Physical Processes Associated with Heavy Snowfall

1. Physical Processes Associated with Heavy Snowfall EAX Winter Weather Seminar November 10, 2004 Edited by Suzanne Fortin, SOO EAX Contributions by many to be named later...

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

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

4. 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) • Avertical temperature profile conducive to the efficient production of dendritic crystals, high snow:liquid equivalent ratios, and little/no melting of crystals

5. A Conceptual Model: Plan View of Key Processes NW SE

6. 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

7. 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

8. C 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

9. 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

10. TROugh of Warm air ALoft (TROWAL) What is a TROWAL?Penner (1955, Q.J. RMS) Apex of warm sector Cold air Warm Air Cold Air Market 2002

11. Conceptual Model of a TROWAL Associated With a Warm-Type Occlusion Graphic courtesy of COMET From Martin (1999, MWR)

12. GOES-8 IR satellite image for 10 November 1998 1515 UTC JMS Trowal PIA

13. Theta-E Cross-Section (JMS – PIA)

14. RUC 2 Initialization 650 mb Theta-E Valid 1500 UTC 10 November 1998 JMS PIA

15. 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 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 F>0 frontogenesis, F<0 frontolysis

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)

17. Dynamics of Frontogenesis Ageostrophic circulation develops as a response to increasing temperature gradient.

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

19. 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

20. 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

21. 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

22. 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

23. Frontogenesis and Symmetric Instability

24. 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: 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)

26. Nicosia and Grumm Model for EPV Reduction Near Extratropical Cyclones Graphic courtesy of COMET

27. 12-13 to 1 11-12 to 1

28. 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). Origination of the Liquid Ratio Problem

29. 12-13 to 1 11-12 to 1

30. http://www.eas.slu.edu/CIPS/Research/slr/slrmap.htm

31. 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 )

32. 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

33. 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.