Physical processes and downstream impacts of extratropical transition

1. John R. Gyakum1 Ron McTaggart-Cowan2 1McGill University 2University of Quebec at Montreal Physical processes and downstream impacts of extratropical transition

2. 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

3. 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

4. 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.

5. Extended Tropical Lifecycle (McTaggart-Cowan et al. 2006) Juan • GOES water vapour image (0015 UTC, 29 September 2003)

6. 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

7. 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 East of centre

8. 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

9. 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 intensifying LC1-A type ET.

10. 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.

11. T S 968 S 965 M 977 S 968 S 974 S 967 S 972 S 964 D W 1000 D M 979 D D D D Trough Phasing (Ritchie and Elsberry 2006) Initial locations of the TC relative to a midlatitude trough. S=strong re-int. M=moderate re-int. W=weak re-int D=dissipate

12. 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

13. 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

14. 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

15. 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

16. 120h 156h 192h 240h 325 350 375 K Idealized Downstream Impacts (Riemer 2006) white contours: surface pressure <995 hPa black contours: wind speed >40 m/s at 200 hPa

17. day 2 day 4 day 6 day 8 Idealized Downstream Impacts (Riemer 2006) smaller, weaker tropical cyclone larger, stronger tropical cyclone m/s Hovmöller plot (meridional wind on 200 hPa at 45oN)

18. 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)

19. Diabatic Rossby Waves (Moore and Montgomery 2005) • Positive low level PV • Southerly flow to east of the DRV centre

20. 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

21. Diabatic Rossby Waves (Moore and Montgomery 2005) • Rapidly moving low centre is difficult to forecast because of strong diabatic forcing

22. 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

23. 09/09 08/09 07/09 06/09 05/09 04/09 03/09 02/09 01/09 31/08 29/08 30/08 Ensemble Estimate of Downstream Predictability (Harr et al. 2006)

24. 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

25. Analysis of Downstream Impacts (Anwender et al. 2006) Hurricane Maemi (2003) Hovmoller diagram of 500 hPa RMS differences between ensemble members with/out perturbations

26. 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

27. 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 Katrina’s ET

28. 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.

29. 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?

30. 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? • What dynamical processes control the distribution and amount of track-relative precipitation during ET?

31. 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.