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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Physical processes and
downstream impacts of extratropical
transition
John R. Gyakum1
Ron McTaggart-Cowan2
1McGill University
2University of Quebec at Montreal
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Introduction/Motivation
• Review of relevant research occurring since
the publication of the ET review paper by
Jones et al. (2003, W and F) highlights:
– wide range of scales associated with physical
processes during ET
– potential for ET events to have significant impact
on weather events far downstream
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Physical Processes
• Occur over scales ranging from microscale
(e.g. sea spray) to planetary scale (e.g.
regime transitions forced by ET)
• Physical processes are difficult to model or
diagnose – often involve phase transitions
• Can lead to rapid evolution of vortex structure
and/or intensity
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Extended Tropical Lifecycle
(McTaggart-Cowan et al. 2006)• Hurricane Juan (2003), maintained its tropical
characteristics into Atlantic Canada, and attendant
colder waters.
• Hurricane-strength winds are maintained above the
statically-stable PBL.
• Anomalously-strong ridging in the western Atlantic
is associated with TC maintenance over Atlantic
Canada.
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
GOES water vapour image (0015 UTC, 29 September 2003)
Extended Tropical Lifecycle
(McTaggart-Cowan et al. 2006)
Juan
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Hurricane Michael research aircraft
observations (Abraham et al. 2004)Airborne radar
reflectivity and
dropsonde-
derived isotachs
Jet associated
with PBL
decoupling and
dry, convectively
unstable air
wrapping into core
centre
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
• Stabilized PBL over
cool SSTs allows spin-
up of circulation
• Cool air aloft reduces
stability and allows jet
to expand in the
vertical as momentum
is re-distributed by
convection
Hurricane Michael research aircraft
observations (Abraham et al. 2004)
Dropsonde winds
East of centre
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
• Investigates baroclinic wave / TC phase dependence for redevelopment
• Baroclinic wave structures resemble LC1 developments (Thorncroft et al. 1993)
• Finds two categories of interaction:
– LC1-A: TC is steered northward ahead of the
upstream trough and reintensifies
– LC1-B: trough-relative TC position precludes
strong interaction
Trough Phasing (Weindl 2004)
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Trough Phasing (Weindl 2004)
Initial vortex locations relative to the
baroclinic waves for intensifying LC1-A
type ET.
For LC1-A, the low is
located farther to the
east and north, and is
steered northward in
advance of the
trough. It also
develops as it
interacts with the
positive PV anomaly.
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Trough Phasing (Weindl 2004)For LC1-B, the initial position of the vortex precludes a strong interaction with the positive PV anomaly, and the low passes to the west of the trough with little development.
Initial vortex locations relative to the
baroclinic waves for non-intensifying
LC1-B type ET.
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Trough Phasing (Ritchie and
Elsberry 2006)
S974D D M
979D
S968
T
S965
S968S
967S964
D D D
W1000
S972
M977
Initial locations of
the TC relative to a
midlatitude trough.
S=strong re-int.
M=moderate re-int.
W=weak re-int
D=dissipate
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Atmosphere-Ocean coupled
dynamics (Ren et al. 2004)• Sensible and latent
heat fluxes for
uncoupled and coupled
simulations of
Hurricane Earl (1998)
• Wind-induced SST
cooling reduces heat
fluxes in the coupled
simulation
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Atmosphere-Ocean coupled
dynamics (Ren et al. 2004)• Latent fluxes dominate over sensible
fluxes by nearly an order of magnitude
• Reduction in latent fluxes by wind-
induced SST cooling translates into
decreased redevelopment
• Hurricane Earl (1998) in the coupled
simulation is about 4 hPa and 2 m/s
weaker than in the uncoupled runs with
fixed SSTs
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Downstream Impacts• The effects of ET have been shown to
influence the flow both upstream and downstream of the ET event
• Generation of Rossby wave trains can influence the midlatitude circulation on hemispheric scales
• Recent studies suggest that this impact may be long-lived and influence seasonal climate
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Idealized Downstream Impacts
(Riemer 2006)• Employs an MM5 channel model with an idealized
initial state consisting of a straight jet and a TC
• Development downstream is found to depend only weakly on the strength of the TC, but strongly on the strength of the midlatitude jet
• Primary downstream impacts are:
– generation of a system in the poleward jet exit
– excitation of a Rossby wave train
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Idealized Downstream Impacts
(Riemer 2006)
120h 156h
192h 240h
325 350 375 K
black contours: wind speed >40 m/s at 200 hPa white contours: surface pressure <995 hPa
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Idealized Downstream Impacts
(Riemer 2006)
day 2
day 4
day 6
day 8
m/s
smaller, weaker tropical cyclone
Hovmöller plot (meridional wind on 200 hPa at 45oN)
larger, stronger tropical cyclone
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
• Propagates by creating convection downstream
• Requires little upper-level forcing, but strong
baroclinicity
• Grows as a result of an approximate phase locking
and mutual amplification of two diabatically-
generated PV anomalies
• Excellent example is that of the Lothar (1999) storm
(Wernli et al. 2002)
Diabatic Rossby Waves (Moore and
Montgomery 2005)
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Diabatic Rossby Waves (Moore and
Montgomery 2005)• Positive low level PV
• Southerly flow to east of the DRV centre
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Diabatic Rossby Waves (Moore and
Montgomery 2005)• Rising motion and latent heating to the
east of the DRV centre
• Development of an outflow PV minimum
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Diabatic Rossby Waves (Moore and
Montgomery 2005)• Rapidly moving low centre is difficult to
forecast because of strong diabatic forcing
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Ensemble Estimate of Downstream
Predictability (Harr et al. 2006)
• Use ensemble measures to assess the downstream
predictability impacts of W-Pac ET
• Sequential cluster analyses from the 120h to 24h
forecast lead times shows that the number of likely
outcomes of ET is closely tied to predictability
• Deterministic prediction of the TC lifecycle – and its
impact on the midlatitude flow – is challenging,
making the ensemble more robust over many cases
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Ensemble Estimate of Downstream
Predictability (Harr et al. 2006)
29/0830/08
31/0801/09
02/0903/09
04/09
05/09
06/09
07/09
08/09
09/09
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Analysis of Downstream Impacts
(Anwender et al. 2006)• Use modifications to the ECMWF ensemble initial
state perturbation scheme to investigate the effects
of near-TC uncertainty on downstream development
• Adding near-TC perturbations impacts the
development of the ET-forced Rossby wave in
perturbed members
• Perturbations increase membership in clusters with
amplified near-surface and upper air patterns
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Analysis of Downstream Impacts
(Anwender et al. 2006)
Hovmoller diagram of 500 hPa RMS
differences between ensemble
members with/out perturbations
Hurricane Maemi (2003)
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
• A discrete diabatically generated warm pool shed from the South Asian Anticyclone is shown to interact with the upper level remnants of Katrina following ET
• The resulting mid-latitude anticyclonic feature:
– reduces predictability over the North Atlantic
– assists with development of Nate and Maria
– blocks the flow over the Atlantic for several days
Hemispheric Impacts of ET
(McTaggart-Cowan et al. 2006)
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Hemispheric Impacts of ET
(McTaggart-Cowan et al. 2006)
• A transient warm pool associated with Hurricane Katrina (2005)
is shown to perturb the midlatitude flow on a hemispheric scale
for a period of nearly 1 month centered on Katrina’s ET
Katrina
blocking
TC genesisDT temperature
Streamfunction
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Summary
• Scale of physical processes involved in the ET
process ranges from microscale to planetary
scale – most are associated with phase
changes and are difficult to model/diagnose
• Downstream impacts of ET have been well
documented, and appear to be of greater
importance than conventional wisdom
otherwise may have suggested.
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Recommendations for future
research directions• What are the origins of the varying ‘flavors’ of ET,
and is there any means of identifying the physical
mechanisms that allow a subset of these storms to
reintensify explosively?
• To what extent do differences in the mean
environmental conditions across various ocean
basins contribute to the various ‘flavors’ of ET?
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
• What is the sensitivity of the downstream response
to the upstream state and the TC during ET?
• What are the relative contributions to ET from
sensible and latent heat fluxes versus momentum
transports?
• What dynamical processes control the distribution
and amount of track-relative precipitation during
ET?
Recommendations for future
research directions
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IWTC-VI Topic 2.5 November 2006 – Costa Rica
Additional ReferencesWernli, H., S. Dirren, M. A. Liniger, and M. Zillig, 2002: Dynamical aspects of
the life-cycle of the winter storm “Lothar” (24-26 December 1999). Quart J.
Roy. Meteor. Soc., 128, 405-429.
All other references are contained in the IWTC-VI
report for Topic 2.5.