Midlevel Ventilation’s Constraint on Tropical Cyclone Intensity by Brian Hong-An Tang B.S. Atmospheric Science, University of California Los Angeles, 2004 B.S. Applied Mathematics, University of California Los Angeles, 2004 Submitted to the Department of Earth, Atmospheric and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Atmospheric Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2010 c Massachusetts Institute of Technology 2010. All rights reserved. Author.............................................. ................ Department of Earth, Atmospheric and Planetary Sciences June 14, 2010 Certified by.......................................... ................ Kerry A. Emanuel Breene M. Kerr Professor of Atmospheric Science Director, Program in Atmospheres, Oceans and Climate Thesis Supervisor Accepted by.......................................... ............... Maria Zuber Earle Griswold Professor of Geophysics and Planetary Science Head, Department of Earth, Atmospheric and Planetary Sciences 2 Midlevel Ventilation’s Constraint on Tropical Cyclone Intensity by Brian Hong-An Tang Submitted to the Department of Earth, Atmospheric and Planetary Sciences on June 14, 2010, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Atmospheric Science Abstract Midlevel ventilation, or the flux of low-entropy air into the inner core of a tropical cyclone (TC), is a hypothesized mechanism by which environmental vertical wind shear can constrain a TC’s intensity. An idealized framework is developed to assess how ventilation affects TC intensity via two pathways: downdrafts outside the eyewall and eddy fluxes directly into the eyewall. Three key aspects are found: ventilation has a detrimental effect on TC intensity by decreasing the maximum steady state intensity, imposing a minimum intensity below which a TC will unconditionally decay, and providing an upper ventilation bound beyond which no steady TC can exist. Based on the idealized framework, a ventilation index is derived that is equal to the environmental vertical wind shear times the midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on the present- day climatology of tropical cyclogenesis and the distribution of TC intensification. Additionally, changes in the ventilation index are also examined in general circulation models (GCMs) between the late 20th century and the late 22nd century. Individual GCMs indicate potential regional shifts in preferred locations of tropical cyclogenesis and changes in TC intensity statistics due to shifts in the seasonal ventilation index, but a statistically significant projection cannot be given. The GCMs do show a robust increase in the midlevel entropy deficit and potential intensity nearly everywhere in the tropics. Lastly, an axisymmetric model with parameterized ventilation is used to examine the sensitivity of TC intensity to the strength and location of the ventilation and to examine the findings of the idealized framework. Increasing the strength of the ventilation and placing the ventilation at lower to middle levels results in a greater decrease in the quasi-steady intensity, whereas upper-level ventilation has little effect on the intensity. For strong ventilation, an oscillatory intensity regime materializes and is tied to transient convective bursts and strong downdrafts into the boundary layer. The sensitivity of TC intensity to ventilation can be viewed in the context of a modified thermal wind relation or the fractional Carnot efficiency of the inner-core. 3 Thesis Supervisor: Kerry A. Emanuel Title: Breene M. Kerr Professor of Atmospheric Science Director, Program in Atmospheres, Oceans and Climate 4 Acknowledgments It is hard to believe it has been six years in the Green Building already. I know I will look back upon my graduate years and realize how special they were. This would not have been possible without the great support and friendship of many individuals. First, I would like to thank my advisor, Kerry Emanuel. I knew upon coming to MIT that I would learn a great deal from him given his expertise in the field, and my expectations were greatly exceeded. He has given me guidance in both my generals research and dissertation that have been invaluable to my progress, but at the same time has let me find my own way, which will benefit me greatly after I leave MIT. Not only has he advised me well in matters of science, but his ability to communicate with such eloquence has truly inspired me to do the same. Additionally, Kerry has given me opportunities few graduates have to teach, attend conferences/workshops, and participate in a field campaign. My thesis committee has been invaluable to me. Alan Plumb is an excellent teacher, and I’ve benefited greatly from his class in atmospheric dynamics. I will never forget the time Alan and I were preparing a two layer convection experiment for the undergraduate fluids lab. We accidentally made the bottom layer so salty and dense that when we set out to figure out what temperature would be needed to cause the system to start convecting, we calculated it would have to be above boiling much to the laughter of the class. I would also like to thank John Molinari for giving me plenty of encouragement and also giving observational fodder that these nebulous equations on the following pages might actually apply to reality! Lastly, my research was greatly aided by Mike Montgomery. I would like to especially thank Mike and Michael Riemer for looking out for me during my trip to Guam, for hosting my trip out to Monterey in 2009, and for providing me data from their 3D hurricane simulations. I was able to fine tune and think much more critically about my research than any time before my visit out there. I love to teach and enjoyed TAing both graduate and undergraduate courses in the department. This would not have been possible without the support of Lodovica 5 Illari and John Marshall. The undergraduate lab course (12.310) sets a new paradigm for teaching atmospheric science and oceanography to students, and it was a pleasure to be part of this innovative and unique experience. I do apologize again for the water spills, cords wrapping around rotating tables, and speeding up the rotation rate of the Hadley Cell demo, but at least I avoided setting the place on fire. My colleagues and the administrative staff in the program have also been part of why my graduate years have been so memorable. Starting from when I first arrived MIT, I would like to thank Robert Korty and Bill Boos for their advice during my first couple of years. I knew I could always rely on Jon Moskaitis, Cegeon Chan, and Garrett Marino to start up a conversation about the weather, as we are all professed weather junkies. We suffered through many humbling days in the National Collegiate Weather Forecast Competition and WxChallenge, but victory was sweet when it came. Thanks to Kelly Klima and Tim Whitcomb for being great friends, as we toughed it out through classes together. I lost count how many times Dan Gombos and I ate at the student center and Quiznos over the years talking about anything from poker to ensembles. Angela Zalucha has been my ski buddy and brunch buddy, particularly because she is always at the front of the line. Lastly, Mary Elliff always put the students first, and she made a huge difference. Finally, I would like to thank my family for being there for me through the years. I’ve always had the unconditional love of my family, and my success is largely due to this. My parents have asked nothing of me except to do the best I can at whatever I decide to do. For this, I dedicate this large stack of paper (a.k.a. thesis) to them. This work is supported by NSF grant ATM-0850639. P.S. Go crEAPS! 6 Contents 1 Introduction 21 1.1 Motivation................................. 21 1.1.1 IntensityPrediction. 21 1.1.2 DefiningShear .......................... 25 1.2 Background ................................ 27 1.2.1 Observations of Sheared Tropical Cyclones . .. 27 1.2.2 Numerical Modeling Studies of Sheared Tropical Cyclones .. 31 1.2.3 VortexRossbyWaves. 34 1.3 HypothesisandObjectives . 39 2 A Theoretical Framework for Ventilation Modified Intensity 41 2.1 Introduction................................ 41 2.2 Carnotenginemodification. 44 2.3 TheoreticalFramework . 46 2.3.1 Subcloudlayerentropy . 48 2.3.2 Freetroposphereentropy. 53 2.3.3 Subcloudlayerangularmomentum . 54 2.3.4 Steady-stateintensity. 55 2.4 BehaviorofaventilatedTC . 57 2.4.1 Energybudget .......................... 60 2.4.2 Midlevel pathway in Hurricane Bonnie (1998) . 61 2.4.3 Interactivethermodynamicefficiency . 63 2.5 Sensitivitytoparameters. 66 7 2.5.1 Potentialintensity . 66 2.5.2 Alphaandgamma ........................ 66 2.6 Conclusions ................................ 68 3 A Ventilation Index for Tropical Cyclones: Climatology and Projec- tions 73 3.1 Introduction................................ 73 3.1.1 GenesisandIntensity. 74 3.1.2 ChangeswithClimate . 76 3.2 VentilationIndex ............................. 78 3.3 VentilationClimatology . 80 3.3.1 Genesis .............................. 82 3.3.2 Intensity.............................. 84 3.4 VentilationinGlobalClimateModels . .. 89 3.4.1 1981-2000 ............................. 90 3.4.2 2181-2200 ............................. 95 3.4.3 Comparison with Emanuel et al. (2008) (E08) . 103 3.5 Conclusions ................................ 104 4 Ventilation in an Axisymmetric Tropical Cyclone