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. 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

7. 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

15. 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

18. 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

19. 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

20. 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

25. 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 Katrina’s 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

30. 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

31. 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.

32. 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?

35. 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.