TC Lifecycle and Intensity Changes Part I: Genesis

1. TC Lifecycle and Intensity Changes Part I: Genesis Hurricane Katrina (2005) August 24-29 M. D. Eastin

2. Outline • Tropical Cyclone Genesis • Large-Scale Factors • Easterly Waves and MCVs • CISK Mechanism • WISHE Mechanism • VHT Mechanism M. D. Eastin

3. TC Genesis • Genesis: The transformation of a “disorganized” cold-core convective • system into a self-sustaining synoptic-scale warm-core vortex • with a cyclonic circulation at the surface • Necessary (but not sufficient) Conditions: • Pre-existing convection • Significant planetary vorticity • Favorable wind shear pattern • Moist Mid-troposphere • Warm ocean with deep mixed layer • Conditionally unstable atmosphere M. D. Eastin

4. TC Genesis • Pre-existing Convection: • Source of latent heating • Persistent heating in one area will • lower the local surface pressure • and begin to converge air toward • the low pressure • (recall the hypsometric equation) M. D. Eastin

5. TC Genesis • Significant Planetary Vorticity: • Convection near the equator results in • little if any rotation in the low-level inflow • Convection off the equator will contain rotation • in the low level inflow due to appreciable • Coriolis forcing • Systems need to be ~5º off the equator in order • to have a chance for development M. D. Eastin

6. TC Genesis • Favorable Wind Shear Pattern: • Wind shear is often defined as the vector difference • between winds at two altitudes (850 and 200 mb) • Low magnitudes of shear (< 20 knots) are desired Good – latent heat can concentrate in one area Bad – convection torn apart High westerly shear Low easterly shear M. D. Eastin

7. TC Genesis • Moist Mid-Troposphere: • Dry air will lead to evaporation and cooling • Cooling produces a surface high pressure, • low-level divergence, sinking air, and a • suppression of convection Gray/Blue Areas = Moist Strong downdrafts = Outflow Boundaries Red Areas = Dry GOES Water Vapor Image M. D. Eastin

8. TC Genesis • Warm Ocean: • Allows for sensible and latent heat fluxes from • the ocean in order to sustain deep convection • SSTs > 26.5ºC is the rule Standard Flux Equations Deep Convection The inflowing air gains heat and moisture only if the ocean is warmer and moister than the air L M. D. Eastin

9. TC Genesis • Deep Oceanic Mixed Layer: • Mixed layer: Nearly isothermal ocean layer from • the surface to a depth where temperatures cool • rapidly (the thermocline) • Strong winds churn up cool water from the • thermocline or below • Deeper mixed layers prevent the cooling of • surface waters • Cold surface waters limit (or reverse) sensible • and latent heat fluxes, reducing convection Mixed Layer M. D. Eastin

10. TC Genesis • Conditionally Unstable Atmosphere: • Lapse rate between the dry adiabatic • and moist adiabatic lapse rates • Parcels become unstable only when lifted • to their Level of Free Convection (LFC) • Further ascent produces latent heat • release and locally warm air • (lowers surface pressure) • Frictional convergence produces lift Sounding on a Skew-T M. D. Eastin

11. Easterly Waves • Origin: Develop over sub-Saharan Africa from • instabilities along the African Easterly Jet • Basics: • Wavelengths of ~3000 km • Move westward at 6-8 m/s • 60-80 easterly waves cross the Atlantic • each year between July and October • 7-9 develop into tropical cyclones • Why do we care about easterly waves? • Often emerge over warm waters with convection • Like mid-latitude synoptic waves, have preferred • regions of lift (east of the trough): helps generate • persistent convection in the same location • Often contain mid-level (but not surface) vortices • Systems “pre-conditioned” for successful genesis M. D. Eastin

12. Mesoscale Convective Vortices (MCVs) • Origin: Develop within persistent mesoscale • convection from heating aloft (convection) • and cooling below (cold downdrafts) • Basics: • Confined to mid-levels with little or no • signature at the surface • Often present in easterly waves • Dynamically stable (last several days) • Multiple convective cycles • Can emerge from the continental U.S. • and developed into tropical cyclones • (e.g. Hurricane Danny 1997) • Why do we care about MCVs? • Often emerge over warm waters with convection • Systems “pre-conditioned” for successful genesis Typical MCV Cross-Section Positive Vorticity Negative Vorticity Warm Cold M. D. Eastin

13. TC Genesis One of the greatest enigmas of tropical meteorology: How do we transform a cold-core synoptic-scale disturbance with a mid-level vortex to a warm-core system with a surface vortex? “This question has been asked at every tropical cyclone conference since the dawn of time.” (Dr. Bill Gray, 2003) M. D. Eastin

14. Genesis via the CISK Mechanism • Convective Instability of the Second Kind (CISK): • First proposed by Jule Charney in 1964 • Assumes the atmosphere is conditionally unstable • Requires the presence of a finite amplitude synoptic • scale disturbance (easterly wave) • Assumes latent heat release results from synoptic-scale • frictional convergence • Remaining question: How does the surface vortex form? Jule Charney M. D. Eastin

15. Genesis via the CISK Mechanism 1 2 Friction with surface causes inflow into the disturbance to be “deflected” inward toward the surface center. Mass continuity dictates upward motion must result. This process is called “Ekman Pumping” Upward motion causes saturation and thus latent heat release. If conditionally unstable, upward motion will continue and enhance secondary circulation. Vortex will stretch, which will develop and intensify low-level cyclonic vorticity (through conservation of angular momentum) Latent Heat Release L Charney and Eliassen (1964) showed that CISK developed a TC with a diameter of 100 km in 2.5 days (similar to observations) M. D. Eastin

16. Genesis via the WISHE Mechanism • Wind Induced Surface Heat Exchange (WISHE): • First proposed by Kerry Emanuel in 1986 • Assumes the tropical atmosphere is not conditionally • unstable, but rather near neutral to moist convection • (i.e. the thermodynamic profile is moist adiabatic) • Assumes the primary instability is the thermodynamic • difference between ocean and the boundary layer air • (i.e. sensible and latent heat fluxes are crucial) • Genesis requires the presence of a finite amplitude • disturbance (i.e. an easterly wave or MCV) • Remaining question: How does the surface vortex form? Kerry Emanuel M. D. Eastin

17. Genesis via the WISHE Mechanism a. Prior convective cycle creates a MCV. Continued stratiform rain leads to cooling and a mesoscale downdraft, which transports the mid-level vorticity and low-θe air to the surface b. New surface cyclone envokes sensible and latent heat fluxes. Frictional driven inflow begins to warm and moisten, and develop new convection. c. Downdrafts disappear, convection regularly occurs in near neutral air, warm core gradually develops, further vortex intensification near the surface M. D. Eastin

18. Genesis via the VHT Mechanism • Vortical Hot Towers (VHT): • First proposed by Mike Montgomery in 2004 • Assumes the atmosphere is conditionally unstable • Assumes the preferred route to genesis is from multiple • “merger events” between convective-scale cumulonimbus • towers that possess intense cyclonic vorticity • Genesis requires the presence of a finite amplitude • disturbance (easterly wave or MCV) for a background • vorticity source • Remaining question: How does the surface vortex form? Mike Montgomery M. D. Eastin

19. Genesis via the VHT Mechanism a. Hot towers (buoyant updrafts) develop and feed off the conditional instability. Minimal low-level vorticity. b. Upward acceleration leads to vorticity stretching and low-level convergence (via angular momentum conservation) of background vorticity Considerable low-level vorticity M. D. Eastin

20. Genesis via the VHT Mechanism Shear Vector • Observational Evidence: • Tropical Storm Gustav (2002) • Vertically sheared from the northeast • Exposed low-level circulation • Convection confined to the southwest • Episodic convective bursts (hot towers) • developed multiple low-level vortices that • rotated around to the northeast Low-level vortices M. D. Eastin

21. Genesis via the VHT Mechanism • Low-level vorticity maxima associated with two • distinct hot towers are present • Roughly 0.5 hrs later the maxima have merged • into a single stronger low-level vorticity maximum • The low-level vortex develops through multiple • merger events. z = 0.67 km M. D. Eastin

22. TC Lifecycle and Intensity Changes Part I: Genesis • Summary • Necessary Large-Scale Conditions • Pre-existing convection • Significant planetary vorticity • Favorable wind shear pattern • Moist Mid-troposphere • Warm ocean with deep mixed layer • Conditionally unstable atmosphere • Easterly Waves (origin, structure, importance) • Mesoscale Convective Vortices (origin, structure, importance) • Genesis Mechanisms • CISK (assumptions, physical processes) • WISHE (assumptions, physical processes) • VHTs (assumptions, physical processes) M. D. Eastin

23. References Charney, J. G., and A. Eliassen, 1964: On the growth of the hurricane depression. J. Atmos. Sci., 21, 68-75. Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady-state maintenance., J. Atmos. Sci., 43, 585-604. Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev.,96, 669-770. Hendricks, E. A., M. T, Montgomery, and C. A. Davis, 2004: On the role of “vortical” hot towers in formation of tropical cyclone Diana (1984), J. Atmos. Sci., 61, 1209-1231. Montgomery, M. T., M. E. Nicholls, T. A. Cram, and A. B. Saunders, 2006: A vortical hot tower route to tropical cyclogenesis. J. Atmos. Sci., 63, 355-386. M. D. Eastin