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

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

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

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

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

Cyclone Parameter -V T : Thermal Wind Warm-core example: Floyd 14 Sep 1999 Focus here on hPa -V T L >> 0

Cyclone Parameter -V T : Thermal Wind Cold-core example: Cleveland Superbomb 26 Jan 1978 Focus here on hPa -V T L << 0

Cyclone phase diagram: B Vs. -V T L Asymmetric cold- core Symmetric cold-core Asymmetric warm-core Symmetric warm-core

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

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

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

Asymmetric cold-core Symmetric cold-core Asymmetric warm-core 0000 UTC 16 Sept 1999 B -V T L Phase diagnosis: extratropical transition begins NHC Best-track: Hurricane 950hPa /90knots

Asymmetric cold-core Symmetric cold-core Asymmetric warm-core 1200 UTC 16 Sept 1999 B -V T L Phase diagnosis: hybrid cyclone NHC Best-track: Hurricane 967hPa /70knots

Asymmetric cold-core Symmetric cold-core 1200 UTC 17 Sept 1999 B -V T L Phase diagnosis: extratropical transition completion NHC Best-track: Extratropical 984hPa /45knots

Asymmetric cold-core Symmetric cold-core 1200 UTC 19 Sept 1999 B -V T L Phase diagnosis: asymmetric, cold-core

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

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)

Summary The reverse (subtropical or tropical) transition can also be diagnosed or forecast by also looking at –V T L Vs. -V T U : 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

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?

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

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

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

Cyclone parameter -V T : Thermal Wind  Z = Z MAX -Z MIN : isobaric height difference within 500km radius Proportional to geostrophic wind (V g ) magnitude  Z = d f |V g | / g where d=distance between height extrema, f=coriolis, g=gravity Vertical profile of Z MAX -Z MIN is proportional to thermal wind (V T ) if d is constant: -V T 0 = Warm-core 500km Z MIN Z MAX e.g. 700hPa height hPa: -V T L hPa: -V T U

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

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

Symmetric warm-core evolution: Hurricane Mitch (1998) B Vs. -V T L -V T L B SYMMETRIC WARM-CORE

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

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