Extratropical & Tropical Transition: Two trajectories through cyclone ...

Extratropical Transition: One Trajectory through a

Cyclone Phase Space2 May 2002

Robert Hart and Jenni EvansDepartment 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 900-600hPa mean thickness field (shaded) asymmetry within 500km radius:

Cyclone Parameter B: Thermal Asymmetry

LEFThPahPaRIGHThPahPa 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


Symmetric cold-core


Symmetric warm-core

1200 UTC 9 Sept 1999

B

-VTL



Symmetric cold-core

0000 UTC 15 Sept 1999

B

-VTL

• Phase diagnosis: very strong, symmetric warm-core

• NHC Best-track: Hurricane 933hPa /115knots


Symmetric cold-core


0000 UTC 16 Sept 1999

B

-VTL

• Phase diagnosis: extratropical transition begins



Symmetric cold-core


1200 UTC 16 Sept 1999

B

-VTL

• Phase diagnosis: hybrid cyclone



Symmetric cold-core

1200 UTC 17 Sept 1999

B

-VTL

• Phase diagnosis: extratropical transition completion

• NHC Best-track: Extratropical 984hPa /45knots


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 VpZZ

-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.

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

L

-VTL

B

SYMMETRIC WARM-CORE

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.

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

Extratropical & Tropical Transition: Two trajectories through cyclone ...

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