A Cyclone Phase Space Derived from Thermal Wind & Thermal Asymmetry.
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Transcript of A Cyclone Phase Space Derived from Thermal Wind & Thermal Asymmetry.
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A Cyclone Phase Space Derived from Thermal Wind &
Thermal Asymmetry
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Robert HartDepartment of Meteorology
Penn State [email protected]
http://eyewall.met.psu.edu/cyclonephase
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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.
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“Conventional” Cyclones
Tropical cyclone
Symmetric warm-core
Low-moderate?
Charney & Eliassen (1964) Kuo (1965) Ooyama (1964, 1969) Emanuel (1986)
Type:
Structure:
Predictability:
Basic Theory:
Extratropical cyclone
Asymmetric cold-core
Moderate-high?
Bjerknes & Solberg (1922) Charney (1947) Sutcliffe (1947) Eady (1949)
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Research has shown that the distribution of cyclones is not limited to these two discrete groups.
Tannehill (1938)
Pierce (1939)
Knox (1955)
Sekioka (1956a,b;1957)
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)
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Example: Separate the 5 tropical cyclones from the 5 extratropical.
Images courtesy NCDC
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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?
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Non-conventional cyclones: ExamplesChristmas 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.
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TimeL
Extratropical cyclone
Forecast skill and/or innate predictability (?)
L
Dominant lifecycle?
Transitions?
Tropical cyclone
Hybrid evolution?
Lifecycle Type
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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?
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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.
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Method:Characteristic 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)
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• Defined using storm-relative 900-600hPa mean thickness field (shaded) asymmetry within 500km radius:
Cyclone Parameter B: Thermal Asymmetry
3160
m32
60mL
Cold Warm
LEFThPahPa
RIGHThPahPa ZZZZB 900600900600
B >> 0: Frontal B0: Nonfrontal
B=100m in this example
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Cyclone Parameter B: Thermal Asymmetry
L L L
Developing Mature Occlusion
B >> 0 B > 0 B 0
Conventional Extratropical cyclone: B varies
L L L
Forming Mature Decay
Conventional Tropical cyclone: B 0
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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 Vp
ZZ
-VT < 0 = Cold-core, -VT > 0 = Warm-core
500km
ZMIN
ZMAX
e.g. 700hPa height
900-600hPa: -VTL
600-300hPa: -VTU
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Cyclone Parameter -VT: Thermal Wind
Warm-core example: Hurricane Floyd 14 Sep 1999
Two layers of interest:
-VTU >> 0
-VTL >> 0
Tropospheric warm core
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Cyclone Parameter -VT: Thermal Wind
Cold-core example: Cleveland Superbomb 26 Jan 1978
-VTU << 0
-VTL << 0
Two layers of interest:
Note: horizontal tilt of cyclone is necessarily associated with a strong cold-core structure & is captured well by the method
Tropospheric cold core
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Constructing 3-D phase space from cyclone parameters: B, -VT
L, -VTU
A trajectory within 3-D generally too complex to readily visualize
Take two cross sections:
B
-VTL
-VTU
-VTL
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Results:
Conventional cyclone “trajectories” through the phase space
Tropical Cyclone: Mitch (1998)
Extratropical cyclone: December 1987 (Schultz & Mass 1993)
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Symmetric warm-core evolution:Hurricane Mitch (1998) B Vs. -VT
L
-VTL
B
SYMMETRIC WARM-CORE
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Symmetric warm-core evolution:Hurricane Mitch (1998) -VT
L Vs. -VTU
Upward warm core development maturity, and decay.
With landfall, warm-core weakens more rapidly in lower troposphere than upper.
-VTL
-VTU
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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.
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Asymmetric cold-core evolution:Extratropical cyclone -VT
L Vs. -VTU
-VTL
-VTU
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Results:
Non-conventional cyclone “trajectories” through the phase space
Extratropical transition: Floyd (1999)
Tropical transition: Olga (2001)
Extratropical transition: Floyd (1999)
(Sub)tropical transition: Olga (2001)
Warm seclusion: Ocean Ranger (1982) (Kuo et al. 1992)
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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)
Extratropical transition begins when B=10m
Extratropical transition ends when –VT
L < 0
-VTL
B
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Warm-to-cold core transition: Extratropical Transition of Hurricane Floyd (1999)
-VTL Vs. -VT
U
-VTL
-VTU
Upward warm core development maturity, and decay.
Extratropical transition here drives a conversion from warm to cold core aloft first, then downward.
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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.
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Warm-seclusion of an extratropical cyclone: “Ocean Ranger” cyclone of 1982
-VTU Vs. -VT
L
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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
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15-year cyclone phase inhabitance
Few TCs!
B Vs. -VTL
-VTU Vs. -VT
L
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Mean cyclone intensity (MSLP)
within phase space
B Vs. -VTL
-VTU Vs. -VT
L
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Mean cyclone intensity change (hPa/6hr) within
phase space
B Vs. -VTL
-VTU Vs. -VT
L
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Summary of cyclone types within the phase space
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Summary of cyclone types within the phase space
?Polar lows?
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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
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Multiple model solutions Multiple Phase DiagramsExample: Hurricane Erin (2001)
AVN NGP
UKM
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Cyclone Phase Forecasting: EnsemblingConsensus Mean & Forecast Envelope
AVN+NOGAPS+UKMET
Z
A
-VTL
B
C
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Phase space limitations
• Cyclone phase diagrams are dependent on the quality of the analyses upon which they are based.
• Three dimensions (B, -VTL, -VT
U) 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.
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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)
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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?
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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.
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Separate the 5 tropical cyclones from the 5 extratropical.
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