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Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic

Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic. Subtropical Storm Sean 8 November 2011. 28N. 72W. 68W. 6 4 W. Alicia M. Bentley, Daniel Keyser, and Lance F. Bosart University at Albany, SUNY 16 th Cyclone Workshop 27 September 2013

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Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic

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  1. Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic Subtropical Storm Sean 8 November 2011 28N 72W 68W 64W Alicia M. Bentley, Daniel Keyser, and Lance F. Bosart University at Albany, SUNY 16th Cyclone Workshop 27 September 2013 Research support provided by NSF Grant AGS-0935830

  2. Subtropical Cyclones Operational Definition • “A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones.” • “Unlike tropical cyclones, subtropical cyclones derive a significant portion of their energy from baroclinic sources…often being associated with an upper-level low or trough.” − National Hurricane Center Online Glossary (2012)

  3. Subtropical Cyclones DiabaticEnergySources TCs Subtropical cyclones Frontal cyclones BaroclinicEnergy Sources Adapted from Fig. 9 in Beven (2012)30th Conference on Hurricanes and Tropical Meteorology

  4. Motivation • There is currently no objective set of characteristics used to define subtropical cyclones (STCs) • The hybrid nature of STCs makes them likely candidates to become tropical cyclones (TCs) via the tropical transition (TT) process • Few studies address the relationship between STCs, TC development, and high-impact weather events

  5. Outline • Adapt Davis (2010) methodology for STC identification • Refine objective STC identification technique and apply to North Atlantic baroclinically influenced tropical cyclogenesis cases to construct STC climatology (1979–2010) • Perform a cyclone-relative composite analysis of the upper-tropospheric features linked to STC formation • Discussion and Conclusions

  6. Adapt Davis (2010) Methodology • Davis (2010) methodology: • Based on Ertel potential vorticity (PV) • Formulated in terms of two PV metrics that quantify the relative contributions of baroclinicprocesses and condensation heating to the evolution of individual cyclones • Davis (2010) methodology is similar to Hart (2003) cyclone phase space diagrams

  7. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) 425 hPa Potential temperature anomaly Length of 6° of latitude absolute vorticity

  8. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PVanomaly) 425 hPa Potential temperature anomaly Length of 6° of latitude absolute vorticity Ertel PV anomaly PV1/PV2 : measure of the contribution of lower-troposphericbaroclinic processes relative to the contribution of condensation heating

  9. Adapt Davis (2010) Methodology 200 hPa 925 hPa

  10. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) 200 hPa 925 hPa Lower-tropospheric baroclinic processes (PV1)

  11. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PV anomaly) 200 hPa 925 hPa Lower-tropospheric baroclinic processes (PV1)

  12. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PV anomaly) 200 hPa 500 hPa Midtroposphericlatent heat release (PV2) 925 hPa Lower-tropospheric baroclinic processes (PV1) PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

  13. Adapt Davis (2010) Methodology • Introduce additional metric to diagnose upper-tropospheric dynamical processes • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly) Ertel PV anomaly 300 hPa Length of 6° of latitude

  14. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PV anomaly) 200 hPa 500 hPa Midtroposphericlatent heat release (PV2) 925 hPa Lower-tropospheric baroclinic processes (PV1) PV1/PV2 : measure of the contribution of lower-troposphericbaroclinic processes relative to the contribution of condensation heating

  15. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PV anomaly) • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly) Upper-troposphericdynamical processes(PV3) 200 hPa 500 hPa Midtroposphericlatent heat release (PV2) 925 hPa Lower-tropospheric baroclinic processes (PV1) PV1/PV2 : measure of the contribution of lower-troposphericbaroclinic processes relative to the contribution of condensation heating

  16. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PV anomaly) • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly) Upper-troposphericdynamical processes(PV3) 200 hPa 500 hPa Midtroposphericlatent heat release (PV2) 925 hPa Lower-tropospheric baroclinic processes (PV1)

  17. Adapt Davis (2010) Methodology • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly) • Midtroposphericlatent heat release:(interior PV anomaly) • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly) Upper-troposphericdynamical processes(PV3) 200 hPa 500 hPa Midtroposphericlatent heat release (PV2) 925 hPa Lower-tropospheric baroclinic processes (PV1) PV3/PV2 : measure of the contribution of upper-troposphericdynamical processes relative to the contribution of condensation heating

  18. STC Identification • Apply adapted Davis (2010) methodology to a subset of North Atlantic baroclinically influenced tropical cyclogenesis cases identified in McTaggart-Cowan et al. (2013) • IBTrACS (v03r03) • North Atlantic tropical cyclogenesis cases1948–2010: 816 cases1979–2010: 460 cases (period of CFSR) • 36 h backward trajectories obtained using a reverse steering flow calculation (McTaggart-Cowan et al. 2008) and added to IBTrACS

  19. STC Identification N = 460 Strong TT Weak TT Trough induced Low-level baroclinicNonbaroclinic

  20. STC Identification N = 460 Strong TT Weak TT Trough induced Low-level baroclinicNonbaroclinic Category Description Strong TT Upper-level disturbance without strong lower-level thermal gradients Weak TT Upper-level disturbance with moderate lower-level thermal gradients Trough induced Upper-level disturbance without appreciable lower-level thermal gradients Low-level baroclinic Strong lower-level thermal gradients without an upper-level disturbance Nonbaroclinic No appreciable baroclinic influences

  21. STC Identification N = 460 Strong TT Weak TT Trough induced Low-level baroclinicNonbaroclinic Category Description Strong TT Upper-level disturbance without strong lower-level thermal gradients Weak TT Upper-level disturbance with moderate lower-level thermal gradients Trough induced Upper-level disturbance without appreciable lower-level thermal gradients Low-level baroclinic Strong lower-level thermal gradients without an upper-level disturbance Nonbaroclinic No appreciable baroclinic influences

  22. STC Identification N = 460 Strong TT Weak TT Trough induced Low-level baroclinicNonbaroclinic Category Description Strong TT Upper-level disturbance without strong lower-level thermal gradients Weak TT Upper-level disturbance with moderate lower-level thermal gradients Trough induced Upper-level disturbance without appreciable lower-level thermal gradients Low-level baroclinic Strong lower-level thermal gradients without an upper-level disturbance Nonbaroclinic No appreciable baroclinic influences

  23. STC Identification N = 460 Strong TT Weak TT Trough induced Low-level baroclinicNonbaroclinic

  24. STC Identification N = 222 Strong TT Weak TT Trough induced Low-level baroclinicNonbaroclinic

  25. STC Identification • Identify STC signature in PV metrics to refine objective identification technique • Determine the time and position when individual cyclones became STCs using objective identification technique to construct STC climatology (1979–2010) • Perform a cyclone-relative composite analysis to document the structure, motion, and evolution of the upper-tropospheric features linked to STC formation

  26. STC Identification • Identify STC signature in PV metrics to refine objective identification technique • Determine the time and position when individual cyclones became STCs using objective identification technique to construct STC climatology (1979–2010) • Perform a cyclone-relative composite analysis to document the structure, motion, and evolution of the upper-tropospheric features linked to STC formation

  27. STC Identification STC Sean (2011) 1745 UTC 7 November 2011 PV3/PV2 PVU EX LOSTC TS EX Image courtesy of NASA Goddard MODIS Rapid Response Team 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  28. STC Identification STC Sean (2011) 1745 UTC 7 November 2011 PV3/PV2 PVU EX LOSTC TS EX Image courtesy of NASA Goddard MODIS Rapid Response Team 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  29. STC Identification STC Sean (2011) Criteria: PV3/PV2 PVU EX LOSTC TS EX 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  30. STC Identification STC Sean (2011) Criteria: • Slope of PV3/PV2 < 0 at t = 0 h, 6 h PV3/PV2 PVU EX LOSTC TS EX 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  31. STC Identification STC Sean (2011) Criteria: • Slope of PV3/PV2 < 0 at t = 0 h, 6 h • PV3 > 0 and PV2 > 0 at t = 0 h PV3/PV2 PVU EX LOSTC TS EX 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  32. STC Identification STC Sean (2011) Criteria: • Slope of PV3/PV2 < 0 at t = 0 h, 6 h • PV3 > 0 and PV2 > 0 at t = 0 h • Slope of PV3 < 0 at t = 6h, 12 h PV3/PV2 PVU EX LOSTC TS EX 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  33. STC Identification STC Sean (2011) Criteria: • Slope of PV3/PV2 < 0 at t = 0 h, 6 h • PV3 > 0 and PV2 > 0 at t = 0 h • Slope of PV3 < 0 at t = 6h, 12 h • Not a hurricane • Not a tropical storm • Not a tropical depression for ≥ 12 h PV3/PV2 PVU EX LOSTC TS EX 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov PV1 PV2 PV3 PV3/PV2

  34. STC Identification STC Sean (2011) Criteria: • Slope of PV3/PV2 < 0 at t = 0 h, 6 h • PV3 > 0 and PV2 > 0 at t = 0 h • Slope of PV3 < 0 at t = 6h, 12 h • Not a hurricane • Not a tropical storm • Not a tropical depression for ≥ 12 h PV3/PV2 PVU EX LOSTC TS EX 9 Nov 7 Nov 8 Nov 10 Nov 12 Nov 11 Nov = STC identified PV1 PV2 PV3 PV3/PV2

  35. STC Climatology (1979–2010) N = 105 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  36. STC Climatology (1979–2010) N = 105 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  37. STC Climatology (1979–2010) N = 34 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  38. STC Climatology (1979–2010) N = 56 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  39. STC Climatology (1979–2010) N = 15 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  40. STC Climatology (1979–2010) N = 125 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  41. STC Climatology (1979–2010) N = 105 ~3 STCs/year Strong TT Weak TT Trough Induced Average Strong TT Average Weak TT Average Trough Induced Average STC

  42. Conclusions • STCs have characteristics of both tropical and extratropicalcyclones and are likely candidates to become TCs via TT • Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes during baroclinically influenced tropical cyclogenesis events • Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during STC formation • Cyclone-relative composite analysis revealed that STCs form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events

  43. Questions? ambentley@albany.edu • STCs have characteristics of both tropical and extratropicalcyclones and are likely candidates to become TCs via TT • Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes during baroclinically influenced tropical cyclogenesis events • Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during STC formation • Cyclone-relative composite analysis revealed that STCs form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events Special Thanks: Chris Davis and Ron McTaggart-Cowan

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