Transcript A Cyclone Phase Space Derived from Thermal Wind & Thermal

A Cyclone Phase Space
Derived from
Thermal Wind &
Thermal Asymmetry
Robert Hart
Department of Meteorology
Penn State University
[email protected]
http://eyewall.met.psu.edu/cyclonephase
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.
“Conventional” Cyclones
Type:
Tropical cyclone
Extratropical cyclone
Structure:
Symmetric warm-core
Asymmetric cold-core
Predictability: Low-moderate?
Moderate-high?
Basic Theory: Charney & Eliassen (1964)
Bjerknes & Solberg (1922)
Charney (1947)
Sutcliffe (1947)
Eady (1949)
Kuo (1965)
Ooyama (1964, 1969)
Emanuel (1986)
Research has shown that the distribution of cyclones is not
limited to these two discrete groups.
Tannehill (1938)
Billing et al. (1983)
Bosart & Lackmann (1995)
Pierce (1939)
Gyakum (1983a,b)
Beven (1997)
Knox (1955)
Sardie & Warner (1983)
Harr & Elsberry (2000)
Sekioka (1956a,b;1957)
Smith et al. (1984)
Harr et al. (2000)
Palmén (1958)
Rasmussen & Zick (1987)
Klein et al. (2000)
Hebert (1973)
Emanuel & Rotunno (1989) Miner et al. (2000)
Kornegay & Vincent (1976) Rasmussen (1989)
Smith (2000)
Brand & Guard (1978)
Bosart & Bartlo (1991)
Thorncroft & Jones (2000)
Bosart (1981)
Kuo et al. (1992)
Hart & Evans (2001)
DiMego & Bosart (1982a,b) Reed et al. (1994)
Reale & Atlas (2001)
Example: Separate the 5 tropical cyclones from the 5 extratropical.
Images
courtesy
NCDC
Non-conventional cyclones: Examples
1938 New England Hurricane
?
940hPa
Pierce 1939
•
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?
Non-conventional cyclones: Examples
Christmas 1994
Hybrid New England Storm
NCDC
•
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.
Lifecycle Type
Time
L
L
Dominant
lifecycle?
Transitions?
Extratropical
cyclone
Hybrid
evolution?
Tropical
cyclone
Forecast skill and/or innate predictability (?)
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?
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.
Method:
Characteristic cyclone parameters

Desire cyclone parameters that can uniquely
diagnose & distinguish the full range of cyclones

Fundamental parameters that describe the threedimensional structural evolution of storms:
1)
Asymmetry (frontal vs. nonfrontal)
2)
Thermal wind (cold vs. warm core)
Cyclone Parameter B: Thermal Asymmetry
• Defined using storm-relative 900-600hPa mean
thickness field (shaded) asymmetry within 500km
radius:
B  Z 600hPa  Z 900hPa
RIGHT
L
B >> 0:
 Z 600hPa  Z 900hPa
LEFT
B=100m in this example
Cold
Warm
Frontal
B0: Nonfrontal
Cyclone Parameter B: Thermal Asymmetry
Conventional Tropical cyclone: B  0
Forming
L
Mature
Decay
L
L
Conventional Extratropical cyclone: B varies
Developing
L
B >> 0
Mature
L
B>0
Occlusion
L
B0
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
Cyclone Parameter -VT:
Thermal Wind
Warm-core example:
Hurricane Floyd 14 Sep 1999
Two layers of interest:
-VTU >> 0
-VTL >> 0
Tropospheric warm core
Cyclone Parameter -VT:
Thermal Wind
Cold-core example:
Cleveland Superbomb 26 Jan 1978
Two layers of interest:
-VTU << 0
-VTL << 0
Tropospheric cold core
Note: horizontal tilt of cyclone
is necessarily associated with a
strong cold-core structure & is
captured well by the method
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
Results:
Conventional cyclone “trajectories”
through the phase space

Tropical Cyclone:
Mitch (1998)

Extratropical cyclone:
December 1987
(Schultz & Mass 1993)
Symmetric warm-core evolution:
Hurricane Mitch (1998) B Vs. -VTL
B
SYMMETRIC WARM-CORE
-VTL
Symmetric warm-core evolution:
Hurricane Mitch (1998) -VTL Vs. -VTU
Upward warm
core
development
maturity, and
decay.
-VTU
With landfall,
warm-core
weakens more
rapidly in lower
troposphere
than upper.
-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
Asymmetric cold-core evolution:
Extratropical cyclone -VTL Vs. -VTU
-VTU
-VTL
Results:
Non-conventional cyclone “trajectories”
through the phase space
  Extratropical
transition:
Extratropical
transition:
  (Sub)tropical
transition:
Tropical transition:
Floyd
(1999)
Floyd
(1999)
Olga
(2001)
Olga
(2001)

Ocean Ranger (1982)
Warm seclusion:
(Kuo et al. 1992)
Warm-to-cold core transition:
Extratropical Transition of Hurricane Floyd (1999)
B Vs. -VTL
Provides for
objective
indicators of
extratropical
transition
lifecycle.
B
Extratropical transition
ends when –VTL < 0
Extratropical transition
begins when B=10m
-VTL
Provides for a
method of
comparison to
satellite-based
diagnoses of
extratropical
transition from
Harr & Elsberry
(2000), Klein et
al. (2000)
Warm-to-cold core transition:
Extratropical Transition of Hurricane Floyd (1999)
-VTL Vs. -VTU
Upward warm
core
development
maturity, and
decay.
-VTU
Extratropical
transition here
drives a
conversion
from warm to
cold core aloft
first, then
downward.
-VTL
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.
Warm-seclusion of an extratropical cyclone:
Ranger” cyclone of 1982
-VTU Vs. -VTL
“Ocean
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°-70°N, 120°-0°W
• Some tracking errors for fast-moving cyclones
• Insufficient resolution for TCs  poor
climatology
15-year cyclone
phase inhabitance
Few TCs!
B Vs. -VTL
-VTU Vs. -VTL
Mean cyclone
intensity (MSLP)
within phase space
B Vs. -VTL
-VTU Vs. -VTL
Mean cyclone
intensity change
(hPa/6hr) within
phase space
B Vs. -VTL
-VTU Vs. -VTL
Summary of cyclone types within the phase space
Summary of cyclone types within the phase space
?Polar lows?
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
Multiple model solutions Multiple Phase Diagrams
Example: Hurricane Erin (2001)
NGP
AVN
UKM
Cyclone Phase Forecasting: Ensembling
Consensus Mean & Forecast Envelope
AVN+NOGAPS+UKMET
B
Z
C
A
-VTL
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.
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)
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?
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.
Pierce, C. H., 1939: The meteorological history of the New England hurricane of Sept. 21, 1938. Mon. Wea. Rev., 67, 237-285.
Reale, O. and R. Atlas, 2001: Tropical cyclone-like vortices in the extratropics: Observational evidence and synoptic analysis. Weather and
Forecasting, 16, 7-34.
Schultz, D. M. and C.F. Mass, 1993: The occlusion process in a midlatitude cyclone over land. Mon. Wea. Rev., 121, 918-940.
Separate the 5 tropical cyclones from the 5 extratropical.
Images
courtesy
NCDC
Noel (2001)
Unnamed TC
(1991)
Michael (2000)
“Perfect” Storm
(1991)
Extratropical
Low
President’s Day
Blizzard (1979)
Floyd (1999)
Gloria (1985)
Superstorm
of 1993