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Cool-Season Regime Transition and its Impact on Northeast Precipitation

Cool-Season Regime Transition and its Impact on Northeast Precipitation. Heather Archambault Lance Bosart, Daniel Keyser, Anantha Aiyyer NWS Focal Point: Rich Grumm 3 November 2004 Department of Earth and Atmospheric Sciences, University at Albany, State University of New York.

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Cool-Season Regime Transition and its Impact on Northeast Precipitation

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  1. Cool-Season Regime Transition and its Impact on Northeast Precipitation Heather Archambault Lance Bosart, Daniel Keyser, Anantha Aiyyer NWS Focal Point: Rich Grumm 3 November 2004 Department of Earth and Atmospheric Sciences, University at Albany, State University of New York

  2. Research Motivation • Meteorological wisdom: increased threat of a major storm during large–scale regime transition • Past research points to a connection between synoptic-scale cyclogenesis and reconfiguration of the planetary-scale flow (e.g. Colucci 1985) • Dave Groenert (CSTAR, 2002) documented an apparent tendency for increased precipitation in the Northeast during phase transitions of the North Atlantic Oscillation

  3. Presentation Overview • Research methodology • Statistical look at cool-season (Nov – Apr) weather regime transitions and Northeast precipitation anomalies • Composite analyses of one kind of regime transition

  4. Objectively Defining a “Weather Regime” • 4 weather regimes were defined using teleconnection indices: + NAO – NAO + PNA – PNA • Only extreme events were used (Index anomaly ≥ |1 standard deviation|)

  5. 1000 – 500 hPa Thickness AnomalyWinters (+ NAO) n = 724 Courtesy: Anantha Aiyyer & Eyad Atallah Decameters

  6. 1000 – 500 hPa Thickness AnomalyWinters (– NAO) n = 814 Courtesy: Anantha Aiyyer & Eyad Atallah Decameters

  7. 1000 – 500 hPa Thickness AnomalyWinters (+ PNA) n = 803 Courtesy: Anantha Aiyyer & Eyad Atallah Decameters

  8. 1000 – 500 hPa Thickness AnomalyWinters (– PNA) n = 805 Courtesy: Anantha Aiyyer & Eyad Atallah Decameters

  9. Defining a Weather Regime Change Regime Change Criteria: 1. A significant change in teleconnection index magnitude (|2 stdev|) over a 7 – day period 2. Teleconnection index phase change 3. Index at start of transition must be strongly positive or negative (|Index|>1 stdev)

  10. Creating a Teleconnection Index Database • Calculated daily NAO and PNA indices for a 56-year period (1948 – 2003) using formulas outlined by the Climate Diagnostics Center • Dataset: Twice-daily 500 hPa geopotential heights from 2.5° x 2.5° NCAR/NCEP reanalysis • Used this database to identify all cool-season regime transitions

  11. Creating a Northeast Precipitation Index 1. Using the Unified Precipitation Dataset, calculated domain-average daily precipitation over a 56-year period (1948 –2003) • Domain: Grid size: 0.25° x 0.25°

  12. Creating a Northeast Precipitation Index Example: Top 25 cool – season Northeast precipitation events

  13. Creating a Northeast Precipitation Index 2. Used a simple square-root transformation to normalize the daily precipitation data: y = x0.5 3. Calculated 7-day running sum of precipitation for each day: Sum (d+0) = Precip(d-3) + … + Precip(d+3)

  14. Creating a Northeast Precipitation Index 4. Calculated 7-day precipitation anomaly centered on each day in the time–series: Precipitation Anomaly = [Actual Value]7-day sum – [Climo Value]7-day sum ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– [Climatological Standard Deviation]7-day sum

  15. The NAO and Northeast Precipitation

  16. The NAO and Northeast Precipitation • The phase of the NAO does not appear to be correlated with Northeast precipitation • The tendency of the NAO may be correlated with Northeast precipitation 18 of 25 top Northeast precipitation events: negative NAO tendencies average NAO tendency: -0.45 over 7 days • Illustrates the motivation behind this research

  17. The PNA and Northeast Precipitation

  18. The PNA and Northeast Precipitation •  17 of 25 events: positive PNA transitions Top 20 Precip. Events – 500 hPa Heights

  19. The PNA and Northeast Precipitation • As with the NAO, tendency of the PNA may be more important Top 25 Northeast Precipitation Events • Average PNA tendency: +0.77 over 7 days • 3 events involved PNA “regime changes”

  20. Regime Change and Northeast Precipitation • Looked at teleconnection indices during major Northeast precipitation events • Now, another perspective: • What is climatological Northeast precipitation anomaly during cool-season regime change?

  21. NAO Regime Change and Precipitation

  22. NAO Regime Change and Precipitation • Signals:  “Negative” NAO regime transitions: a somewhat wetter than normal period  strongest signal: late fall – mid-winter  a weakening North Atlantic jet may be associated with enhanced Northeast precipitation  “Positive” NAO regime transitions: a somewhat drier than normal period  a strengthening North Atlantic jet may be associated with drier weather in the Northeast

  23. PNA Regime Change and Precipitation

  24. PNA Regime Change and Precipitation Signals:  “Positive” PNA regime transitions: in Fall and Spring, tend to have enhanced precip • Neither + nor – PNA transitions are associated with enhanced precipitation in Winter • “Negative” PNA regime transitions: a drier than normal period  associated with a building ridge in the East

  25. Composite Analyses • Goal: analyze forcing for ascent during negative NAO regime transitions occurring in November, December, and January • Only included regime transitions that were exactly 7 days long: 20 cases  Analyses don’t contain the most extreme regime transitions

  26. Composite Analyses • Sutcliffe approximation of omega:  advection of vorticity by the thermal wind (shown by 1000-500 hPa thickness)

  27. Composite Cases: Day -3: 500 hPa 1. 5. 9. 13. 17. 2. 6. 10. 14. 18. 3. 7. 11. 15. 19. 4. 8. 12. 16. 20.

  28. Day - 3

  29. Day - 2

  30. Day - 1

  31. Day + 0

  32. Day + 1

  33. Day + 2

  34. Day + 3

  35. Negative NAO Regime Change Composite • Relatively weak mean offshore flow • Weak surface trough over Northeast • Days +0 to +3: block develops in the eastern Atlantic and sharpens the northwesterly flow over the Northeast:  offshore flow shuts off precipitation • *No “smoking gun” storm affecting the Northeast…in the mean

  36. Conclusions Statistical Results: • Negative NAO regime transition corresponds to enhanced precipitation in the Northeast from fall to mid-winter • Positive PNA regime transition corresponds to enhanced precipitation in the Northeast during fall and spring • d/dt |index|  more important than index

  37. Conclusions (cont.) Composite Analysis: • Many different flavors to a negative NAO • No big storm over the Northeast in composite

  38. Caveats and Conclusions • PNA and NAO used as first order approximations of weather regimes: clear limitations • Advantage: strong correlation between observed NAO, PNA and 7 – 10 day MRF ensemble forecasts  “Regime changes” are fairly well-predicted  Models forecasting a negative NAO/positive PNA transition should be a heads-up to forecasters

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