A Cyclone Phase Space Derived from Thermal Wind

1
A Cyclone Phase Space Derived from Thermal
Wind Thermal Asymmetry
2
Robert HartDepartment of MeteorologyPenn State
Universityhart_at_ems.psu.eduhttp//eyewall.met.ps
u.edu/cyclonephase
3
Introduction The Problem

• Tropical and extratropical cyclones historically
  have been viewed as two discrete, mutual
  exclusive cyclone groups.
• Warm SSTs, increased surface fluxes, enhanced
  convection, enhanced latent heat release
  warm-seclusion within extratropical cyclones can
  blur that once-perceived fine line between
  tropical and extratropical cyclones.
• Cyclones that have aspects of both tropical and
  extratropical cyclones are difficult to
  completely explain by individual development
  theories.
• Yet, synthesizing tropical cyclone
  extratropical cyclone development theories is
  difficult.
• Cyclone predictability (both numerically and in
  reality) is likely related to cyclone phase.
• Current diagnosis and forecast methods do not
  adequately address such a gray area of cyclone
  development cyclone transition.

4
Conventional Cyclones
Type Structure Predictability
Basic Theory
Extratropical cyclone Asymmetric
cold-core Moderate-high? Bjerknes Solberg
(1922) Charney (1947)
Sutcliffe (1947) Eady
(1949)
Tropical cyclone Symmetric warm-core Low-moderate?
Charney Eliassen (1964) Kuo (1965)
Ooyama (1964, 1969)
Emanuel (1986)
5

• Research has shown that the distribution of
  cyclones is not limited to these two discrete
  groups.

Tannehill (1938) Pierce (1939) Knox
(1955) Sekioka (1956a,b1957) Palmén
(1958) Hebert (1973) Kornegay Vincent
(1976) Brand Guard (1978) Bosart (1981) DiMego
Bosart (1982a,b)
Billing et al. (1983) Gyakum (1983a,b) Sardie
Warner (1983) Smith et al. (1984) Rasmussen
Zick (1987) Emanuel Rotunno (1989) Rasmussen
(1989) Bosart Bartlo (1991) Kuo et al.
(1992) Reed et al. (1994)
Bosart Lackmann (1995) Beven (1997) Harr
Elsberry (2000) Harr et al. (2000) Klein et al.
(2000) Miner et al. (2000) Smith
(2000) Thorncroft Jones (2000) Hart Evans
(2001) Reale Atlas (2001)
6
Example Separate the 5 tropical cyclones from
the 5 extratropical.
Images courtesy NCDC
7
Non-conventional cyclones Examples
1938 New England Hurricane

• Began as intense tropical cyclone
• Rapid transformation into an intense frontal
  cyclone over New England (left)
• Enormous damage (3.5 billion adjusted to 1990).
  10 of trees downed in New England. 600 lives
  lost.
• At what point between tropical extratropical
  structure is this cyclone at?

Pierce 1939
8
Non-conventional cyclones Examples
Christmas 1994 Hybrid New England
Storm

• Gulf of Mexico extratropical cyclone that
  unexpectedly acquired partial tropical
  characteristics (Beven 1997)
• A partial eye-like structure was observed when
  the cyclone was just east of Long Island
• Wind gusts of 50-100mph observed across southern
  New England
• Largest U.S. power outage (350,000) since Andrew
  in 1992
• Forecast 6hr earlier chance of light rain,
  winds of 5-15mph.

NCDC
9
Lifecycle Type
Time
L
L
Dominant lifecycle?
Transitions?
Tropical cyclone
Extratropical cyclone
Hybrid evolution?
Forecast skill and/or innate predictability (?)
10
Questions

• Is it reasonable to expect that there is a
  continuum of cyclones, rather than two discrete
  groups?
• Previous research has suggested such a continuum
  (Beven 1997 Reale Atlas 2001)
• How do we describe this continuum objectively
  practically?
• By relaxing our current view of all cyclones as
  only tropical or extratropical, can we gain a
  better diagnosis understanding of cyclone
  development non-conventional cyclones?

11
Goal A more flexible approach to cyclone
characterization

• To describe the basic structure of tropical,
  extratropical, subtropical, warm-seclusion, and
  hybrid cyclones simultaneously using a cyclone
  phase space leading to
• Improved, unified diagnosis understanding of
  the broad spectrum of cyclones
• Objective classification, improved forecasting
  estimation of predictability, more stringent
  verification.

12
MethodCharacteristic cyclone parameters

• ? Desire cyclone parameters that can uniquely
  diagnose distinguish the full range of cyclones
  
• Fundamental parameters that describe the
  three-dimensional structural evolution of storms
• 1) Asymmetry (frontal vs. nonfrontal)
• 2) Thermal wind (cold vs. warm core)

13
Cyclone Parameter B Thermal Asymmetry

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

3160m
B100m in this example
3260m
L
Cold Warm
B gtgt 0 Frontal B?0 Nonfrontal
14
Cyclone Parameter B Thermal Asymmetry
Conventional Tropical cyclone B ? 0
Forming Mature Decay
L
L
L
Conventional Extratropical cyclone B varies
Developing Mature Occlusion
L
L
L
B gtgt 0 B gt 0 B ? 0
15
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
16
Cyclone Parameter -VT Thermal Wind
Warm-core example Hurricane Floyd 14
Sep 1999
Two layers of interest
-VTU gtgt 0
-VTL gtgt 0
?Tropospheric warm core
17
Cyclone Parameter -VT Thermal Wind
Cold-core example Cleveland
Superbomb 26 Jan 1978
Two layers of interest
-VTU ltlt 0
-VTL ltlt 0
?Tropospheric cold core
Note horizontal tilt of cyclone is necessarily
associated with a strong cold-core structure is
captured well by the method
18
Constructing 3-D phase space from cyclone
parameters B, -VTL, -VTU
A trajectory within 3-D generally too complex to
readily visualize ? Take two cross sections
B
-VTU
-VTL
-VTL
19
ResultsConventional cyclone trajectories
through the phase space
? Tropical Cyclone Mitch (1998) ? Extratropical
cyclone December 1987 (Schultz Mass 1993)
20
Symmetric warm-core evolutionHurricane Mitch
(1998) B Vs. -VTL
SYMMETRIC WARM-CORE
21
Symmetric warm-core evolutionHurricane Mitch
(1998) -VTL Vs. -VTU
Upward warm core development maturity, and
decay. With landfall, warm-core weakens more
rapidly in lower troposphere than upper.
22
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.
23
Asymmetric cold-core evolutionExtratropical
cyclone -VTL Vs. -VTU
24
ResultsNon-conventional cyclone
trajectories through the phase space

• ? Extratropical transition Floyd (1999)
• (Sub)tropical transition Olga (2001)
• Warm seclusion Ocean Ranger (1982)
  (Kuo et al. 1992)

? Extratropical transition Floyd
(1999) ? Tropical transition Olga (2001)
25
Warm-to-cold core transition
Extratropical Transition of
Hurricane Floyd (1999) B Vs. -VTL
?Provides for objective indicators of
extratropical transition lifecycle. ?Provides
for a method of comparison to satellite-based
diagnoses of extratropical transition from Harr
Elsberry (2000), Klein et al. (2000)
26
Warm-to-cold core transition
Extratropical Transition of
Hurricane Floyd (1999) -VTL Vs. -VTU
Upward warm core development maturity, and
decay. Extratropical transition here drives a
conversion from warm to cold core aloft first,
then downward.
27
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.
28
Warm-seclusion of an extratropical cyclone
Ocean Ranger cyclone of 1982 -VTU
Vs. -VTL
29
Cyclone phase climatology

• 1986-2000 NCEP Reanalysis (2.5 resolution)
• Compared to 1 for operational analyses
• 20 vertical levels
• Approximately 15,000 cyclones
• Domain 10-70N, 120-0W
• Some tracking errors for fast-moving cyclones
• Insufficient resolution for TCs ? poor climatology

30
15-year cyclone phase inhabitance
B Vs. -VTL
-VTU Vs. -VTL
31
Mean cyclone intensity (MSLP) within phase space
B Vs. -VTL
-VTU Vs. -VTL
32
Mean cyclone intensity change (hPa/6hr) within
phase space
B Vs. -VTL
-VTU Vs. -VTL
33
Summary of cyclone types within the phase space
34
Summary of cyclone types within the phase space
?Polar lows?
35
Real-time Cyclone Phase Analysis Forecasting

• Phase diagrams produced in real-time for various
  operational and research models.
• Provides insight into cyclone evolution that may
  not be apparent from conventional analyses
• Can be used to aid anticipation of phase changes,
  especially extratropical (sub)tropical
  transition.
• Were used experimentally during 2001 hurricane
  season.
• Web site http//eyewall.met.psu.edu/cyclonephase

36
Multiple model solutions? Multiple Phase
DiagramsExample Hurricane Erin (2001)
NGP
AVN
UKM
37
Cyclone Phase Forecasting EnsemblingConsensus
Mean Forecast EnvelopeAVNNOGAPSUKMET
38
Phase space limitations

• Cyclone phase diagrams are dependent on the
  quality of the analyses upon which they are
  based.
• Three dimensions (B, -VTL, -VTU) are not expected
  to explain all aspects of cyclone development
• Other potential dimensions static stability,
  long-wave pattern, jet streak configuration,
  binary cyclone interaction, tropopause
  height/folds, surface moisture availability,
  surface roughness...
• However, the chosen three parameters represent a
  large percentage of the variance explain the
  crucial structural changes.

39
Summary

• A continuum of cyclone phase space is proposed,
  defined, explored.
• A unified diagnosis method for basic cyclone
  structure is possible.
• Conventional tropical extratropical cyclone
  lifecycles are well-defined within the phase
  space.
• Unconventional lifecycles (extratropical
  transition, tropical transition, hybrid cyclones)
  are resolved within the phase space.
• Describing and explaining cyclone evolution is
  not limited to the two textbook examples provided
  by historic cyclone development theory.
• The phase diagram can be applied to forecast data
  to arrive at estimates for forecast cyclones
  evolution, providing guidance for complex
  cyclones that was otherwise unavailable.
• Objective estimates for the timing of
  extratropical and tropical transition of cyclones
  is now possible. (NHC, CHC)

40
Future Work

• Continued use of the phase space to understand
  complex cyclone evolutions, including examination
  of dynamics as phase changes.
• Evaluation of the phase space to diagnose phase
  transition tropical and extratropical
• Hart Evans (2002 AMS Hurricanes Thursday
  presentation)
• Can it be used to anticipate (sub)tropical
  transition (e.g. Olga 2001)
• Examine the impact of a synthetic (bogus) vortex
  on the phase evolution
• Can phase evolution be used to diagnose when a
  bogus should be ceased?
• Examine the predictability within phase space
  what models are most skilled at forecasting
  extratropical transition, tropical transition,
  and phase in general?
• Is predictability related to phase or phase
  change?

41
Acknowledgments References

• Penn State University Jenni Evans, Bill
  Frank, Nelson Seaman, Mike Fritsch
• SUNY Albany Lance Bosart, John Molinari
• University of Wisconsin/CIMSS Chris Velden
• National Hurricane Center (NHC) Jack Beven,
  Miles Lawrence
• Canadian Hurricane Center (CHC) Pete Bowyer
• NCDC for the online database of satellite
  imagery, NCEP for providing real-time analyses,
  NCAR/ NCEP for their online archive of reanalysis
  data through CDC, and Mike Fiorino for providing
  NOGAPS analyses
• Beven, J.L. II, 1997 A study of three hybrid
  storms. Proc. 22nd Conf. On Hurricanes and
  Tropical Meteorology, Fort Collins, CO, Amer.
  Meteor. Soc., 645-6.
• Harr, P. and R. L. Elsberry, 2000 Extratropical
  transition of tropical cyclones over the western
  North Pacific. Part I. Evolution of structural
  characteristics during the transition process.
  Mon. Wea. Rev., 128, 2613-2633.
• Klein, P., P. Harr, and R. Elsberry, 2000
  Extratropical transition of western north Pacific
  tropical cyclones An overview and conceptual
  model of the transformation stage. Wea. And
  Forecasting, 15, 373-396.
• Kuo, Y.-H., R. J. Reed, and S. Low-Nam, 1992
  Thermal structure and airflow in a model
  simulation of an occluded marine cyclone. Mon.
  Wea. Rev., 120, 2280-2297.

42
Separate the 5 tropical cyclones from the 5
extratropical.
Unnamed TC (1991)
Michael (2000)
Images courtesy NCDC
Noel (2001)
Perfect Storm (1991)
Extratropical Low
Presidents Day Blizzard (1979)
Floyd (1999)
Superstorm of 1993
Gloria (1985)