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Tropical Transition
Climatology
R. McTaggart-Cowan, L. F. Bosart, C. A. Davis and G. Deane
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.
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
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
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
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
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
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)
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)
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)
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)
The Question
Do TCs initiated by TT events have different fundamental characteristcs that those whose genesis follows a more traditional “tropical”
pathway?
Genesis frequencyPeak intensityStorm longevityLikelihood of ET... ?
Data and MethodologyDefine 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
Data and MethodologyDefine 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)
Data and MethodologyDefine 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
Data and MethodologyDatasets (1948-2004)
– NCEP/NCAR Reanalysis
– NHC Best Track
Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h
Data and MethodologyDatasets (1948-2004)
– NCEP/NCAR Reanalysis
– NHC Best Track
Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h
Data and MethodologyDatasets (1948-2004)
– NCEP/NCAR Reanalysis
– NHC Best Track
Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h
Data and MethodologyCompute 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
Data and MethodologyCompute 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
Data and Methodology
Q-vector for:
T-36h
Time series of Q-vector metric for Hurricane Diana (1984)
Data and Methodology
Q-vector for:
T-30h
Time series of Q-vector metric for Hurricane Diana (1984)
Data and Methodology
Q-vector for:
T-24h
Time series of Q-vector metric for Hurricane Diana (1984)
Data and Methodology
Q-vector for:
T-18h
Time series of Q-vector metric for Hurricane Diana (1984)
Data and Methodology
Q-vector for:
T-12h
Time series of Q-vector metric for Hurricane Diana (1984)
Data and Methodology
Q-vector for:
T-06h
Time series of Q-vector metric for Hurricane Diana (1984)
Data and Methodology
Q-vector for:
T-00h
Time series of Q-vector metric for Hurricane Diana (1984)
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
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
Results - ClassificationThe optimal division of the dataset is:
3 Thickness Groups 7 Q-vector Groups
Time Time
T-36h T-00hT-36hT-36h T-00h
Qvec
Thic
k
Results - ClassificationPhysically based synthesis of the groups
yields 6 basic categories:
Results - ClassificationTotal 591 storms in the NHC Atlantic archive:
Strong TT
Weak TT
Trough Induced
Perturbed
Wave Induced
Tropical Development
Genesis Category Events
83
94
23
64
76
251
% Total
14
16
4
11
13
42
30%
Results – Genesis Locations
Strong TT Weak TT Tr Induced
TropicalWave InducedPerturbed
Results – Track Density
Strong TT Weak TT Tr Induced
TropicalWave InducedPerturbed
Results – Maximum Intensity
Strong TT Weak TT Tr Induced
TropicalWave InducedPerturbed
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
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
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
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)
Review of TT
Components of TT:– Low latitude trough
– Lower/midlevel vortex
Review of TT
Components of TT:– Low latitude trough
– Lower/midlevel vortex
Review of TT
Components of TT:– Low latitude trough
– Lower/midlevel vortex
Review of TT
Components of TT:– Low latitude trough
– Lower/midlevel vortex