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TC Lifecycle and Intensity Changes Part II: Intensification

TC Lifecycle and Intensity Changes Part II: Intensification. Hurricane Katrina (2005) August 24-29. Outline. Tropical Cyclone Intensification Large-Scale Factors Symmetric Route Asymmetric Route Maximum Potential Intensity (MPI) Eyewall Replacement Cycles

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TC Lifecycle and Intensity Changes Part II: Intensification

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

  2. Outline • Tropical Cyclone Intensification • Large-Scale Factors • Symmetric Route • Asymmetric Route • Maximum Potential Intensity (MPI) • Eyewall Replacement Cycles • Role of Trough Interactions • Role of Upper Ocean Features • Rapid Intensification M. D. Eastin

  3. TC Intensification • Intensity change can be a slow and steady process or it can occur rapidly over the course of several hours • Forcing exists on multiple scales • Seasonal (SST, relative humidity) • Synoptic (wind shear) • Mesoscale (convective features, MCV, eyewall cycles) • Microscales (air-sea interface, water phase changes) • Complex interactions exist between the scales • Very difficult forecast problem!!! M. D. Eastin

  4. TC Intensification: Large Scale Factors • Conditions favorable for intensification: • Favorable wind shear pattern • Moist mid-troposphere • Warm ocean with deep mixed layer • Enhanced outflow • Persistent deep convection near the cyclone center • Conditions favorable for weakening would be the opposite M. D. Eastin

  5. Symmetric Route to Intensification • Local Heat and Momentum Sources: • In 1982, Lloyd Shapiro and Hugh Willoughby examined • the response of “balanced” (slowly evolving), symmetric • hurricanes to local sources of heat and momentum • Idealized study (built upon many before) • Symmetric vortex is in thermal wind balance • The eyewall is a uniform ring of convection • Local heat sources (mimic latent heat release in • convection) • Local momentum sources (mimic vertical advection • of momentum to upper levels by convection) • In hurricane-like vortices, the local sources induce secondary circulations that can slowly intensify the vortex ...How? Hugh Willoughby No Picture Available Lloyd Shapiro M. D. Eastin

  6. Symmetric Route to Intensification • Local Heat Sources: • Heating produces a local temperature • anomaly (like a buoyant updraft) which • disturbs the local pressure surfaces • This effect on the local pressure surfaces • induces an local secondary circulation • In hurricanes, the inner circulation is more • confined with radius than the outer Streamfunction response to a local heat source (mathematical solution) Streamfunction response to a local heat source in the mid-level eyewall (numerical simulation) H Adiabatic Warming Adiabatic Warming L Note the difference between the two circulations M. D. Eastin

  7. Symmetric Route to Intensification • Local Heat Sources: • The sinking branches adiabatically • warm the air (further pressure • decreases) • The radial confinement of the inner • circulation limits the warming to a • smaller area than that associated • with the outer circulation Change in pressure and tangential wind by local heat source in the mid-level eyewall (numerical simulation) Lowers pressure in the eye Streamfunction response Increases winds in the eyewall Radius of the local heat source is denoted M. D. Eastin

  8. Symmetric Route to Intensification • Local Momentum Sources: • Increased tangential momentum results • in a “super-gradient” state and an outward • acceleration up the pressure gradient • This acceleration produces an local • secondary circulation to conserve mass Streamfunction response to a local momentum source (mathematical solution) H Streamfunction response to a local momentum source in the upper-level eyewall (numerical simulation) Gradient Balance PGF Centrifugal Force L H PGF Super Gradient State L Centrifugal Force M. D. Eastin

  9. Symmetric Route to Intensification • Local Momentum Sources: • The lower circulation’s inflow • conserves angular momentum • (increases the tangential wind) • The upper circulation’s descent • results in adiabatic warming • confined in the eye • (lowers pressure) Change in pressure and tangential wind by local momentum source in the upper-level eyewall (numerical simulation) Lowers pressure in the eye Streamfunction response Increases winds in the eyewall Radius of local momentum source is denoted M. D. Eastin

  10. Asymmetric Route to Intensification • Convective Bursts: • In 1960, Joanne Malkus (Simpson) and Herbert Riehl • first suggested that hurricane evolution was linked • to a few, asymmetric, intense cumulonimbus clouds, • which they called “hot towers”, that carried a large • fraction of the high-θe inflow aloft in undiluted updrafts • Observational study • Eyewall convection was often asymmetric with • many localized updraft cores • Convection was often episodic with “bursts” • These “convective bursts” increase the latent heating aloft and the asymmetric secondary circulations that can intensify the vortex...How? Joanne Simpson Herbert Riehl M. D. Eastin

  11. Asymmetric Route to Intensification Convective Burst in Hurricane Bonnie (1998) on 23 August M. D. Eastin

  12. Asymmetric Route to Intensification • Convective Bursts: • Overshooting and diverging convection at • upper levels drives asymmetric mesoscale • descent (adiabatic warming) in the eye, • which lowers the pressure, increasing the • pressure gradient and tangential winds • A recent survey of convective bursts: • 80% of TCs have at least one “burst” • 70% of TCs intensify after a “burst” Conceptual Model of Convective Burst M. D. Eastin

  13. Maximum Potential Intensity • Maximum Potential Intensity (MPI) • Theoretical maximum intensity a TC could • achieve if environmental conditions were • infinitely perfect • Emanuel (1988) • MPI is primarily a function of SST and • the mean outflow temperature at the • top of the eyewall • No eye subsidence • Holland (1998) • MPI is primarily a function of • environmental CAPE • Incorporates eye subsidence for • strong hurricanes MPI computed for Typical Conditions Note: Observed values should be higher since the dynamical environment will limit TC intensities M. D. Eastin

  14. Eyewall Replacement Cycles • Eyewall Replacement Cycles: • Outer eyewall develops and begins to contract • Inner eyewall begins to dissipate • Maximum winds decrease • Minimum central pressure increases • Outer eyewall continues to contract • Maximum winds increase • Minimum central pressure decreases Hurricane Gilbert (1988) Radar at 2300 UTC 13 September Tangential Winds 11-16 September M. D. Eastin

  15. Eyewall Replacement Cycles • Eyewall Replacement Cycles: Statistics • More common in intense tropical cyclones • Process typically takes 36 hours • Survey of multiple eyewall structures in • TCs with maximum winds > 120 knots • (Category 345) during 1997-2002 • 40% of Atlantic hurricanes • 60% of East Pacific hurricanes • 70% of West Pacific typhoons • Significant factor in TC intensity changes • Results in an outward expansion of the • wind field (i.e., TC grows in size) and an • “annular” (or symmetric) wind field • An eyewall replacement cycle contributed • the weakening of Katrina (2005) just prior • to landfall near New Orleans M. D. Eastin

  16. Eyewall Replacement Cycles Eyewall Replacement Cycles: Hurricane Ivan (2004) Note the overall expansion of the wind field after 6 EWRCs Inner eyewall Secondary eyewall Third eyewall From Sitkowski et al. (2011) M. D. Eastin

  17. Role of Trough Interactions • Basic Idea: • Upper tropospheric troughs can promote • intensification by enhancing the • upper-level divergence and outflow • Troughs can also promote weakening by • enhancing the vertical shear • experienced by the TC • What are the differences between “good” • and “bad” troughs (for intensification)? • Hanley et al. (2001): • Examined 146 TCs which interacted with upper-level troughs • 68% of the TCs intensified • Composited the large-scale flow with respect to each TC center Vorticity Cross-Section Upper-level Trough Hurricane Dennis (1999) M. D. Eastin

  18. Role of Trough Interactions • Favorable Trough Interactions: • Trough potential vorticity (PV) • maximum comes within 400 km • of TC center, but rarely closer • Troughs are generally small in size • Outflow is enhanced • Mean vertical wind shear between • 850 and 200 mb is less than 8 m/s Composite 200 mb Flow and Potential Vorticity Note: Asterick denotes TC center M. D. Eastin

  19. Role of Trough Interactions • Unfavorable Trough Interactions: • Trough potential vorticity (PV) • maximum comes within 100 km • of TC center • Troughs are generally larger in size • Mean vertical wind shear between • 850 and 200 mb is greater than 10 m/s Composite 200 mb Flow and Potential Vorticity Note: Asterick denotes TC center M. D. Eastin

  20. Role of Upper Ocean Features • Deep Warm Currents and Eddies: • A shallow oceanic mixed layer can easily • be eroded by TC induced upwelling of • cold water, resulting in cold SSTs and • and the potential weakening of the TC • A deep oceanic mixed layer will experience • less upwelling of cold water, resulting in • higher SSTs, and a better chance for • intensification • Deep warm water matters, not just SST SST on 8-25-05 Depth of 26ºC on 8-25-05 M. D. Eastin

  21. Role of Upper Ocean Features Common Deep Warm Currents and Eddies: Gulf Stream Warm Core Eddies (Rings) Loop Current Trajectories of NOAA buoys from 1978-2003 M. D. Eastin

  22. Rapid Intensification (RI) • Definition and Statistics: • Increase in maximum wind speed of 15.4 m/s (30 knots) over a 24 hour period • A survey of Atlantic basin TCs (1989-2000) • All category 4 and 5 hurricanes underwent a period of RI during their life • ~60% of all hurricanes undergo a period of RI • ~30% of all tropical storms undergo RI • When is Rapid Intensification more likely? • Storm is far from it’s MPI (weak system) • Storm over high SST and deep warm oceanic mixed layer • Higher than normal mid-tropospheric humidity • Low vertical wind shear M. D. Eastin

  23. Rapid Intensification (RI) • Hurricane Opal (1995) • Weak hurricane stalled in southern Gulf of Mexico • Moved rapidly NE during the night of 4 October • Rapidly intensified from 965 to 916 mb in 14 hours • Coastal residents not warned appropriately (unexpected intensification) M. D. Eastin

  24. Rapid Intensification (RI) Hurricane Opal (1995) M. D. Eastin

  25. Rapid Intensification (RI) • Forecasting: 37-GHz Imagery • Kieper and Jiang (2012) evaluated • precipitation patterns prior to and • during RI for 84 Atlantic TCs • Rapid intensification often occurred • 6-12 hrs after the first appearance • of a “ring pattern” in the 37-GHz • passive microwave (SSMI) imagery • (75% of all RI cases in 2003-2007) Ring of shallow precipitation around a small “eye” M. D. Eastin

  26. TC Lifecycle and Intensity Changes Part II: Intensification • Summary • Large-Scale Factors • Symmetric Intensification (assumptions, physical processes, cases) • Intensification via Hot Towers (assumptions, physical processes) • MPI (basic idea) • Eyewall Replacement Cycles (process, impacts) • Upper-level Trough Interactions (favorable/unfavorable, impacts) • Upper Ocean Features (examples, physical processes, impacts) • Rapid Intensification (definition, favorable situations, forecasting) M. D. Eastin

  27. References Bosart, L. A., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. Black, 2000: Environmental influences on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico. Mon. Wea. Rev., 128, 322-352 Emanuel, K. A., 1988: The maximum intensity of hurricanes. J. Atmos. Sci., 45, 1143-1155. Hanley, D. E., J. Molinari, and D. Keyser, 2001: A composite study of of the interactions between tropical cyclones and upper-tropospheric troughs. Mon. Wea. Rev., 129, 2570-2584. Heymsfield, G. M., J. B. Halverson, J. Simpson, L. Tian, and T. P. Bui, 2001: ER-2 Doppler radar investigations of the eyewall of Hurricane Bonnie during the Convection and Moisture Experiment-3. J. Appl. Met., 40, 1310-1330. Holland, G. J., 1997: The maximum potential intensity of tropical cyclones. J. Atmos. Sci., 54, 2519-2541. Kaplan, J., and M. DeMaria, 2003: Large-scale characteristics of rapidly intensifying tropical cyclones in the north Atlantic basin. Wea. Forecasting, 18, 1093-1108. Kieper, M., and H. Jiang, 2012: Predicting tropical cyclone rapid intensification using the 37-GHz ring pattern identified from passive microwave measurements, Geophysical Research Letters, 39, L13804. Kossin, J. P., and M. D. Eastin, 2001: Two distinct regimes in the kinematic and thermodynamic structure of the hurricane eye and eyewall. J. Atmos. Sci., 58, 1079-1090. Kossin, J. P., and M. Sitkowski, 2012: Predicting hurricane intensity and structure changes associated with eyewall replacement cycles, Wea. Forecasting, 27, 484-488. Knaff, J. A., M. DeMaria, and J. P. Kossin, 2003: Annular hurricanes. Wea. Forecasting, 18, 204–223. Malkus, J., and H. Riehl, 1960: On the dynamics and energy transformations in steady-state hurricanes. Tellus, 12, 1–20. Moeller, D. J., and M. T. Montgomery, 1999: Vortex Rossby Waves and hurricane intensification in a barotropic model. J. Atmos. Sci., 56, 1674-1687. M. D. Eastin

  28. References Montgomery, M. T., and R. J. Kallenbach, 1997: A theory for vortex Rossby waves and its application to spiral bands and intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc., 123, 435–465. Shapiro, L. J., and H. E. Willoughby, 1982: The response of balanced hurricanes to local sources of heat and momentum. J. Atmos. Sci., 39, 378–394. Sitkowski, M. J. P. Kossin, and C. M. Rozoff, 2011: Intensity nad structure changes during eyewall replacement cycles. Mon. Wea. Rev., 139, 3829-3847. Sitkowski, M. J. P. Kossin, and C. M. Rozoff, 2011: Intensity nad structure changes during eyewall replacement cycles. Mon. Wea. Rev., 139, 3829-3847. Willoughby, H. E., and M. L. Black, 1992: The concentric eyewall cycle of Hurricane Gilbert. Mon. Wea. Rev., 120, 947-957 M. D. Eastin

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