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

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Presentation on theme: "Tropical Transition Climatology R. McTaggart-Cowan, L. F. Bosart, C. A. Davis and G. Deane."— Presentation transcript:

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 ( ) – 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 ( ) – 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 ( ) – 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: hPa thickness and winds. Bottom left: 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

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: hPa thickness and winds. Bottom left: 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

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 % Total %

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...

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


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