Objectives

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Objectives

• Combine features and processes already discussed,
  plus a couple others, to define a forecast
  process for heavy snow in the EAX CFWA.
  

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Outline

• Requirements for heavy snow
• Define processes associated with heavy snow
• Processes in place, but how much?
• Archived event

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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
  snowliquid equivalent ratios, and little/no
  melting of crystals

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A Conceptual Model Plan View of Key Processes
NW
SE
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Physical Processes Critical to the Production of
Heavy Banded Snowfallin 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
  

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Character of upper level wave
CCB
heavy banded snow
xxxxx deformation zone
Strong extratropical cyclone with deep, closed ULL
Frontal zone with modest surface cyclone with
open upper level wave
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Progressive S/W trough Short time scale (h) for precipitation Westward extension of comma
head often disconnected from main precipitation
shield Weak easterly flow in CCB is enhanced b
y the eastward motion of the system
Often a non-occluded system with inverted trough
north of low
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Slow-moving upper-level system Long-lasting snow
event ( 12 h) Extensive comma head Strong easte
rly flow in CCB, north of warm front
Surface system is typically occluded
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What is a TROWAL?Penner (1955, Q.J. RMS)
TROugh of Warm air ALoft (TROWAL)
Apex of warm sector

• Cold air

Warm Air
Cold Air
Market 2002
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Conceptual Model of a TROWAL Associated
With a Warm-Type Occlusion
Graphic courtesy of COMET
From Martin (1999, MWR)
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GOES-8 IR satellite image for 10 November 1998
1515 UTC
JMS
Trowal
PIA
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Theta-E Cross-Section (JMS PIA)
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RUC 2 Initialization 650 mb Theta-E
Valid 1500 UTC 10 November
1998
JMS
PIA
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Frontogenesis
F ?v/?y ??/?y ?w/?y ??/?z Term A
Term B- 1/Cp (Po/P)k ?/?y (dQ/dt) -?/?y(Kh
?2?/?y2)Term C Term DA
effect of horizontal temperature gradientB
tilting of the vertical temperature gradient onto
a horizontal planeC horizontal variation in
diabatic heating/coolingD sub-grid scale
horizontal temperature gradient
F0 frontogenesis, F 16
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 direct
thermal circulation (warm air rising and cold air
sinking)
Horizontal Vorticity
Sawyer (1956), Eliassen (1962)
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Dynamics of Frontogenesis
Ageostrophic circulation develops as a response
to increasing temperature gradient.
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Dynamics of Frontogenesis
When we talk about frontogenesis forcing, its
the resulting ageostrophic circulation we are
most interested in for precipitation forecasting.
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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.
  Precipitation bands tend to align with ?

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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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Qn
Q Qs 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
  

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Qs
Q Qs Qn

• 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
  

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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

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Frontogenesis and Symmetric Instability
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Two-Dimensional Form of EPV Equation
Interpretive Form Derived from Martins (1992)
3-D EPV equation, Moore and Lambert (1993),
assumed geostrophic flow, neglected vertical
contribution and neglected y terms to get
A B C D
Term 1
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)
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Nicosia and Grumm Model for EPV Reduction Near
Extratropical Cyclones
Graphic courtesy of COMET
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12-13 to 1
11-12 to 1
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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).

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12-13 to 1
11-12 to 1
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http//www.eas.slu.edu/CIPS/Research/slr/slrmap.ht
m
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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.
• 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.

• Average SLR for southeastern Wisconsin with
  various storm tracks (Adapted from Harms, 1970 )
  

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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

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References

• 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 Symmetric
  Instability in the Midwest Snowstorm of 30-31
  January 1982, MWR, Vol. 116, 2155-2171.
• 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.