Extratropical Transition: One Trajectory through a
Cyclone Phase Space
2 May 2002
Robert Hart and Jenni Evans
Department of Meteorology
Penn State University
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 900-600hPa mean thickness field (shaded) asymmetry within 500km radius:
Cyclone Parameter B: Thermal Asymmetry
LEFThPahPa
RIGHThPahPa ZZZZB 900600900600
3160
m32
60mL
Cyclone Parameter B: Thermal Asymmetry
L L L
Developing(B>>0) Mature(B>0) Occlusion(B0)Conventional
Extratropical cyclone: B varies
L L L
Forming (B0) Mature(B0) Decay(B0)Conventional
Tropical cyclone: B 0
Cyclone Parameter -VT: Thermal WindWarm-core example: Floyd 14 Sep 1999
Focus here on 900-600hPa
-VTL >> 0
Cyclone Parameter -VT: Thermal WindCold-core example: Cleveland Superbomb 26 Jan 1978
Focus here on 900-600hPa
-VTL << 0
Cyclone phase
diagram: B Vs. -
VTL
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
-VTL
Asymmetric cold-core
Asymmetric warm-core
Symmetric cold-core
0000 UTC 15 Sept 1999
B
-VTL
• 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
-VTL
• 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
-VTL
• Phase diagnosis: hybrid cyclone
• NHC Best-track: Hurricane 967hPa /70knots
Asymmetric cold-core
Symmetric cold-core
1200 UTC 17 Sept 1999
B
-VTL
• Phase diagnosis: extratropical transition completion
• NHC Best-track: Extratropical 984hPa /45knots
Asymmetric cold-core
Symmetric cold-core
1200 UTC 19 Sept 1999
B
-VTL
• 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 –VTL Vs. -VT
U: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
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 -VT: Thermal Wind
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
Vertical profile of ZMAX-ZMIN is proportional to thermal wind (VT) if d is constant:
||ln
)(T
MINMAX Vp
ZZ
-VT < 0 = Cold-core, -VT > 0 = Warm-core
500km
ZMIN
ZMAX
e.g. 700hPa height
900-600hPa: -VTL
600-300hPa: -VTU
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. -VT
L
-VTL
-VTU
Tropical transition begins when –VT
L > 0
(subtropical status)
Tropical transition completes when –VT
U > 0
(tropical status)
-VTU Vs. –VT
L
can show tendency toward a shallow or even deep warm-core structure when conventional analyses of MSLP, PV may be ambiguous or insufficient.
Asymmetric cold-core evolution: Extratropical Cyclone B Vs. -VT
L
-VTL
B
Increasing B as baroclinic development occurs.
After peak in B, intensification ensues followed by weakening of cold-core & occlusion.