IWTC-VI Topic 2.5November 2006 – Costa Rica Physical processes and downstream impacts of...
-
date post
21-Dec-2015 -
Category
Documents
-
view
230 -
download
1
Transcript of IWTC-VI Topic 2.5November 2006 – Costa Rica Physical processes and downstream impacts of...
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
IWTC-VI Topic 2.5 November 2006 – Costa Rica
Outline of discussion
• Introduction/Motivation
• Physical Processes
• Downstream Impacts
• Summary
• Recommendations for future research
directions
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) highlights:
– wide range of scales associated with physical
processes during ET
– potential for ET events to have significant impact
on weather events far downstream
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
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)
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
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
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
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.
IWTC-VI Topic 2.5 November 2006 – Costa Rica
GOES water vapor image (0015 UTC, 29 September 2003)
Extended Tropical Lifecycle
(McTaggart-Cowan et al. 2006)
Juan
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)
IWTC-VI Topic 2.5 November 2006 – Costa Rica
Trough Phasing (Weindl 2004)
Initial vortex locations relative to the
baroclinic waves for non-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.
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 intensifying LC1-B
type ET.
IWTC-VI Topic 2.5 November 2006 – Costa Rica
Sea-spray impacts on ET (Perrie et
al. 2005)• 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:
SLP Wind
Earl
Danielle
CoastalEarl
Danielle
Coastal
Sensitivity
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
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
IWTC-VI Topic 2.5 November 2006 – Costa Rica
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
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
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 E of centre
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
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
IWTC-VI Topic 2.5 November 2006 – Costa Rica
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
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
IWTC-VI Topic 2.5 November 2006 – Costa Rica
Ensemble Estimate of Downstream
Predictability (Harr et al. 2006)
• Typhoon Saola is poorly rep-
resented by deterministic forecasts
– even the 12h forecast has
significant intensity and track errors
Analysis
60h Forecast
36h Forecast
12h Forecast
• Spread of SD
from ensemble
shows down-
stream growth
in uncertainty
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
IWTC-VI Topic 2.5 November 2006 – Costa Rica
Analysis of Downstream Impacts
(Anwender et al. 2006)
Hovm
oller diagram of 500 hP
a RM
S
differences between ensem
ble
mem
bers with/out perturbations
Hurricane Maemi (2003)
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)
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
IWTC-VI Topic 2.5 November 2006 – Costa Rica
• 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
Seasonal Impact of ET (Hart 2006)
IWTC-VI Topic 2.5 November 2006 – Costa Rica
Seasonal Impact of ET (Hart 2006)
• Midlatitude tropospheric thickness is reduced following inactive
ET seasons; however, the response is not symmetric since the
anomalies are spatially smaller following inactive seasons
Active seasons
Inactive seasons
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.
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?
IWTC-VI Topic 2.5 November 2006 – Costa Rica
• 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?
Recommendations for future
research directions
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?
Recommendations for future
research directions
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.