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

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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.

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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

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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

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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

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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

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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

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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)

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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)

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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)

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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)

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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... ?

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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

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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)

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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

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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

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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

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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

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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

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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

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

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

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

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

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

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

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

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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

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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

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

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

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Results - Classification Total 591 storms in the NHC Atlantic archive: Strong TT Weak TT Trough Induced Perturbed Wave Induced Tropical Development Genesis CategoryEvents % Total %

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Results – Genesis Locations Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

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Results – Track Density Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

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Results – Maximum Intensity Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

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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

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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...

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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

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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)

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

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

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

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

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