogenetically),
always, irrespective of isotherm orientation.
F0

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

Horizontal Deformation (cont.)
F

Vertical Vorticity
F=0
Pure vorticity acts to rotate isotherms, cannot
tighten or weaken them.

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).

Dynamics of Frontogenesis
(vertical circulation)
Flow field
dominated by
deformation.

Dynamics of Frontogenesis (cont.)
Ageostrophic circulation develops as a response to increasing
temperature gradient.

Dynamics of Frontogenesis (cont.)
When we talk about frontogenesis forcing, it’s the resulting ageostrophic
circulation we are most interested in for precipitation forecasting.

Forecasting Applications

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

Common Synoptic Patterns
TWO CLASSES OF BANDS:
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 surface cyclone, only a
low-mid tropospheric baroclinic zone
‣ Bands associated with surface cyclogenesis
‣ Bands not associated with surface cyclogenesis

I. CYCLOGENETIC PATTERN
NW of surface cyclone --“wrap around precipitation”
Mature Stage
Decaying Stage

Northwest of Strong Cyclone 1/6/02

Snowfall Accumulations

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 common in the Central and Northern Plains with
boundaries.
quasi-stationary arctic

Within Strong E-W Frontal Zone
3/13/02

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.

700mb 00z 15 OCT 01

Surface 15 OCT 2001
00z
12z

925mb 12z 15 OCT 01
Large-scale deformation field - eastern KS and western MO

18z 15 OCT 01
18z mosaic base reflectivity and
surface observations
18z 600mb
Frontogenesis

Kirksville, MO (ASOS) Hourly Rainfall
15 October 2001
0.30
0.25
Hourly Precipitation (inches)
0.20
0.15
0.10
0.05
0.00
14 15 16 17 18 19 20 21 22
TIME (UTC)
• 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.

Topeka, KS 12z 15 OCT 01

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.

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.

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.

Sloped Continuity of Frontogenesis Forcing (cont.)
‣ The previous two slides have several important
implications:
1) 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.
2) Vertical averaging should probably be avoided.
3) 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.

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.

Role of Deep-Layer Shear (cont.)
Martin (1998)
‣ Note banding orientation (parallel to isentropes / isotherms).

Vertical Wind Profile and
Idealized Hodographs:
banding
Col point aloft

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”)

Banded – Cold Season 3-10Z 12/29/02

Mosaic Radar 8z 12/29/02
‣ RUC 2h frontogenesis forecast 850mb

1.5 o Base Velocity / VAD – Spokane, WA
0854z 12/29/02
Frontogenesis coincident with col point / straight shear

Banded – Warm Season 12Z 6/27/01
Training thunderstorms, in gravitationally unstable environment
VIS 1500Z
TLX 1459Z

Banded – Translation along short axis
North Dakota 0256z 1/26/03
Two problems for heavy precip:
Moisture starved, and moving fast

Non-Banded 0256z 12/25/02

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.

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.

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.

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.

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:
EPV
=
⎡⎛
g⎢⎜
⎣⎝
∂M
∂p
g
∂θ
∂x
e
⎞
⎟ −
⎠
⎛
⎜
⎝
∂M
∂x
g
∂θ
∂p
(TERM 1) (TERM 2)
e
⎞⎤
⎟⎥
⎠⎦
• The closer EPV is to zero, the more responsive the atmosphere will
be to a given amount of forcing.
• IF EPV

Using EPV to Measure Stability
An example from Moore and Lambert
(1993)

Frontogenesis and Symmetric Instability

Cloud-Layer Stratificaiton Comparison
2-D 750mb
frontogenesis
Bismarck VAD
21z RUC
Forecast valid
at 00z
0018Z 22 Oct 02
ND
MT

ETA 0h EPV 00z 10/22/02
700mb (thick dashed
line)
600mb (thin dashed
line)
Multiple bands exist here
in negative EPV regime
over Montana.
0018Z 22 Oct 02

00z Soundings 10/22/02
Great Falls, MT
Bismarck, ND
700-500mb lapse rate: 6.7 C/km
700-500mb lapse rate: 5.1 C/km
850-500mb lapse rate: 3.5 C/km

Numerical Model Considerations
Date: 7 February 2003 (Case #2)
• Heavy snow band across southern New England
• QPF/ 700mb UVV field: may not tell you what you need to know, even for a
“well-handled” system:
“What you see isn’t always what you get”

2/7/03 09Z RUC Forecast QPF/UVV

2/7/03 09Z RUC Forecast
700mb Warm Advection

2/7/03 Mosaic Radar 1215z-0022z

2/7/03 Mosaic Radar / RUC 700mb F

Profiler – Plymouth, MA

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

Snowfall Accumulations 2/7/03
• Inadequate resolution likely precluded evidence of band in UVV / QPF fields.

Suggested Snow Band Checklist
Presence of (1”/hr):
limited dry air advection in near surface.
near saturated / high low-mid level RH present (east CONUS, 1000-
500mb >85%)
Favorable thermodynamic profile for snow (i.e. cloud top temp

Suggested Snow Band Checklist (cont.)
Enhancement of (1-3”/hr, 5”/hr in extreme cases):
UVV
Saturation through dendrite growth layer (-12 to – 16C) coincident with strong
(high precipitation efficiency)
Presence of negative EPV, elevated potential or slantwise instability
(convective snow potential, band multiplicity)

SUMMARY
When applied within the context of ingredients based forecasting, frontogenesis is
useful for assessing potential for mesoscale banded precipitation areas.
Doesn’t 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