Transcript Extratropical & Tropical Transition: Two trajectories

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
Cyclone Parameter B: Thermal Asymmetry
• Storm-relative 900-600hPa mean thickness field
(shaded) asymmetry within 500km radius:
B  Z 600hPa  Z 900hPa
RIGHT
L
 Z 600hPa  Z 900hPa
LEFT
Cyclone Parameter B: Thermal Asymmetry
Forming (B0)
Conventional
Tropical
cyclone:
B0
L
Developing(B>>0)
Conventional
Extratropical
cyclone:
B varies
L
Mature(B0)
L
Mature(B>0)
L
Decay(B0)
L
Occlusion(B0)
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 coldcore
Asymmetric
warm-core
Symmetric cold-core
Symmetric
warm-core
Case example: Hurricane Floyd (1999)
Extratropical transition (NHC)
Rapid movement &
trough interaction
Category 4 TC
Track
image
from
NHC
BestTrack
Analysis/
web page
1200 UTC 9 Sept 1999
Asymmetric cold-core
B
Asymmetric
warm-core
• Phase diagnosis:
symmetric, moderately
strong warm-core
• NHC Best-track:
Tropical Storm
1000hPa /45knots
Symmetric cold-core
-VTL
Symmetric
warm-core
0000 UTC 15 Sept 1999
Asymmetric cold-core
B
• Phase diagnosis:
very strong, symmetric
warm-core
• NHC Best-track:
Hurricane
933hPa /115knots
Asymmetric
warm-core
Symmetric cold-core
-VTL
0000 UTC 16 Sept 1999
Asymmetric cold-core
B
• Phase diagnosis:
extratropical transition
begins
• NHC Best-track:
Hurricane
950hPa /90knots
Asymmetric
warm-core
Symmetric cold-core
-VTL
1200 UTC 16 Sept 1999
Asymmetric cold-core
B
• Phase diagnosis:
hybrid cyclone
• NHC Best-track:
Hurricane
967hPa /70knots
Asymmetric
warm-core
Symmetric cold-core
-VTL
1200 UTC 17 Sept 1999
Asymmetric cold-core
B
• Phase diagnosis:
extratropical transition
completion
• NHC Best-track:
Extratropical
984hPa /45knots
Symmetric cold-core
-VTL
1200 UTC 19 Sept 1999
Asymmetric cold-core
B
• Phase diagnosis:
asymmetric, cold-core
Symmetric cold-core
-VTL
Erin (2001): NGP
Recent transition cases of similar
trajectory but varied analysis,
geography & season
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. -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:
•
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:
NCEP:
NCAR/CDC:
Chris Velden
Satellite imagery
Real-time gridded analyses & forecasts
NCEP/NCAR Reanalyses
Images
courtesy
NCDC
Unnamed TC
(1991)
Michael (2000)
“Perfect” Storm
(1991)
Noel (2001)
President’s Day
Blizzard (1979)
Extratropical
Low
Floyd (1999)
Gloria (1985)
Superstorm
of 1993
Cyclone parameter -VT: Thermal Wind
e.g. 700hPa height
Z = ZMAX-ZMIN:
500km
ZMAX
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:
 ( Z MAX  Z MIN )
  | VT |
 ln p
900-600hPa: -VTL
600-300hPa: -VTU
-VT < 0 = Cold-core, -VT > 0 = Warm-core
ZMIN
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
-VTU
Tropical transition
begins when –VTL > 0
(subtropical status)
-VTL
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
B
SYMMETRIC WARM-CORE
-VTL
Asymmetric cold-core evolution:
Extratropical Cyclone B Vs. -VTL
Increasing B as
baroclinic
development
occurs.
B
After peak in B,
intensification
ensues followed
by weakening
of cold-core &
occlusion.
-VTL
Cold-core phase diagnosis compared to NHC ET declaration
1979-1993 ECMWF 1.125° Reanalysis [60 storms]