1. Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic Alicia M. Bentley, Daniel Keyser, and Lance F. Bosart University at Albany, SUNY 16 th Cyclone Workshop Sainte-Adele, Quebec, Canada 25 September 2013

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 Diabatic Energy Sources Baroclinic Energy Sources Adapted from Fig. 9 in Beven (2012) 30 th Conference on Hurricanes and Tropical Meteorology TCs Subtropical cyclones Frontal cyclones

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. Adapt Davis (2010) methodology for STC identification –Equations and schematics Case Study: STC Sean (2011) –Track –Anticyclonic wave breaking (AWB) precursor event –Tropical transition (TT) –Application of adapted Davis (2010) methodology for STC identification Discussion and Conclusions Outline

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

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

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

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

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

11. Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly)

12. 500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) 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-tropospheric dynamical processes: (upper-tropospheric PV anomaly) Ertel PV anomaly 300 hPa Length of 6° of latitude

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

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

16. 500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa 300 hPa Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) Upper-tropospheric dynamical processes: (upper-tropospheric PV anomaly) Vertical wind shear Adapt Davis (2010) Methodology Upper- tropospheric dynamical processes (PV3) PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

17. Case Study STC Sean (2011) (6 November – 12 November) –Track –AWB precursor event –Tropical transition (TT) –Time series of PV1–PV3 and PV1/PV2 Images created using 0.5° Climate Forecast System Reanalysis V2 (CFSR v2) dataset

18. Image courtesy of the National Climatic Data Center Tropical cyclone Subtropical cyclone Extratropical cyclone / Remnant low STC Sean (2011): Track

19. STC Sean (2011): Upper-level Precursors Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

20. STC Sean (2011): Upper-level Precursors Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) T1

21. AWB STC Sean (2011): Upper-level Precursors T1 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

22. AWB STC Sean (2011): Upper-level Precursors T1 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

23. T1 STC Sean (2011): Upper-level Precursors T1 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

24. STC Sean (2011): Upper-level Precursors T1 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

25. STC Sean (2011): Upper-level Precursors T1 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

26. STC Sean (2011): Upper-level Precursors T1 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

27. STC Sean (2011): Upper-level Precursors T1 T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

28. STC Sean (2011): Upper-level Precursors T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

29. STC Sean (2011): Upper-level Precursors T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

30. STC Sean (2011): Upper-level Precursors T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

31. STC Sean (2011): Upper-level Precursors T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

32. STC Sean (2011): Upper-level Precursors T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

33. STC Sean (2011): Upper-level Precursors T2 Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 )

34. Position of cross section STC Sean (2011): Tropical Transition PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10 −3 hPa s −1 ), and winds (barbs, kts) 0000 UTC 6 November 2011 33.5°N,70°W23.5°N,70°W

35. Position of cross section STC Sean (2011): Tropical Transition PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10 −3 hPa s −1 ), and winds (barbs, kts) 1200 UTC 7 November 2011 32.5°N,69°W22.5°N,69°W

36. Position of cross section STC Sean (2011): Tropical Transition PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10 −3 hPa s −1 ), and winds (barbs, kts) 0000 UTC 9 November 2011 33°N,70°W23°N,70°W

37. Position of cross section STC Sean (2011): Tropical Transition 1200 UTC 10 November 2011 PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10 −3 hPa s −1 ), and winds (barbs, kts) 25°N,71°W35°N,71°W

38. Position of cross section STC Sean (2011): Tropical Transition 0000 UTC 12 November 2011 PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10 −3 hPa s −1 ), and winds (barbs, kts) 41°N,59.5°W31°N,59.5°W

39. STC Sean (2011): Adapted Davis (2010) PV metrics and vertical wind shear values calculated from 0.5° CFSR v2 dataset PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating 500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa 300 hPa Vertical wind shear Upper- tropospheric dynamical processes (PV3)

40. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 6 November 2011 925–300-hPa vertical wind shear: 24.6 m s −1 T1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV3 PV2 PV1 PV2 PV1/PV2 PVU

41. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 7 November 2011 925–300-hPa vertical wind shear: 13.6 m s −1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV1 PV2 PV3 PV2 PV1

42. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 8 November 2011 925–300-hPa vertical wind shear: 10.3 m s −1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1 PV2

43. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 9 November 2011 925–300-hPa vertical wind shear: 13.9 m s −1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

44. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 10 November 2011 925–300-hPa vertical wind shear: 12.9 m s −1 T2 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

45. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 925–300-hPa vertical wind shear: 18.8 m s −1 T2 1200 UTC 11 November 2011 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

46. DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) 925–300-hPa vertical wind shear: 41.6 m s −1 Calculation Location T2 1200 UTC 12 November 2011 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

47. Conclusions STCs have characteristics of both tropical and extratropical cyclones and are likely candidates to become TCs via TT STCs can form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes to the evolution of STC Sean (2011) Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during TT of STC Sean (2011)

48. Questions? ambentley@albany.edu Special Thanks: Kyle MacRitchie and Matthew Janiga STCs have characteristics of both tropical and extratropical cyclones and are likely candidates to become TCs via TT STCs can form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes to the evolution of STC Sean (2011) Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during TT of STC Sean (2011)