1. Relationships between Large-Scale Regime Transitions and Major Cool-Season Precipitation Events in the Northeast U.S. Heather M. Archambault Daniel Keyser and Lance F. Bosart Department of Earth and Atmospheric Sciences University at Albany, SUNY 26 September 2008 NSF Grant ATM-0434189 NOAA Grant NA07NWS4680001

2. Presentation Overview A. A.Motivation and overview B. B.Methodology C. C.Statistical analyses of cool-season Northeast precipitation during regime transitions D. D.Composite analyses of a major cool-season Northeast precipitation event occurring during a regime transition E. E.Case study: Major Northeast precipitation event of 29–30 Nov 1963 F. F.Summary

3. Motivation Past research indicates that Northeast precipitation is modulated by certain types of large-scale flow regimes (e.g., Leathers et al. 1991; Hurrell 1995; Archambault et al. 2008) Synoptic/planetary-scale flow interactions are an important aspect of regime transitions (e.g., Higgins and Schubert 1994; Benedict et al. 2004; Woollings et al. 2008)

4. Motivation Conventional meteorological wisdom suggests that high-impact weather events tend to be associated with regime transitions A critical scientific and operational challenge is predicting high-impact weather events such as cool-season heavy precipitation associated with extratropical cyclones

5. Motivation Are certain cool-season regime transitions associated with an increased frequency of major Northeast precipitation events? Do major Northeast precipitation events and associated extratropical cyclones play a role in cool-season regime transitions?

6. Key Definitions Large-scale regime: +/  1.0 std dev anomaly of the North Atlantic Oscillation (NAO) or Pacific/North American (PNA) pattern daily index Large-scale regime transition: NAO or PNA daily index change ≥ 2.0 std dev centered on zero occurring in seven or fewer days

7. Overview of Large-Scale Regimes 500-hPa Standardized Geopotential Height Anomalies: Cool Season Source: http://www.cdc.noaa.gov/map/wx/indices.shtml Positive NAO PhasePositive PNA Phase

8. Evolution of a Negative NAO Regime: θ on the Dynamic Tropopause (Benedict et al. 2004) 305 K 335 K Day  8: Weak trough over eastern North America Day  4: Cyclonic wave breaking over western North Atlantic Day 0: Onset of high-latitude blocking pattern Day +4: Decay of high-latitude blocking pattern

9. Datasets Large-Scale Regime Data: Time series of daily standardized cool-season (Nov–Apr) NAO and PNA indices (1 Jan 1948–30 Apr 2003) Data Source: 2.5° NCEP–NCAR reanalysis NAO domainPNA domain

10. Datasets Northeast Precipitation Data: Time series of domain-averaged 24-h cool-season Northeast precipitation (1 Jan 1948–30 Apr 2003) Data Source: 0.25° Unified Precipitation Dataset Northeast domain

11. Statistical Analyses: Dependence of cool-season Northeast precipitation on regime transitions

12. Above-normal Northeast precipitation is associated with +NAO   NAO and  PNA  +PNA regime transitions Composite Northeast Precipitation Anomalies for Cool-Season Regime Transitions Percentages above and below bars indicate statistical significance as calculated from a Student’s t-test.

13. Histogram of 24-h Northeast Precipitation Anomalies during +NAO to  NAO Transitions Major (>+1.6 std dev anomaly) Northeast precipitation events are more frequent compared to climatology n = 225

14. Histogram of 24-h Northeast Precipitation Anomalies during  PNA to +PNA Transitions Moderate Northeast precipitation events are more frequent compared to climatology n = 350

15. Frequency of Major 24-h Northeast Precipitation Events During Regime Transitions Major precipitation events occur during +NAO   NAO transitions at ~twice the climatological frequency

16. Summary of Statistical Results Dependence of cool-season Northeast precipitation on regime transitions: Above-normal precipitation is associated with +NAO   NAO and  PNA  +PNA transitions Major precipitation events occur during +NAO   NAO transitions at ~twice the climatological frequency

17. Composite Analyses of a Major Northeast Precipitation Event Occurring during a +NAO   NAO Regime Transition

18. Composite Case Selection from 30 Major 24-h Northeast Precipitation Events during a +NAO   NAO Regime Transition 1. 1.Two consecutive 24-h major precipitation events are treated as one independent event: 24 events 2. NAO index is positive at precipitation event onset (T+0 h): 17 events 3. NAO index is negative at T+48 h: 14 events 4. 4.500-hPa flow evolution is similar among events: 11 events

19. 1 st set of maps: Midtropospheric flow evolution: 500-hPa geopotential height and height anomaly 2 nd set of maps: Tropopause flow evolution: Potential temperature on DT (2.0 - PVU surface) Data source: 2.5° NCEP–NCAR reanalysis Composite Analyses of a Major Northeast Precipitation Event during a +NAO   NAO Regime Transition

20. Composite +NAO   NAO Regime Transition: T  48 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) n=11 NAO

21. Composite +NAO   NAO Regime Transition: T  24 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) n=11 NAO

22. n=11 Composite +NAO   NAO Regime Transition: T+0 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO

23. n=11 Composite +NAO   NAO Regime Transition: T+24 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO

24. Composite +NAO   NAO Regime Transition: T+48 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) n=11 NAO

25. Composite +NAO   NAO Regime Transition: T+72 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) n=11 NAO

26. Composite +NAO   NAO Regime Transition: T  48 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) n=11 NAO

27. Composite +NAO   NAO Regime Transition: T  24 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) n=11 NAO

28. n=11 Composite +NAO   NAO Regime Transition: T+0 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO

29. n=11 Composite +NAO   NAO Regime Transition: T+24 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO

30. Composite +NAO   NAO Regime Transition: T+48 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) n=11 NAO

31. Composite +NAO   NAO Regime Transition: T+72 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) n=11 NAO

32. Summary of Composite Analyses Midtropospheric flow evolution: – –Ridge amplification associated with major Northeast precipitation event leads to onset of high-latitude blocking over the North Atlantic Tropopause flow evolution: – –Cyclonic wave breaking occurs over western North Atlantic, consistent with negative NAO evolution described by Benedict et al. (2004) and Woollings et al. (2008)

33. Case Study Overview 29–30 Nov 1963: A representative major Northeast precipitation event Ranks second in the 11-event composite in storm total Northeast precipitation Data source: 1.125 ° ERA-40 reanalysis

34. Case Study Overview Case used by Danielsen (1966) to demonstrate reliability of isentropic trajectory analysis

35. Coauthor’s anecdotal recollection (in an email dated 24 Jan 2008): “How ironic that I analyzed this case in my very first synoptic laboratory class with Fred Sanders in September 1966. It was Fred’s ‘bomb-of-the-year’ case from 1963, so chosen because at least a dozen big windows were blown out of the Green Building … on the MIT campus at the height of the storm.” Case Study Overview

36. 36-h 305-K Isentropic Trajectories ending 1200 UTC 30 Nov 1963 455 hPa 715 hPa (Danielsen 1966) >0.5 g kg -1

37. 36-h Kinematic Trajectories ending 1200 UTC 30 Nov 1963 on 305-K Surface Parcel pressure (hPa) Mixing ratio (g kg -1 ) on 305-K surface

38. 30º–50ºN 500-hPa Geopotential Height Hovmöller: 24 Nov–6 Dec 1963 Midtropospheric flow amplifies over eastern Pacific, North America, and central North Atlantic during case study period

39. Case Study +NAO   NAO Regime Transition: T  48 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO 1200 UTC 27 Nov 1963

40. Case Study +NAO   NAO Regime Transition: T  24 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO 1200 UTC 28 Nov 1963

41. Case Study +NAO   NAO Regime Transition: T+0 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO 1200 UTC 29 Nov 1963

42. Case Study +NAO   NAO Regime Transition: T+24 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO 1200 UTC 30 Nov 1963

43. Case Study +NAO   NAO Regime Transition: T+48 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO 1200 UTC 1 Dec 1963

44. Case Study +NAO   NAO Regime Transition: T+72 h Potential temperature on the DT (K; shaded), wind > 50 kt on the DT (barbs) NAO 1200 UTC 2 Dec 1963

45. 36-h Back Trajectory ending 1200 UTC 1 Dec 1963 00Z 30 Nov 12Z 30 Nov 00Z 1 Dec 12Z 1 Dec 12Z 1 Dec 300-hPa Geo. Height (dam) Parcel Pressure (hPa) 600 500 400 300

46. 12Z 1 Dec00Z 1 Dec12Z 30 Nov00Z 30 Nov d(PV)/dt =  0.3 PVU (8 h)  1 0.4 PVU 0.1 PVU PV Evolution Following Parcel

47. Summary of Case Study Results Major Northeast precipitation event of 29–30 Nov 1963: 1. Leads to onset of high-latitude blocking over North Atlantic 2. Occurs in conjunction with cyclonic Rossby wave breaking over western North Atlantic Potential temperature and PV nonconservation suggest that diabatic heating associated with major Northeast precipitation event contributes to onset of high-latitude blocking

48. Summary 1. 1.Cool-season Northeast precipitation is favored during certain regime transitions Statistical analyses: Above-normal precipitation is associated with +NAO   NAO and  PNA  +PNA transitions Major precipitation events occur during +NAO   NAO transitions at ~twice the climatological frequency

49. Summary 2. 2.Major Northeast precipitation events and associated cyclones can contribute to cool-season regime transitions Composite analyses and case study: Ridge amplification induced by major Northeast precipitation events can lead to onset of high-latitude blocking over North Atlantic associated with a +NAO   NAO transition

50. References Archambault, H. M., L. F. Bosart, D. Keyser, and A. R. Aiyyer, 2008: Influence of large-scale flow regimes on cool-season precipitation in the northeastern United States. Mon. Wea. Rev., 136, 2945–2963. Benedict, J. J., S. Lee, and S. B. Feldstein, 2004: Synoptic view of the North Atlantic Oscillation. J. Atmos. Sci., 61, 121–144. Higgins, R. W., and S. D. Schubert, 1994: Simulated life cycles of persistent anticyclonic anomalies over the North Pacific: Role of synoptic-scale eddies. J. Atmos. Sci., 51, 3238–3260. Hurrell, J. W., 1995: Decadal trends in the North Atlantic oscillation: Regional temperatures and precipitation. Science, 269, 676–679. Leathers, D. J., B. Yarnal, and M. A. Palecki, 1991: The Pacific/North American teleconnection pattern and United States climate. Part I: Regional temperature and precipitation associations. J. Climate, 4, 517– 528. Woollings, T., B. Hoskins, M. Blackburn, and P. Berrisford, 2008: A new Rossby wave-breaking interpretation of the North Atlantic Oscillation. J. Atmos. Sci., 65, 609–626.

51. NAO Index and 24-h Domain-Averaged Northeast Precipitation for Composite

52. NAO Index and 24-h Domain-Averaged Northeast Precipitation for Case Study

53. Case Study +NAO   NAO Regime Transition: T  48 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO 1200 UTC 27 Nov 1963

54. Case Study +NAO   NAO Regime Transition: T  24 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO 1200 UTC 28 Nov 1963

55. Case Study +NAO   NAO Regime Transition: T+0 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO 1200 UTC 29 Nov 1963

56. Case Study +NAO   NAO Regime Transition: T+24 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO 1200 UTC 30 Nov 1963

57. Case Study +NAO   NAO Regime Transition: T+48 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO 1200 UTC 1 Dec 1963

58. Case Study +NAO   NAO Regime Transition: T+72 h 500-hPa geopotential height (dam; contoured in black), geopotential height anomaly (dam; shaded) NAO 1200 UTC 2 Dec 1963