A Cyclone Phase Space Derived from Thermal Wind & Thermal Asymmetry

1. A Cyclone Phase Space Derived from Thermal Wind & Thermal Asymmetry

2. Robert HartDepartment of MeteorologyPenn State Universityhart@ems.psu.eduhttp://eyewall.met.psu.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. 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) Research has shown that the distribution of cyclones is not limited to these two discrete groups.

6. Images courtesy NCDC Example: Separate the 5 tropical cyclones from the 5 extratropical.

7. ? 940hPa 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. GoalA 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. 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)

13. Cyclone Parameter B: Thermal Asymmetry • Defined using storm-relative 900-600hPa mean thickness field (shaded) asymmetry within 500km radius: 3160m B=100m in this example 3260m L Cold Warm B >> 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 >> 0 B > 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 d=distance between height extrema, f=coriolis, g=gravity ZMIN Vertical profile of ZMAX-ZMIN is proportional to thermal wind (VT) if d is constant: 900-600hPa: -VTL 600-300hPa: -VTU -VT < 0 = Cold-core, -VT > 0 = Warm-core

16. Cyclone Parameter -VT: Thermal Wind Warm-core example: Hurricane Floyd 14 Sep 1999 Two layers of interest: -VTU >> 0 -VTL >> 0 Tropospheric warm core

17. 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

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. Results:Conventional cyclone “trajectories” through the phase space  Tropical Cyclone: Mitch (1998)  Extratropical cyclone: December 1987 (Schultz & Mass 1993)

20. B -VTL Symmetric warm-core evolution:Hurricane Mitch (1998) B Vs. -VTL SYMMETRIC WARM-CORE

21. -VTU -VTL Symmetric warm-core evolution:Hurricane 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. B -VTL 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. -VTU -VTL Asymmetric cold-core evolution:Extratropical cyclone -VTL Vs. -VTU

24. Results:Non-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. Extratropical transition ends when –VTL < 0 B Extratropical transition begins when B=10m -VTL 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. -VTU -VTL 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. -VTU Tropical transition completes when –VTU > 0 (tropical status) Tropical transition begins when –VTL > 0 (subtropical status) -VTL 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°-70°N, 120°-0°W • Some tracking errors for fast-moving cyclones • Insufficient resolution for TCs  poor climatology

30. Few TCs! 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. Z B C A -VTL Cyclone Phase Forecasting: EnsemblingConsensus Mean & Forecast EnvelopeAVN+NOGAPS+UKMET

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. 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.

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