Physical processes and downstream impacts of extratropical transition

1
Physical processes and downstream impacts of
extratropical transition

• John R. Gyakum1
• Ron McTaggart-Cowan2
• 1McGill University
• 2University of Quebec at Montreal

2
Outline of discussion

• Introduction/Motivation
• Physical Processes
• Downstream Impacts
• Summary
• Recommendations for future research directions

3
Introduction/Motivation

• Review of relevant research occurring since the
  publication of the ET review paper by Jones et
  al. (2003) highlights
• wide range of scales associated with physical
  processes during ET
• potential for ET events to have significant
  impact on weather events far downstream

4
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

5
Diabatic Rossby Waves (Moore and Montgomery 2005)

• 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)

6
Diabatic Rossby Waves (Moore and Montgomery 2005)

• Positive low level PV
• Southerly flow to east of the DRV centre

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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

8
Diabatic Rossby Waves (Moore and Montgomery 2005)

• Rapidly moving low centre is difficult to
  forecast because of strong diabatic forcing

9
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.

10
Extended Tropical Lifecycle (McTaggart-Cowan et
al. 2006)
Juan
GOES water vapor image (0015 UTC, 29 September
2003)
11
Trough Phasing (Weindl 2004)

• 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

12
Trough Phasing (Weindl 2004)
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.
Initial vortex locations relative to the
baroclinic waves for non-intensifying LC1-A type
ET.
13
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 intensifying LC1-B type ET.
14
Sea-spray impacts on ET (Perrie et al. 2005)
SLP
Wind

• Effects of sea spray on model simulations of the
  ET of Hurricanes Earl (1998) and Danielle (1998),
  and rapid coastal development.
• A range of sensitivity is found

Earl
Danielle
Danielle
Sensitivity
Coastal
Earl
Coastal
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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

16
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

17
Hurricane Michael research aircraft observations
(Abraham et al. 2004)
Low level jet on the right side of the storm
reaches 70 m/s at 1500 m.
Dropsonde-derived equivalent potential
temperature transect (E/W)
centre
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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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Hurricane Michael research aircraft observations
(Abraham et al. 2004)

• 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

Dropsonde winds E of centre
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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

21
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

22
Idealized Downstream Impacts (Riemer 2006)

• Hovmoller diagram of 200 hPa meridional wind
  speed
• Solid arrow indicates the displacement of the ET
  system
• Dashed arrow shows propagation of the Rossby
  wave train

23
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

24
Ensemble Estimate of Downstream Predictability
(Harr et al. 2006)
Analysis

• Typhoon Saola is poorly rep-resented by
  deterministic forecasts even the 12h forecast
  has significant intensity and track errors

60h Forecast

• Spread of SD from ensemble shows down-stream
  growth in uncertainty

36h Forecast
12h Forecast
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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

26
Analysis of Downstream Impacts (Anwender et al.
2006)
Hovmoller diagram of 500 hPa RMS differences
between ensemble members with/out perturbations
Hurricane Maemi (2003)
27
Hemispheric Impacts of ET (McTaggart-Cowan et al.
2006)

• 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

28
Hemispheric Impacts of ET (McTaggart-Cowan et al.
2006)
Streamfunction
blocking
Katrina
TC genesis
DT temperature

• 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 Katrinas ET

29
Seasonal Impact of ET (Hart 2006)

• An enhanced number of northern hemisphere ET
  (recurvature) events result in
• an anomalously warm winter season
• a reduced meridional temperature gradient
• a reduction in the number of weak winter cyclones
• Results are symmetric for seasons with an
  anomalously small number of ET events

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Seasonal Impact of ET (Hart 2006)
Inactive seasons
Active seasons

• Midlatitude tropospheric thickness is reduced
  following inactive ET seasons however, the
  response is not symmetric since the anomalies are
  spatially smaller following inactive seasons

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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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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?

33
Recommendations for future research directions

• Is there a significant quantifiable impact of
  these episodic ET events on the general
  circulation on intraseasonal time scales?
• What dynamical processes control the distribution
  and amount of track-relative precipitation during
  ET?

34
Recommendations for future research directions

• 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?

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Additional References
Wernli, 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.