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Extratropical Transition: One Trajectory through a Cyclone Phase Space

Extratropical Transition: One Trajectory through a Cyclone Phase Space. 2 May 2002 Robert Hart and Jenni Evans Department of Meteorology Penn State University. http://eyewall.met.psu.edu/cyclonephase/. http://eyewall.met.psu.edu/cyclonephase/. Which 5 are officially tropical cyclones?.

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Extratropical Transition: One Trajectory through a Cyclone Phase Space

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  1. Extratropical Transition: One Trajectory through a Cyclone Phase Space 2 May 2002 Robert Hart and Jenni Evans Department of Meteorology Penn State University http://eyewall.met.psu.edu/cyclonephase/ http://eyewall.met.psu.edu/cyclonephase/

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

  3. 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 Cyclone phase diagram

  4. Cyclone Parameter B: Thermal Asymmetry • Storm-relative 900-600hPa 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. Warm-core example: Floyd 14 Sep 1999 Cyclone Parameter -VT: Thermal Wind Focus here on 900-600hPa -VTL >> 0

  7. Cold-core example: Cleveland Superbomb 26 Jan 1978 Cyclone Parameter -VT: Thermal Wind Focus here on 900-600hPa -VTL << 0

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

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

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

  11. 0000 UTC 15 Sept 1999 Asymmetric cold-core B Asymmetric warm-core • Phase diagnosis: very strong, symmetric warm-core Symmetric cold-core • NHC Best-track: Hurricane 933hPa /115knots -VTL

  12. 0000 UTC 16 Sept 1999 Asymmetric cold-core B Asymmetric warm-core • Phase diagnosis: extratropical transition begins Symmetric cold-core • NHC Best-track: Hurricane 950hPa /90knots -VTL

  13. 1200 UTC 16 Sept 1999 Asymmetric cold-core B Asymmetric warm-core • Phase diagnosis: hybrid cyclone Symmetric cold-core • NHC Best-track: Hurricane 967hPa /70knots -VTL

  14. 1200 UTC 17 Sept 1999 Asymmetric cold-core B • Phase diagnosis: extratropical transition completion Symmetric cold-core -VTL • NHC Best-track: Extratropical 984hPa /45knots

  15. 1200 UTC 19 Sept 1999 Asymmetric cold-core B • 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: http://eyewall.met.psu.edu/cyclonephase • 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

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

  23. 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: 900-600hPa: -VTL 600-300hPa: -VTU -VT < 0 = Cold-core, -VT > 0 = Warm-core

  24. 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 2. Gulf stream can drive symmetric/warm-core development  Hybrid structure maintained over several days until one ultimately dominates or dissipation occurs

  25. -VTU Tropical transition completes when –VTU > 0 (tropical status) Tropical transition begins when –VTL > 0 (subtropical status) -VTL Cold-to-warm core transition: Tropical Transition of Hurricane Olga (2001) -VTU Vs. -VTL -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.

  26. B -VTL Symmetric warm-core evolution:Hurricane Mitch (1998) B Vs. -VTL SYMMETRIC WARM-CORE

  27. B -VTL 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.

  28. Cold-core phase diagnosis compared to NHC ET declaration1979-1993 ECMWF 1.125° Reanalysis [60 storms]

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