1. Mesoscale Structure of Precipitation Regions in Northeast Winter Storms Matthew D. Greenstein, Lance F. Bosart, and Daniel Keyser Department of Earth and Atmospheric Sciences University at Albany, Albany, NY 12222 David J. Nicosia National Weather Service Binghamton Weather Forecast Office, Johnson City, NY 13790 7 April 2006 CSTAR-II support provided by NOAA Grant NA04NWS4680005

2. Outline Introduction Introduction Case selection Case selection Radar classification Radar classification Cross section analysis Cross section analysis Summary of results Summary of results Future work Future work

3. Introduction Forecasters can predict likely areas of precipitation Forecasters can predict likely areas of precipitation Forecasters cannot always skillfully predict mesoscale features Forecasters cannot always skillfully predict mesoscale features Forecasting mesoscale details adds value to a forecast: Forecasting mesoscale details adds value to a forecast: Prediction of snowfall amount and variability Prediction of snowfall amount and variability Differentiating between high-impact and low-impact snows Differentiating between high-impact and low-impact snows

4. Introduction Precipitation regions have multiple modes (patterns) Precipitation regions have multiple modes (patterns) Goal is to examine ingredients … Goal is to examine ingredients … * Lift * Instability * Moisture * Microphysics * Lift * Instability * Moisture * Microphysics … to find ways of distinguishing the modes … to find ways of distinguishing the modes

5. Introduction: Previous banded studies Matejka, Houze, and Hobbs (1980) Matejka, Houze, and Hobbs (1980) Warm frontal Surge Postfrontal Cold frontal Warm sector

6. Introduction: Previous banded studies Nicosia and Grumm (1999) Nicosia and Grumm (1999)

7. Introduction: Previous banded studies Novak et al. (2004) Novak et al. (2004) BandedNonbanded

8. Introduction: Previous banded studies Novak et al. (2004) Novak et al. (2004) BandedNonbanded

9. Case Selection Cases occur in area bounded by 36.5°N, 50°N, 65°W, and 85°W Cases occur in area bounded by 36.5°N, 50°N, 65°W, and 85°W Within U.S. radar coverage Within U.S. radar coverage 1 October – 30 April 1 October – 30 April No warm sector precipitation No warm sector precipitation P–type predominantly snow P–type predominantly snow “Heavy snow” = 15+ cm in 12 h over area the size of CT “Heavy snow” = 15+ cm in 12 h over area the size of CT No lake effect snows and enhancements No lake effect snows and enhancements Past three winters (2002–3, 2003–4, 2004–5) Past three winters (2002–3, 2003–4, 2004–5)

10. Case Selection Data used Data used NCDC national hourly mosaic reflectivity images NCDC national hourly mosaic reflectivity images Public Information Statements (PNS) Public Information Statements (PNS) Northeast River Forecast Center snowfall maps Northeast River Forecast Center snowfall maps NCDC’s U.S. Storm Events Database NCDC’s U.S. Storm Events Database ASOS reports ASOS reports

11. 20 Cases 26–27 Nov 2002 26–27 Nov 2002 4–6 Dec 2002 4–6 Dec 2002 25–26 Dec 2002 25–26 Dec 2002 2–5 Jan 2003 2–5 Jan 2003 6–7 Feb 2003 6–7 Feb 2003 15–18 Feb 2003 15–18 Feb 2003 6 Mar 2003 6 Mar 2003 5–8 Dec 2003 5–8 Dec 2003 13–15 Dec 2003 13–15 Dec 2003 14–15 Jan 2004 14–15 Jan 2004 27–28 Jan 2004 27–28 Jan 2004 16–17 Mar 2004 16–17 Mar 2004 18–19 Mar 2004 18–19 Mar 2004 19–20 Jan 2005 19–20 Jan 2005 22–23 Jan 2005 22–23 Jan 2005 24–25 Feb 2005 24–25 Feb 2005 28 Feb–2 Mar 2005 28 Feb–2 Mar 2005 8–9 Mar 2005 8–9 Mar 2005 11–13 Mar 2005 11–13 Mar 2005 23–24 Mar 2005 23–24 Mar 2005

12. Radar Classification 2km WSI NOWrad mosaics * 15-min resolution * 3 levels of quality control * Composite reflectivity2km WSI NOWrad mosaics * 15-min resolution * 3 levels of quality control * Composite reflectivity 1.Uniform 2.Classic Band 3.Transient Band 4.Bandlets 5.Fractured 6.Unclassifiable Multiple modes may exist in a storm’s lifecycle and at one time

13. Radar Classification: Uniform 1200 UTC 27 Nov 2002

14. Radar Classification: Classic Band 1900 UTC 7 Feb 2003

15. Radar Classification: Transient Band 1200–2100 UTC 16 Feb 2003 Evolving Band

16. Radar Classification: Transient Band 1600 UTC 6 Dec 2003 Broken Band

17. Radar Classification: Transient Band 2115 UTC 14 Dec 2003 Messy Band

18. Radar Classification: Bandlets 1500 UTC 17 Feb 2003

19. Radar Classification: Fractured 1500 UTC 16 Mar 2004

20. Cross Section Analysis Previous research: frontogenesis in the presence of weak moist symmetric stability yields bands Previous research: frontogenesis in the presence of weak moist symmetric stability yields bands Negative saturation equivalent potential vorticity (EPV*) indicates conditional slantwise instability (CSI) and/or conditional upright instability (CI) Negative saturation equivalent potential vorticity (EPV*) indicates conditional slantwise instability (CSI) and/or conditional upright instability (CI) CI dominates CSI CI dominates CSI EPV* = – g (ζ ·  θ e ), where ζ is the absolute vorticity vector *

21. Cross Section Analysis 32–km North American Regional Reanalysis (NARR) 32–km North American Regional Reanalysis (NARR) Cross sections contain … Cross sections contain … Saturation equivalent potential temperature – θ e (K) Saturation equivalent potential temperature – θ e (K) Relative humidity (%) Relative humidity (%) 2D Petterssen Frontogenesis (ºC 100 km -1 3 h -1 ) 2D Petterssen Frontogenesis (ºC 100 km -1 3 h -1 ) Saturation equivalent potential vorticity - EPV* (PVU) (calculated with the full wind) Saturation equivalent potential vorticity - EPV* (PVU) (calculated with the full wind) Vertical motion (μb s -1 ) Vertical motion (μb s -1 ) Dendritic growth zone, i.e., −12ºC and −8ºC isotherms Dendritic growth zone, i.e., −12ºC and − 1 8ºC isotherms *

22. Cross Section Analysis: Classic Band Strong, steep, surface-based frontogenesis Strong, tilted ascent rooted in the boundary layer Weakly positive EPV* CI unimportant 2100 UTC 7 Feb 2003

23. Cross Section Analysis: Uniform Weak, flat frontogenesis Upright ascent Ascent strength not a factor Weakly positive & negative EPV* has no effect No CI 2100 UTC 22 Jan 2005

24. Cross Section Analysis: Transient Band Weak, decoupled frontogenesis  Inhibits continuous boundary layer moisture feed Weakly positive EPV* seen in all modes 1500 UTC 16 Feb 2003

25. Cross Section Analysis: Bandlets Frontogenesis lifts air parcels to CI region Escalator- elevator 0000 UTC 1 Mar 2005

26. Cross Section Analysis: Fractured Weak, decoupled, fragmented frontogenesis Separate EPV mins and ascent maxes Lower RH 1500 UTC 16 Mar 2004

27. Summary of Results: Distinguishing features FrontogenesisOther Uniformweak and/or flat or none no CI Classic Band strong and steep; surface-based ascent indicates frontal circulation dominates Transient Band decoupledω not well rooted in B.L.; CI alters precip pattern Bandletsthin, weak, or none; surface-based CI / escalator–elevator; deep CI = messier pattern Fracturedfragmented; decoupled CI or frontogenesis enhances precip; lower RH ‡ = some look like a band CI enhances updrafts & downdrafts ‡ = some look like a band CI enhances updrafts & downdrafts ‡ ‡

28. Summary of Results: Nondistinguishing features Ascent strength * * Ascent strength * Uniform: −4 to −24 μb s -1 * Classic band: ≤ −20 μb s -1 Intersection of max ascent with DGZ –100 hPa in most cases) Depth of DGZ (~50–100 hPa in most cases) Intersection of max ascent with CI region Intersection of max ascent with CI region RH patterns RH patterns Reduced EPV* * All cases contain EPV* 0–0.25 PVU (WMSS) and CSI * Shape and location of reduced EPV* regions Reduced EPV* * All cases contain EPV* 0–0.25 PVU (WMSS) and CSI * Shape and location of reduced EPV* regions

29. Summary of Results: Nondistinguishing features From plan-view analyses… From plan-view analyses… QG–forcing ratio: DCVA / (DCVA + WAA) Depths of reduced EPV* satisfying various criteria –0.25, or EPV* ≤ 0, ≤ 0.25, 0–0.25, or ≤ −0.25 + RH ≥ 70% + Ascent Max vertical speed shear Max vertical speed shear 850–500 hPa vertical speed shear 850–500 hPa vertical speed shear

30. Summary of Results: EPV* vs. EPV Reasons for EPV Reasons for EPV Symmetric instability theory: thermal wind balance Symmetric instability theory: thermal wind balance M g more accurately captures growing instability M g more accurately captures growing instability Reasons for EPV* Better representation of curved flow Assumption that time scale of convection << time scale for large-scale environmental changes not valid?  Potential for slantwise convection better found by using an evolving and unbalanced environment? * g*g

31. EPV EPV**g 0600 UTC 23 Jan 2005

32. Summary of Results: EPV* vs. EPV EPV produces a messier pattern with more negative values, especially in dry areas EPV produces a messier pattern with more negative values, especially in dry areas EPV “bull’s-eyes” line up with band positions EPV “bull’s-eyes” line up with band positions * g*g *g Because… 1) Value does not seem to matter 2) WMSS is a necessary but not distinguishing factor 3) CI plays an important role Because… 1) Value does not seem to matter 2) WMSS is a necessary but not distinguishing factor 3) CI plays an important role Use EPV* because it produces a cleaner image Use EPV* because it produces a cleaner image If classic band is indicated, use EPV for position If classic band is indicated, use EPV for position* g

33. Summary of Results: Conceptual Models

38. Summary of Results: Flowchart

39. Future Work Is the “fractured” mode really just a hybrid of “bandlets” & “transient bands” but with drier spaces? Is the “fractured” mode really just a hybrid of “bandlets” & “transient bands” but with drier spaces? Prove decoupled frontogenesis hypothesis Prove decoupled frontogenesis hypothesis Investigate band lag Investigate band lag Examine the EPV “bull’s-eyes” Examine the EPV “bull’s-eyes”*g

40. Special Thanks Lance and Dan Lance and Dan David Ahijevych (NCAR) David Ahijevych (NCAR) Kevin Tyle Kevin Tyle Alan Srock Alan Srock Anantha Aiyyer Anantha Aiyyer Keith Wagner Keith Wagner Celeste, Sharon, and Lynn Celeste, Sharon, and Lynn My parents My parents

41. Questions? Comments? e-mail: greenstein@atmos.albany.edu