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? 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
Cyclone Parameter B: Thermal Asymmetry Storm-relative 900-600hPa mean thickness field (shaded) asymmetry within 500km radius: 3160m 3260m L
Cyclone Parameter B: Thermal Asymmetry Forming (B0) Mature(B0) Decay(B0) Conventional Tropical cyclone: B 0 L L L Developing(B>>0) Mature(B>0) Occlusion(B0) Conventional Extratropical cyclone: B varies L L L
Cyclone Parameter -VT: Thermal Wind Warm-core example: Floyd 14 Sep 1999 Focus here on 900-600hPa -VTL >> 0
Cyclone Parameter -VT: Thermal Wind Cold-core example: Cleveland Superbomb 26 Jan 1978 Focus here on 900-600hPa -VTL << 0
Cyclone phase diagram: B Vs. -VTL Asymmetric cold-core Asymmetric warm-core Symmetric warm-core Symmetric cold-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 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
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 933hPa /115knots -VTL
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 950hPa /90knots -VTL
1200 UTC 16 Sept 1999 B -VTL Asymmetric cold-core Asymmetric warm-core Phase diagnosis: hybrid cyclone Symmetric cold-core NHC Best-track: Hurricane 967hPa /70knots -VTL
1200 UTC 17 Sept 1999 B -VTL Asymmetric cold-core Phase diagnosis: extratropical transition completion Symmetric cold-core -VTL NHC Best-track: Extratropical 984hPa /45knots
1200 UTC 19 Sept 1999 B -VTL Asymmetric cold-core Phase diagnosis: asymmetric, cold-core Symmetric cold-core -VTL
Erin (2001): NGP Michelle (2001): AVN Recent transition cases of similar trajectory but varied analysis, geography & season 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 –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
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
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)
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
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) -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.
Symmetric warm-core evolution: Hurricane Mitch (1998) B Vs. -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.
Cold-core phase diagnosis compared to NHC ET declaration 1979-1993 ECMWF 1.125° Reanalysis [60 storms]