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Cool-Season Regime Transition and its Impact on Precipitation in the Northeastern United States

Cool-Season Regime Transition and its Impact on Precipitation in the Northeastern United States. Heather Archambault Daniel Keyser, Lance Bosart Department of Earth and Atmospheric Sciences, University at Albany, SUNY CSTAR Focal Point: Rich Grumm

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Cool-Season Regime Transition and its Impact on Precipitation in the Northeastern United States

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  1. Cool-Season Regime Transition and its Impact on Precipitation in the Northeastern United States Heather Archambault Daniel Keyser, Lance Bosart Department of Earth and Atmospheric Sciences, University at Albany, SUNY CSTAR Focal Point: Rich Grumm Central Pennsylvania Weather Forecast Office State College, PA NOAA Grant NA07WA0458

  2. Research Motivation • Is Northeast precipitation influenced by large-scale weather regimes? • Positive correlation between the phase of the North Atlantic Oscillation (NAO) and East Coast precipitation (Hurrell 1995) • Weak negative correlation between the phase of the Pacific/North American (PNA) pattern and Northeast precipitation (Leathers et al. 1991) • No prior studies on relationships between regime transitions and Northeast precipitation

  3. Research Motivation • Can synoptic-scale storms that produce significant precipitation in the Northeast play a role in large-scale weather regime transitions? • Synoptic-scale/planetary scale interactions linked to evolution of large-scale weather regimes (e.g., Colucci 1985; Dole 1986; Cash and Lee 2001) • Rossby wave-breaking attributed to development of both phases of the North Atlantic Oscillation (Franzke et al. 2003; Benedict et al. 2004)

  4. Research Goals • Develop objective definitions for large-scale regimes and regime transitions • Establish statistical relationships between Northeast precipitation and large-scale regimes/large-scale regime transitions • Use composite analyses to explore these relationships • Determine whether Northeast storms/regime transition relationships are associative or cause and effect

  5. Data and Methodology: Key Definitions • Large-Scale Regimes: +1/−1 standard deviation (σ) anomalies of the NAO or PNA • Persistent Large-Scale Regimes: Regimes lasting seven days • Large-Scale Regime Transitions: NAO or PNA index changes of 2σ centered on zero occurring over a seven-day period

  6. Data and Methodology: Large-Scale Regimes 1000–500 hPa Thickness Anomalies: Winter − + − + − + − + Positive NAO Negative NAO Courtesy: Anantha Aiyyer & Eyad Atallah

  7. Data and Methodology: Large-Scale Regimes 1000–500 hPa Thickness Anomalies: Winter − + − + − + − + Positive PNA Negative PNA Courtesy: Anantha Aiyyer & Eyad Atallah

  8. Data and Methodology: Datasets • Regimes: Following CDC method, calculated daily standardized NAO and PNA indices using 500 hPa geopotential heights from NCEP–NCAR reanalysis dataset • 2.5°x 2.5° resolution, 1948–2003 • Northeast Precipitation: Calculated domain-average daily Northeast precipitation from the NCEP Unified Precipitation Dataset • 0.25°x 0.25° resolution, 1948–2003

  9. Results: Relationships Between Large-Scale Regimes and Northeast Precipitation

  10. Cool-Season Precipitation Anomalies During Persistent Large-Scale Regimes

  11. Cool-Season Precipitation Anomalies During Persistent Large-Scale Regimes • Northeast precipitation is above normal during negative PNA and positive NAO regimes • Northeast precipitation is below normal during positive PNA regimes

  12. NAO, PNA Indices during Major Cool-Season Precipitation Events Are NAO and PNA indices typically positive or negative during major cool-season precipitation events in the Northeast?

  13. NAO Index During Top 25 Cool-Season Precipitation Events

  14. NAO Index During Top 25 Cool-Season Precipitation Events 6 events > +1.0 NAO Index 17 25 8 13 2 16 7 11 15 20 12 5 events < −1.0 NAO Index

  15. NAO/Precipitation Relationships • Major precipitation events in the Northeast appear to occur independently of the phase of the NAO • This finding is consistent with results showing that cool-season precipitation is marginally enhanced during persistent positive NAO regimes

  16. PNA Index During Top 25 Cool-Season Precipitation Events

  17. PNA Index During Top 25 Cool-Season Precipitation Events 7 events > +1.0 PNA Index 20 22 2 13 24 5 25 4 1 event < −1.0 PNA Index

  18. PNA/Precipitation Relationships • Major precipitation events in the Northeast appear to occur preferentially during the positive phase of the PNA • Shown earlier: Cool-season precipitation is enhanced in the Northeast during persistent negative PNA regimes and suppressed during positive PNA regimes • How are these results consistent?

  19. PNA/Precipitation Relationships • Contrast top cool-season precipitation events that occurred during strong +PNA regimes with top events that occurred during strong −PNA regimes (PNA > +2.0 vs. PNA < −2.0) • Use three types of composite analyses (centered at midpoints of events) that show: 1. The large-scale pattern 2. The upper-level jet structure 3. Dynamical forcing for ascent

  20. Top Negative PNA Precipitation Events

  21. Top Positive PNA Precipitation Events

  22. Top Positive PNA Precipitation Events #2

  23. Mean Northeast Precipitation:1.5 cm / 24 h Composite PNA:−2.3 std. dev. Composite Negative PNA Storms (Top 5) 500 hPa Height (dam), Height Anomaly (dam)

  24. Mean Northeast Precipitation:2.3 cm / 24 h Composite PNA:+2.3 std. dev. Composite Positive PNA Storms (Top 5) 500 hPa Height (dam), Height Anomaly (dam)

  25. Mean Northeast Precipitation:1.5 cm / 24 h Composite PNA: −2.3 std. dev. Composite Negative PNA Storms (Top 5) 300 hPa Height (dam), Wind Speed (m s-1)

  26. Mean Northeast Precipitation: 2.3 cm / 24 h Composite PNA: +2.3 std. dev. Composite Positive PNA Storms (Top 5) 300 hPa Height (dam), Wind Speed (m s-1)

  27. Mean Northeast Precipitation:1.5 cm / 24 h Composite PNA:−2.3 std. dev. L H L H H Composite Negative PNA Storms (Top 5) 1000–500 hPa Thick (dam), 700 hPa Abs. Vort. (10-5 s-1), MSLP (hPa)

  28. Mean Northeast Precipitation:2.3 cm / 24 h Composite PNA:+2.3 std. dev. L L H L H Composite Positive PNA Storms (Top 5) 1000–500 hPa Thick (dam), 700 hPa Abs. Vort. (10-5 s-1), MSLP (hPa)

  29. PNA/Precipitation Relationships • Better dynamics for storms during +PNA regimes than −PNA regimes • Why, then, is precipitation enhanced during persistent −PNA regimes? • During −PNA regimes, above-normal southwesterly flow from the Gulf of Mexico may enhance Northeast precipitation • During +PNA regimes, the jet stream is suppressed to the south and typically allows cool, dry air to prevail over the Northeast (Leathers et al. 1991)

  30. Summary of Relationships between Large-Scale Regimes and Northeast Precipitation • Positive NAO and negative PNA regimes are associated with enhanced Northeast precipitation • The PNA is typically positive during major precipitation events in the Northeast • Major precipitation events in the Northeast appear to occur independently of the phase of the NAO

  31. Relationships Between Large-Scale Regime Transitions and Northeast Precipitation

  32. Cool-Season Precipitation Anomalies During Large-Scale Regime Transitions

  33. Cool-Season Precipitation Anomalies During Large-Scale Regime Transitions Above-normal Northeast precipitation: • Transition periods from negative PNA to positive PNA • Transition periods from positive NAO to negative NAO Below-normal Northeast precipitation: • Transition periods from positive PNA to negative PNA

  34. NAO Index Trends During Major Cool-Season Precipitation Events How does the NAO behaveduring periods surrounding major cool-season precipitation events in the Northeast?

  35. Seven-Day NAO Index Tendency During Top 25 Cool-Season Precipitation Events

  36. Seven-Day NAO Index Tendency During Top 25 Cool-Season Precipitation Events 16 11 21 25 4 events > +1.0 NAO Tendency 14 12 15 9 13 3 23 8 events < − 1.0 NAO Tendency 22

  37. Relationships between NAO Changes and Northeast Precipitation • The phase of the NAO typically becomes more negative in the seven-day period surrounding a major precipitation event in the Northeast • This result is consistent with earlier results showing cool-season precipitation is enhanced in the Northeast during regime transitions from the positive to negative phase of the NAO

  38. Top Precipitation Events Occurring During Negative NAO Transitions

  39. Top Precipitation Events Occurring During Negative NAO Transitions #3

  40. Composite 500 hPa Heights, Anomalies During Transitions from +NAO to −NAO

  41. Mean Northeast Precipitation:0.4 cm / 24 h Composite NAO:+1.2 std. dev. Composite Negative NAO Transition: Day 1 500 hPa Height (dam), Height Anomaly (dam)

  42. Mean Northeast Precipitation:0.2 cm / 24 h Composite NAO:+1.0 std. dev. Composite Negative NAO Transition: Day 2 500 hPa Height (dam), Height Anomaly (dam)

  43. Mean Northeast Precipitation:0.8 cm / 24 h Composite NAO:+0.9 std. dev. Composite Negative NAO Transition: Day 3 500 hPa Height (dam), Height Anomaly (dam)

  44. Mean Northeast Precipitation:2.0 cm / 24 h Composite NAO:+0.6 std. dev. Composite Negative NAO Transition: Day 4 500 hPa Height (dam), Height Anomaly (dam)

  45. Mean Northeast Precipitation:0.7 cm / 24 h Composite NAO:−0.3 std. dev. Composite Negative NAO Transition: Day 5 500 hPa Height (dam), Height Anomaly (dam)

  46. Mean Northeast Precipitation:0.1 cm / 24 h Composite NAO:−1.3 std. dev. Composite Negative NAO Transition: Day 6 500 hPa Height (dam), Height Anomaly (dam)

  47. Mean Northeast Precipitation:0.3 cm / 24 h Composite NAO:−1.5 std. dev. Composite Negative NAO Transition: Day 7 500 hPa Height (dam), Height Anomaly (dam)

  48. Composite 300 hPa Heights, Wind Speeds During Transitions from +NAO to −NAO

  49. Mean Northeast Precipitation:0.4 cm / 24 h Composite NAO:+1.2 std. dev. Composite Negative NAO Transition: Day 1 300 hPa Height (dam), Wind Speed (m s-1)

  50. Mean Northeast Precipitation:0.2 cm / 24 h Composite NAO:+1.0 std. dev. Composite Negative NAO Transition: Day 2 300 hPa Height (dam), Wind Speed (m s-1)

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