Extratropical Transition: One Trajectory through a Cyclone Phase Space

1
Extratropical Transition One Trajectory
through a Cyclone Phase Space

• 2 May 2002
• Robert Hart and Jenni Evans
• Department of Meteorology
• Penn State University

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

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Cyclone Parameter B Thermal Asymmetry

• Storm-relative 900-600hPa mean thickness field
  (shaded) asymmetry within 500km radius

3160m
3260m
L
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Cyclone Parameter B Thermal Asymmetry
Forming (B?0) Mature(B?0)
Decay(B?0)
Conventional Tropical cyclone B ? 0
L
L
L
Developing(Bgtgt0) Mature(Bgt0)
Occlusion(B?0)
Conventional Extratropical cyclone B
varies
L
L
L
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Cyclone Parameter -VT Thermal Wind
Warm-core example Floyd 14 Sep 1999
Focus here on 900-600hPa
-VTL gtgt 0
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Cyclone Parameter -VT Thermal Wind
Cold-core example Cleveland Superbomb 26 Jan 1978
Focus here on 900-600hPa
-VTL ltlt 0
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Cyclone phase diagram B Vs. -VTL
Asymmetric cold-core
Asymmetric warm-core
Symmetric warm-core
Symmetric cold-core
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Case example Hurricane Floyd (1999)
Track image from NHC Best-Track Analysis/web page
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Phase diagnosis symmetric, moderately strong
warm-core
1200 UTC 9 Sept 1999
Asymmetric cold-core
B
Asymmetric warm-core
Symmetric cold-core
Symmetric warm-core

• NHC Best-track Tropical Storm 1000hPa
  /45knots

-VTL
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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
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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
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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
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1200 UTC 17 Sept 1999
Asymmetric cold-core
B

• Phase diagnosis extratropical transition
  completion

Symmetric cold-core
-VTL

• NHC Best-track Extratropical 984hPa
  /45knots

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1200 UTC 19 Sept 1999
Asymmetric cold-core
B

• Phase diagnosis asymmetric, cold-core

Symmetric cold-core
-VTL
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Michelle (2001) AVN
Recent transition cases of similar trajectory but
varied analysis, geography season
Vance (1999) NGP
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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)

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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/cyclo
  nephase
• Intercomparison of phase diagrams from many
  forecast models may provide measure of lifecycle
  predictability uncertainty ? ensembling

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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?

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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

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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

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(No Transcript)
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Unnamed TC (1991)
Michael (2000)
Images courtesy NCDC
Perfect Storm (1991)
Noel (2001)
Presidents Day Blizzard (1979)
Extratropical Low
Floyd (1999)
Superstorm of 1993
Gloria (1985)
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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
ddistance between height extrema, fcoriolis,
ggravity
ZMIN
Vertical profile of ZMAX-ZMIN is proportional to
thermal wind (VT) if d is constant
900-600hPa -VTL 600-300hPa -VTU
-VT lt 0 Cold-core, -VT gt 0 Warm-core
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Other Paths to Transition Extended hybrid
status.
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
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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.
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Symmetric warm-core evolutionHurricane Mitch
(1998) B Vs. -VTL
SYMMETRIC WARM-CORE
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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.
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Cold-core phase diagnosis compared to NHC ET
declaration1979-1993 ECMWF 1.125 Reanalysis 60
storms