1. Extratropical Transition: One Trajectory through a Cyclone Phase Space
2 May 2002
Robert Hart and Jenni Evans
Department of Meteorology
Penn State University

2. Which 5 are officially tropical cyclones?
Images courtesy NOAA/NCDC

3. Cyclone phase diagram
Generalized, continuum approach to describing cyclone structure proposed schematically by Beven (1997) and also recently suggested by Reale and Atlas (2001).
Objectively defined phase diagram proposed in Hart (2002, MWR and Poster P1.28).
Provides considerably more freedom than two discrete groups of tropical, extratropical cyclones
Cyclones described here using objective physically insightful parameters

4. Cyclone Parameter B: Thermal Asymmetry
Storm-relative hPa mean thickness field (shaded) asymmetry within 500km radius:
3160m
3260m L

5. Cyclone Parameter B: Thermal Asymmetry
Forming (B0) Mature(B0) Decay(B0)
Conventional Tropical cyclone: B  0 L L L
Developing(B>>0) Mature(B>0) Occlusion(B0)
Conventional Extratropical cyclone: B varies L L L

6. Cyclone Parameter -VT: Thermal Wind
Warm-core example: Floyd 14 Sep 1999
Focus here on hPa
-VTL >> 0

7. Cyclone Parameter -VT: Thermal Wind
Cold-core example: Cleveland Superbomb 26 Jan 1978
Focus here on hPa
-VTL << 0

8. Cyclone phase diagram: B Vs. -VTL
Asymmetric cold-core
Asymmetric warm-core
Symmetric warm-core
Symmetric cold-core

9. Case example: Hurricane Floyd (1999)
Track image from NHC Best-Track Analysis/web page
Extratropical transition (NHC)
Category 4 TC
Rapid movement & trough interaction

10. Phase diagnosis: symmetric, moderately strong warm-core
1200 UTC 9 Sept 1999
Asymmetric cold-core B
Asymmetric warm-core
Phase diagnosis: symmetric, moderately strong warm-core
Symmetric cold-core
Symmetric warm-core
NHC Best-track: Tropical Storm 1000hPa /45knots
-VTL

11. 0000 UTC 15 Sept 1999 B -VTL Asymmetric cold-core
Asymmetric warm-core
Phase diagnosis: very strong, symmetric warm-core
Symmetric cold-core
NHC Best-track: Hurricane hPa /115knots
-VTL

12. 0000 UTC 16 Sept 1999 B -VTL Asymmetric cold-core
Asymmetric warm-core
Phase diagnosis: extratropical transition begins
Symmetric cold-core
NHC Best-track: Hurricane hPa /90knots
-VTL

13. 1200 UTC 16 Sept 1999 B -VTL Asymmetric cold-core
Asymmetric warm-core
Phase diagnosis: hybrid cyclone
Symmetric cold-core
NHC Best-track: Hurricane hPa /70knots
-VTL

14. 1200 UTC 17 Sept 1999 B -VTL Asymmetric cold-core
Phase diagnosis: extratropical transition completion
Symmetric cold-core
-VTL
NHC Best-track: Extratropical hPa /45knots

15. 1200 UTC 19 Sept 1999 B -VTL Asymmetric cold-core
Phase diagnosis: asymmetric, cold-core
Symmetric cold-core
-VTL

16. Erin (2001): NGP
Michelle (2001): AVN
Recent transition cases of similar trajectory but varied analysis, geography & season
Vance (1999): NGP

17. Summary
Extratropical transition is correctly identified within the phase space as the conversion: symmetric/warm-core  asymmetric cold-core
Objective diagnoses (and forecast guidance when applied to model output) for the commencement & completion of extratropical transition possible
Allows for comparison to satellite & model diagnostics presented by Harr & Elsberry (2000) and Klein et al. (2000)

18. Summary
The reverse (subtropical or tropical) transition can also be diagnosed or forecast by also looking at –VTL Vs. -VTU: Karen, Olga, Noel (2001)
Phase diagrams are being produced in real-time and were used experimentally by CHC, NHC during the 2001 season:
Intercomparison of phase diagrams from many forecast models may provide measure of lifecycle predictability & uncertainty  ensembling

19. Future work
Further dynamical insight provided by other measures? e.g. Thermal vorticity (Darr 2002)
Examine phase predictability
Impact of synthetic bogus on phase evolution:
Delay or acceleration of transitions?

20. Future work
Can phase diagram be used to indicate when bogussing should cease?
Synoptic evaluation of common trajectories
Dynamics evolution along phase trajectory
Dynamics of hybrid cyclones

21. Acknowledgments
Penn State University: Jenni Evans, Bill Frank, Mike Fritsch, Nelson Seaman
SUNY Albany: Lance Bosart, John Molinari
University of Wisconsin/CIMSS: Chris Velden
National Hurricane Center (NHC): Jack Beven, Richard Pasch, Miles Lawrence, Lixion Avila
Canadian Hurricane Center (CHC): Pete Bowyer
Lawrence Livermore National Lab: Mike Fiorino
NCDC: Satellite imagery
NCEP: Real-time gridded analyses & forecasts
NCAR/CDC: NCEP/NCAR Reanalyses

23. Unnamed TC (1991) Michael (2000) Noel (2001) Extratropical Low
Images courtesy NCDC
“Perfect” Storm (1991)
Noel (2001)
President’s Day Blizzard (1979)
Extratropical Low
Floyd (1999)
Superstorm of 1993
Gloria (1985)

24. Cyclone parameter -VT: Thermal Wind
e.g. 700hPa height
ZMAX
500km
Z = ZMAX-ZMIN:
isobaric height difference within 500km radius
Proportional to geostrophic wind (Vg) magnitude
Z = d f |Vg| / g where
d=distance between height extrema, f=coriolis, g=gravity
ZMIN
Vertical profile of ZMAX-ZMIN is proportional to thermal wind (VT) if d is constant:
hPa: -VTL hPa: -VTU
-VT < 0 = Cold-core, -VT > 0 = Warm-core

25. Other Paths to Transition: Extended hybrid status.
Gabrielle (2001)
Charley (1986)
Results from competing forcings driving vertical structure change:
1. Trough interaction can drive asymmetric/cold-core development Gulf stream can drive symmetric/warm-core development
 Hybrid structure maintained over several days until one ultimately dominates or dissipation occurs

26. Cold-to-warm core transition: Tropical Transition of Hurricane Olga (2001) -VTU Vs. -VTL
Tropical transition begins when –VTL > 0
(subtropical status)
Tropical transition completes when –VTU > 0
(tropical status)
-VTU Vs. –VTL can show tendency toward a shallow or even deep warm-core structure when conventional analyses of MSLP, PV may be ambiguous or insufficient.

27. Symmetric warm-core evolution: Hurricane Mitch (1998) B Vs. -VTL

28. Asymmetric cold-core evolution: Extratropical Cyclone B Vs. -VTL
Increasing B as baroclinic development occurs.
After peak in B, intensification ensues followed by weakening of cold-core & occlusion.

29. Cold-core phase diagnosis compared to NHC ET declaration ECMWF 1.125° Reanalysis [60 storms]