physical processes and downstream impacts of extratropical transition

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


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


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


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.


GOES water vapour image (0015 UTC, 29 September 2003)

Extended Tropical Lifecycle

(McTaggart-Cowan et al. 2006)

Juan


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


• 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


• 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)


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.


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.


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


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


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


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


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



(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



(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


• 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)



Montgomery 2005)• Positive low level PV

• Southerly flow to east of the DRV centre



Montgomery 2005)• Rising motion and latent heating to the

east of the DRV centre

• Development of an outflow PV minimum



Montgomery 2005)• Rapidly moving low centre is difficult to

forecast because of strong diabatic forcing


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


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


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


Analysis of Downstream Impacts

(Anwender et al. 2006)

Hovmoller diagram of 500 hPa RMS

differences between ensemble

members with/out perturbations

Hurricane Maemi (2003)


• 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



Hemispheric Impacts of ET


• 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


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.


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?


• 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


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.

physical processes and downstream impacts of extratropical transition

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