Synoptic composites of the ET lifecycle of North Atlantic TCs: Factors determining post-transition

1. Synoptic composites of the ET lifecycle of North Atlantic TCs: Factors determining post-transition evolution Contributions from: Clark Evans Jenni Evans

2. Motivation: TCs never lose their diapers… TCs that intensify after extratropical transition TCs that weaken after extratropical transition

3. Motivation: Hurricanes cannot be considered passive in midlatitude flow Ivan (2004) Karl (2004)

4. Motivation: The Impact of Hurricane Karl on 72hr Ensemble Forecasts

5. Cyclone Predictability is a function of its structure • Predictability is a function of cyclone structure • Model interpretation/trust is a function of structure • MPI is a function of cyclone structure

6. Motivation: Previous Research The role of the downstream and upstream long-wave patternhas been shown to play a critical role in determining TC evolution after ET (McTaggart-Cowan et al. 2003)Tropical cyclones aren’t passive features upon entering the midlatitude flow The details of tropical development and extratropical transition can dramatically influence the hemispheric long wave patternNWP models often obtain their worst forecast skill when a hurricane is about to enter the midlatitude flow (Harr and Jones 2003).Understanding the reasoning behind these requires improved understanding ofextratropical transition.

7. Some questions raised: • What are the structural aspects of tropical cyclones that favor constructive interaction with a midlatitude trough? • What are the structural aspects of the midlatitude trough that favor constructive interaction with a TC? • What factors determine the rapid or slow transition of a TC? • What factors determine intensification or decay after extratropical transition of the cyclone? • What are the determining factors for post-ET structure of the cyclone: cold core vs. warm-seclusion?

8. 34 North Atlantic Transitioning Cyclones Examined Bonnie (1998) Gordon (2000) Danielle (1998) Isaac (2000) Earl (1998) Michael (2000) Ivan (1998) Nadine (2000) Jeanne (1998) Allison (2001) Karl (1998) Erin (2001) Mitch (1998) Gabrielle (2001) Nicole (1998) Humberto (2001) Cindy (1999) Karen (2001) Dennis (1999) Michelle (2001) Floyd (1999) Cristobal (2002) Gert (1999) Gustav (2002) Harvey (1999) Isidore (2002) Irene (1999) Josephine (2002) Jose (1999) Fabian (2003) Alberto (2000) Isabel (2003) Florence (2000) Kate (2003)

9. Cyclone Phase Space • Unifies the fundamental structural description of cyclones into a multi-dimensional continuum (MWR 2003a,b) : B: 900-600hPa: Storm-relative thermal asymmetry -VTL: 900-600hPa: Thermal wind (cold vs. warm core) -VTU: 600-300hPa: Thermal wind (cold vs. warm core) • Will be focusing on a cross section of B vs –VTL here.

10. Cyclone Phase Space

11. Transition ends (time=TE) when cyclone is cold-core Transition begins (time=TB) when B > 10m => significant thermal gradient. Cyclone Phase Space: ET Example [Floyd]

12. Composite Mean ET Structural Evolution Summary 34-Cyclone Composite Mean Phase NOGAPS-analysis based Trajectory with key milestones labeled TE+24h TE TE+48h TMID TB TE+72h TB-24h TB-72h TB-48h

13. Compositing Method • NOGAPS 1°x1° operational analyses from 34 storms 1998-2003 • Storm-center-relative composites. No coordinate rotation for storm motion. • ±40° longitude • -20° to +30° latitude (never extending outside 0°-90°N) • Raw field and anomaly from NCAR/NCEP Reanalysis2 30-year monthly-mean are both composited.

14. 34-Cyclone Composite Mean Evolution 320K Potential Vorticity TB-24h TB TE TE+24h

15. 34-Cyclone Composite Mean Evolution:320K PV Cross Sections TB TB-24h TE+24h TE

16. Variability About the Composite Mean Boxes represent the calculated one standard deviation spread about the 34-cyclone consensus mean trajectory for each time Considerable variability about mean once transition completed=> posttropical phase can take many forms….

17. Floyd (1999): Non-intensifying cold-core development Hugo (1989): Explosive cold-core development Charley (1986): Schizophrenia

18. Dennis (1999): “ET-Interruptus”. Keith (1988): Explosive warm-seclusion development Cindy (1999): Absorption.

19. Three key subcomposites • Fast [<=12hr] vs. Slow [>=48hr] Transitioning • Post-ET Intensification (N=6) vs. Weakening (N=11) • Post-ET Cold-Core (N=15) vs. Warm-Seclusion (N=6)

20. TB:Fast (left) vs. Slow (right) Transitioning 500mb Height & Anom. SST & Anom. Strengthen (N=6)

21. Strengthen (N=6)

22. TB:Post-ET Weakening (left) vs. Intensification (right) 500mb Height & Anom. SST & Anom. Strengthen (N=6)

23. Post-ET Weakeners (Solid) vs. Intensifiers (Dotted): T-Test: 75%, 90%, 95%, 99% Strengthen (N=6)

24. Two post-ET intensifiers:What determines whether the storm re-acquires warm-core structure? Gustav (2002) Irene (1999)

25. TE: Post-ET Cold-core (left) vs. warm-seclusion (right) 320K PV 320K PV Strengthen (N=6)

26. Post-ET Cold-core (Solid) vs. Warm-seclusion (Dotted): T-Test: 75%, 90%, 95%, 99%

27. Post-ET Cold-core vs. Warm-Seclusion: Statistics T-test: 90% T-test: 95% Strengthen (N=6)

28. Eliassen-Palm (EP) Flux and its Divergence Outward eddy momen-tum flux Outward eddy heat flux Analyses performed by Clark Evans

29. All Strengthening Weakening Cold-Core Warm-Seclusion EP- Flux Outward eddy momen-tum flux Outward eddy heat flux

30. Summary • A well-defined 34-member ensemble mean trajectory through cyclone phase space is calculated for ET in the North Atlantic. • TC diabatic PV destruction aloft leads to a lifting of the mid-latitude tropopause, erosion and narrowing of the approaching trough • TC advects an environment into the trough that has static stability 10-20% lower than prior to TC arrival, enhancing Eady growth rate. • Variability from this mean trajectory is small in the tropical phase, and then increases dramatically once extratropical transition has completed: • cold-core intensifying/decay, warm-seclusion, merger, tropical.

31. Summary • Post-ET Weakening: • Positively tilted UL trough, SSTs near normal and below 26.5C on average • Post-ET Strengthening: • Negatively tilted UL trough, SSTs above normal and above 26.5C on average • Post-ET Cold-core evolution: • Broad UL trough, considerably smaller than average TC size • Post-ET Warm-seclusion evolution: • Narrowing UL trough, considerably larger than average TC size, scale matching (Molinari et al. 1995; Hanley et al. 2001).

32. Summary • Introduction of trough into TC leads to eddy PV forcing. The adiabatic secondary circulation that results (Molinari et al. 1995) attempts to restore thermal wind balance. • The momentum component of the EP forcing far precedes the thermal flux component. • Thus, it appears that the development of frontal structure (increase in ‘B’) within the TC during ET may be initially a consequence of the TC’s adiabatic response to the eddy momentum forcing • This eddy forcing is over a deeper layer and at lower isentropic level than with Elena-type rapid intensification.

33. Summary • Post-ET intensifiers have a marked increase in the magnitude of eddy PV flux compared to weakeners • Post-ET warm seclusion is associated with a narrower depth of cyclonic eddy PV flux at a level comparable to Molinari et al. (1995) • Speculation: Whether a trough interaction with a TC leads to RI or ET and warm-seclusion is a matter of timing of the interaction during the cyc. phase trajectory • With an average track error of 300-500km at 3-5 days, and the subtle sensitivities just shown, it is evident why the long-wave pattern decreases markedly during ET