Objectives
• Combine features and processes already
discussed, plus a couple others, to define
a forecast process for heavy snow in the
EAX CFWA.
Outline
• Requirements for heavy snow
• Define processes associated with heavy
snow
• Processes in place, but how much?
• Archived event
Requirements for Heavy Snow
• Deep layer moisture: from the surface – 500 mb
• A lifting mechanism: both at the synoptic scale and at
the mesoscale
•Instability: not required for light snow (2-4 inches) but
definitely needed for heavy banded snowfall
•A slow-moving system with upstream propagation (i.e.,
new cloud/precipitation development upwind)
•A vertical temperature profile conducive to the efficient
production of dendritic crystals, high snow:liquid
equivalent ratios, and little/no melting of crystals
A Conceptual Model: Plan View of Key Processes
NW
SE
Physical Processes Critical to the
Production of Heavy Banded Snowfall
in the Central United States
• Character of upper level system dictates location and
distribution of banding
• Development of the TROWAL airstream
– cyclonic component of deep-layer warm, moist conveyer belt
(WCB) to northwest of the extratropical cyclone (ETC)
– System relative flow enhancement of CCB
• Mid-level frontogenetical circulation
• Reduction of stability (PI, CSI (EPV), WSS)
• Favorable thermal properties conducive to snow/ice growth
Character of upper level wave
CCB
= heavy banded snow
•x•x•x•x•x =
deformation zone
Strong extratropical
cyclone with deep,
closed ULL
Frontal zone with
modest surface
cyclone with open
upper level wave
Progressive S/W trough; Short time scale (< 12 h) for precipitation
Westward extension of comma head often disconnected from main precipitation shield
Weak easterly flow in CCB – is enhanced by the eastward motion of the system
Often a non-occluded system with inverted trough north of low
Slow-moving upper-level system; Long-lasting snow event (> 12 h)
Extensive comma head; Strong easterly flow in CCB, north of warm front
Surface system is typically occluded
What
is a TROWAL?
TROugh of
Warm
air ALoft (TROWAL)
Penner (1955, Q.J. RMS)
Apex of warm sector
Cold air
Warm
Air
Cold Air
Market 2002
Conceptual Model of a TROWAL Associated
With a Warm-Type Occlusion
Graphic courtesy of COMET
From Martin (1999, MWR)
GOES-8 IR satellite image for 10 November 1998 1515 UTC
JMS
Trowal
PIA
Theta-E Cross-Section (JMS – PIA)
RUC 2 Initialization 650 mb Theta-E
Valid 1500 UTC 10 November 1998
JMS
PIA
Frontogenesis
F = ∂v/∂y ∂θ/∂y + ∂w/∂y ∂θ/∂z
Term A
Term B
- 1/Cp (Po/P)k ∂/∂y (dQ/dt) -∂/∂y(Kh ∂2θ/∂y2)
Term C
Term D
A = effect of horizontal temperature gradient
B = tilting of the vertical temperature gradient onto a horizonta
plane
C = horizontal variation in diabatic heating/cooling
D = sub-grid scale horizontal temperature gradient
F>0 frontogenesis, F<0 frontolysis
Kinematics of Frontogenesis
Strength and
Depth of the
vertical
circulation is
modulated by
static stability
Horizontal Deformation
Horizontal Convergence
the atmospheric response is to create a
Horizontal Vorticity
direct thermal circulation (warm air rising
and cold air sinking)
Sawyer (1956), Eliassen (1962)
Dynamics of Frontogenesis
Ageostrophic circulation develops as a
response to increasing temperature gradient.
Dynamics of Frontogenesis
When we talk about frontogenesis forcing, it’s the resulting
ageostrophic circulation we are most interested in for
precipitation forecasting.
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. Precipitation bands tend to align with θ
Q = Qs + Qn
Qn
▪ Q-Vectors oriented across (normal to) isotherms
(isentropes)
▪ Describes the Vg contribution to the rate
of change of the magnitude of the thermal gradient.
▪ Associated with tangential accelerations
▪ Can indicate direct/indirect circulations by showing
packing(frontogenesis) or unpacking(frontolysis)
of the isotherms(isentropes), i.e., frontogenetical
component
▪ Typically the stronger component with open short waves
Q = Qs + Qn
Qs
▪ Q-Vectors oriented along isotherms(isentropes)
▪ Illustrates turning of the isotherms(isentropes)
▪ Describes the geostrophic contribution to the rate
of change of the direction of the thermal gradient
▪ Associated with centripital accelerations
▪ Tend to identify with synoptic features
▪ Component tends to be stronger in deeper/occluded
Systems
▪ Can be used to identify potential location of a
TROWAL
Conditional Symmetric Instability
• The atmosphere can contain regions of CSI
and convective instability (CI), but since CI
has a faster growth rate (tens of minutes)
relative to CSI (a few hours), it will dominate.
• CSI is favored to occur in regions of:
–
–
–
–
–
–
High vertical wind shear
Weak absolute vorticity (values near zero)
Weak convective stability
High mean relative humidity
Large scale ascent
These conditions are often found in the entrance
region of an upper-level jet streak during the cold
season
Frontogenesis and Symmetric Instability
Two-Dimensional Form of EPV Equation:
Interpretive Form
Derived from Martin’s (1992) 3-D EPV equation, Moore and
Lambert (1993), assumed geostrophic flow, neglected vertical
contribution and neglected ‘y’ terms to get:
 Mg e Mg e 
EPV  g 


x p 
 p x
A
B
Term 1
C
D
Term 2
“Whenever EPV is either zero or negative, and the
atmosphere is nearly saturated, then the atmosphere is
considered to have potential for CSI. CSI occurs whenever
term 1 dominates term 2”. (Weismuller & Zubrick, 1998)
Nicosia and Grumm Model for EPV Reduction Near
Extratropical Cyclones
Graphic courtesy of COMET
12-13 to 1
11-12 to 1
Origination of the Liquid Ratio Problem
•The ten-to-one rule originates from a nineteenth century Canadian
study (1878) in which the observer came to this conclusion after a
long series of experiments (Potter 1965).
•As early as 1875, the United States Weather Bureau provided a
typical snow to liquid ratio (SLR) value of 10 to 1 to its observers.
•A number of studies have shown there is considerable variation
from this estimate depending on location and various environmental
parameters.
•Many NWS offices are aware of the variation in ratios and use
either a climatological value or an empirical method based upon
surface or in-cloud temperatures (Roebber et al 2003).
12-13 to 1
11-12 to 1
http://www.eas.slu.edu/CIPS/Research/slr/slrmap.htm
Ratio typically varies with storm track
•Clipper type storms feature
higher snow to liquid ratios, as
they are colder and contain less
moisture.
•This leads to growth by
deposition.
•Average SLR for southeastern
Wisconsin with various storm
tracks (Adapted from Harms,
1970 )
•Storm tracks that are warmer
or contain more Gulf moisture
feature lower snow to liquid
ratios.
•This leads to growth by
riming, possibly mixed with
sleet.
Acknowledgements
•
•
•
•
•
http://www.eas.slu.edu/CIPS/Presentations
http://www.meted.ucar.edu
http://www.comet.ucar.edu
http://www.spc.noaa.gov
http://www.ncep.noaa.gov
References
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Baxter, M.A., 2003: Winter Storm Forecasting as a Two Step Process: The 26-27 November 2001
Snowstorm, Preprint.
Clark, J.H.E., et al., 2002: A Reexamination of the Mechanisms Responsible for Banded
Precipitation, MWR, Vol. 130, 3074-3086.
Graves, C.E., et al., 2003: Band on the Run – Chasing the Physical Processes Associated with
Heavy Snowfall, BAMS, 990-995.
Martin, J. E., 1998: The Structure and Evolution of Continental Winter Cyclone. Part I: Frontal
Structure and the Occlusion Process, MWR, 303-328.
Martin, J. E., 1998: The Structure and Evolution of Continental Winter Cyclone. Part II: Frontal
Forcing of an Extreme Snow Event, MWR, 329-348.
Moore, J.T. and P. D. Blakley, 1988: The Role of Frontogenetical Forcing and Conditional
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Moore, J.T. and T.E. Lambert. 1993, WAF, Vol 8, No.3, 301-308.
Schultz, D.M. and P.N. Schumacher, 1999: The Use and Misuse of Conditional Symmetric
Instability, MWR, Vol 127, 2709-2732.
Nicosia, D.J. and R.H. Grumm, 1999: Mesoscale Band Formation in Three Major Northeastern
United States Snowstorms, WAF, Vol. 14, 346-368.
Weismueller, J.L. and S.M. Zubrick, 1998: Evaluation and Application of Conditional Symmetric
Instabiiity, Equivalent Potential Vorticity, and Frontogenetical Forcing in the Operational Forecast
Environment, WAF, Vol. 13, 84-100.