1. Tropical Transition Climatology R. McTaggart-Cowan, L. F. Bosart, C. A. Davis and G. Deane

2. Outline  Review of TT: – Role of midlatitude trough – SEC vs. WEC precursors  Data and methodology: – Motivation for climatology – Selection of metrics and groups (LCA)  Results from TT climatology: – Group membership and physical properties Hurricane Michael (18 October 2000) – Category 1. NOAA SeaWiFS imagery.

3. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Low latitude trough – Related to TUTT – Provides downshear QG ascent forcing – Reduces column stability – Cyclonic relative vorticity > local f

4. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - SEC – Wind >10 m/s - WISHE – Elevates near-surface equivalent potential temperature – Redistributes PV and momentum through convection to reduce shear

5. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - SEC – Wind >10 m/s - WISHE – Elevates near-surface equivalent potential temperature – Redistributes PV and momentum through convection to reduce shear

6. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - SEC – Wind >10 m/s - WISHE – Elevates near-surface equivalent potential temperature – Redistributes PV and momentum through convection to reduce shear

7. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - SEC – Wind >10 m/s - WISHE – Elevates near-surface equivalent potential temperature – Redistributes PV and momentum through convection to reduce shear

8. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - WEC – Wind <10 m/s – Weak baroclinic or remnant MCV – Couples with trough forcing to focus ascent – Stretching increases intensity (WISHE)

9. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - WEC – Wind <10 m/s – Weak baroclinic or remnant MCV – Couples with trough forcing to focus ascent – Stretching increases intensity (WISHE)

10. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - WEC – Wind <10 m/s – Weak baroclinic or remnant MCV – Couples with trough forcing to focus ascent – Stretching increases intensity (WISHE)

11. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower vortex - WEC – Wind <10 m/s – Weak baroclinic or remnant MCV – Couples with trough forcing to focus ascent – Stretching increases intensity (WISHE)

12. The Question Do TCs initiated by TT events have different fundamental characteristcs that those whose genesis follows a more traditional “tropical” pathway? Genesis frequency Peak intensity Storm longevity Likelihood of ET... ?

13. Data and Methodology  Define objective indicators of TT: – Upper level Q-vector convergence – Lower level thermal asymmetry  Both are indicators of key components of the Davis and Bosart (2004) TT conceptual model

14. Data and Methodology  Define objective indicators of TT: – Upper level Q-vector convergence – Lower level thermal asymmetry  Q-vector convergence represents: – Trough-induced synoptic scale ascent – Mid-level moistening (reduces downdrafts)

15. Data and Methodology  Define storm-centred objective indicators of TT: – Upper level Q-vector convergence – Lower level thermal asymmetry  Thermal asymmetry represents: – Baroclinicity of percursor vortex – Focusing mechanism for ascent – Discriminates between baroclinic and MCV precursors in WEC cases

16. Data and Methodology  Datasets (1948-2004) – NCEP/NCAR Reanalysis – NHC Best Track  Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h

17. Data and Methodology  Datasets (1948-2004) – NCEP/NCAR Reanalysis – NHC Best Track  Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h

18. Data and Methodology  Datasets (1948-2004) – NCEP/NCAR Reanalysis – NHC Best Track  Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h

19. Data and Methodology  Compute storm- centered diagnostics at 6- hourly intervals along the back- trajectory Sample T-12h diagnostic plots for Hurricane Diana (1984). Top left: DT potential temperature and winds. Top right: 1000-700 hPa thickness and winds. Bottom left: 850-700 hPa relative vorticity and nondivergent winds. Bottom right: Q-vectors, Q-vector divergence and relative humidity (contoured). Plots from T-72h to T+24h are available at http://www.atmos.albany.edu/facstaff/rmctc/ttclim/indexd.php

20. Data and Methodology  Compute storm- centered diagnostics at 6- hourly intervals along the back- trajectory Sample T-12h diagnostic plots for Hurricane Diana (1984). Top left: DT potential temperature and winds. Top right: 1000-700 hPa thickness and winds. Bottom left: 850-700 hPa relative vorticity and nondivergent winds. Bottom right: Q-vectors, Q-vector divergence and relative humidity (contoured). Plots from T-72h to T+24h are available at http://www.atmos.albany.edu/facstaff/rmctc/ttclim/indexd.php

21. Data and Methodology Q-vector for: T-36h Time series of Q-vector metric for Hurricane Diana (1984)

22. Data and Methodology Q-vector for: T-30h Time series of Q-vector metric for Hurricane Diana (1984)

23. Data and Methodology Q-vector for: T-24h Time series of Q-vector metric for Hurricane Diana (1984)

24. Data and Methodology Q-vector for: T-18h Time series of Q-vector metric for Hurricane Diana (1984)

25. Data and Methodology Q-vector for: T-12h Time series of Q-vector metric for Hurricane Diana (1984)

26. Data and Methodology Q-vector for: T-06h Time series of Q-vector metric for Hurricane Diana (1984)

27. Data and Methodology Q-vector for: T-00h Time series of Q-vector metric for Hurricane Diana (1984)

28. Data and Methodology  Genesis events categorized by the T-36h to T-0h trajectories of the metrics – Latent Class Analysis (LCA) – Both the magnitude and the shape of the metric series is considered during grouping  TT is based on the structural evolution of the trough and lower level vortex

29. Data and Methodology  Although events A and C have nearly identical means, the LCA will group A with B because of their similar trajectories – Provides physically consistent groupings  The metrics are conditioned against each other for the final set of groups

30. Results - Classification  The optimal division of the dataset is: 3 Thickness Groups7 Q-vector Groups Time T-36hT-00h T-36h T-00h QvecThick

31. Results - Classification  Physically based synthesis of the groups yields 6 basic categories:

32. Results - Classification  Total 591 storms in the NHC Atlantic archive: Strong TT Weak TT Trough Induced Perturbed Wave Induced Tropical Development Genesis CategoryEvents 83 94 23 64 76 251 % Total 14 16 4 11 13 42 30%

33. Results – Genesis Locations Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

34. Results – Track Density Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

35. Results – Maximum Intensity Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

36. Summary  Objective method for identifying tropical / midlatitude interactions during genesis – Based on TT conceptual model:  Q-vector convergence  Lower/midlevel thickness asymmetry – Groupings based on evolution as well as magnitude of the metrics  Identified 6 genesis modes in 591 cases

37. Conclusions  TT accounts for ~30% of genesis events in the Atlantic Basin  TT events are localized in space near the North American continent  Storms that form from TT tend to be weaker and to have shorter tracks and lifetimes  Strong TT cases are more likely to undergo ET Lots more analysis to be done... http://www.atmos.albany.edu/facstaff/rmctc/ttclim/indexd.php

38. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower/midlevel vortex - SEC – Near-surface windspeed > 10 m/s (WISHE) – Elevates near-surface equivalent potential temperature – Redistributes PV and momentum through convection to reduce shear

39. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex  Lower/midlevel vortex - WEC – Near-surface windspeed < 10 m/s – Weak baroclinic or remnant MCV – Couples with trough forcing to focus ascent – Stretching increases intensity (WISHE)

40. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex

41. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex

42. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex

43. Review of TT  Components of TT: – Low latitude trough – Lower/midlevel vortex