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On the Roles of Radiation and Low-Level Vorticity in Numerical Simulations of Tropical Cyclogenesis

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On the Roles of Radiation and Low-Level Vorticity in Numerical Simulations of Tropical Cyclogenesis On The Roles of Radiation and Low-Level Vorticity in Numerical Simulations of Tropical Cyclogenesis by Warren (Ren) Paul Smith II M.S., University of Colorado Boulder, 2017 B.S., Minnesota State University, Mankato, 2013 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the completion of Doctor of Philosophy Department of Atmospheric and Oceanic Sciences 2019 This thesis entitled: On The Roles of Radiation and Low-Level Vorticity in Numerical Simulations of Tropical Cyclogenesis written by Warren (Ren) Paul Smith II has been approved for the Department of Atmospheric and Oceanic Sciences Dr. Melville Nicholls Prof. Katja Friedrich Date The final copy of this manuscript has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Warren (Ren) Paul Smith II, (Ph.D., Atmospheric and Oceanic Sciences) On The Roles of Radiation and Low-Level Vorticity in Numerical Simulations of Tropical Cyclogenesis Manuscript directed by Dr. Melville Nicholls Before a hurricane forms, a process known as tropical cyclogenesis must occur during which a tropical disturbance transforms into a tropical depression. In order to further un- derstand how and when hurricanes will form, we must start by examining the processes that lead to tropical cyclogenesis. This dissertation employs numerical modeling techniques to highlight the roles that both low-level vorticity and longwave and shortwave radiation play in this transformation. Recent numerical modeling and observational studies indicate the importance of vor- tical hot towers (VHTs) in tropical cyclogenesis. It has recently been recognized that convective-scale downdraft outflows that form within VHTs also preferentially develop pos- itive vertical vorticity around their edges which is considerably larger in magnitude than ambient values. During a numerical simulation of tropical cyclogenesis it is found that par- ticularly strong small-scale low-level convectively-induced positive vertical vorticity anoma- lies (herein termed \LCVAs") occasionally form as convection acts on the enhanced vorticity at the edges of cold pools. The LCVAs studied are considerably deeper than the vorticity produced at the edges of VHT cold pool outflows, and their evolution is associated with persistent convection and vortex merger events which act to sustain them. This dissertation highlights the formation and evolution of two representative LCVAs and discusses the envi- ronmental parameters which eventually become favorable for one LCVA to reach the center of a larger-scale circulation as tropical cyclogenesis occurs. Recent literature also indicates the importance of longwave and shortwave radiation in TCG. Herein we examine the sensitivity of TCG to radiation in both idealized simulations iv for different initial vortex strengths, and in a case study framework of Atlantic Hurricane Matthew (2016) to highlight when during tropical cyclogenesis radiation is most important. It is shown that all else equal, a stronger initial vortex reduces the impact that radiation has on accelerating tropical cyclogenesis. In both modeling frameworks we find that radiation's primary role is to moisten the core of a disturbance, and after sufficient moistening occurs over a deep layer and the winds are sufficiently strong at the surface, radiation no longer plays as significant a role in tropical cyclogenesis. Dedication This dissertation is dedicated to my family, friends, colleagues, and everybody in between for providing an endless stream of support along this incredible journey of more than 20 years of education. This milestone could never have been reached without you. Thank you. \It's a dangerous business, Frodo, going out your door. You step onto the road, and if you don't keep your feet, there's no knowing where you might be swept off to." - J.R.R. Tolkien, The Lord of the Rings vi Acknowledgements I would like to give a big thank you to my Ph.D. research advisor Dr. Melville Nicholls of CIRES/ATOC for all of his guidance and support through this long journey. I would also like to thank the rest of my Ph.D. committee members for their willingness to serve on my committee and their helpful questions and comments every step of the way: Drs. John Cassano of CIRES/ATOC, Katja Friedrich of ATOC, Roger Pielke Sr. of CIRES/ATOC, and Gregory Thompson of NCAR. I would also like to thank ATOC Program Assistant Laurie Conway for all of her assistance with department requirements through the different phases of graduate school. Finally, I would like to thank my colleague Robinson Wallace for his help with developing the IDL visualization code used to produce nearly every figure herein and my colleague Dr. Matthew Tooth for proofreading this document. This work was supported by two NSF Grants: NSF AGS 1445875 and NSF AGS 1415244. I would also like to acknowledge high-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation. vii Contents Chapter 1 Introduction 1 1.1 Motivation and Background . 1 1.2 Two Pathways to Tropical Cyclogenesis Emerge . 2 1.3 The Role of Radiation . 4 1.4 Project Summary and Overview . 6 2 On the Creation and Evolution of Small-Scale Low-Level Vorticity Anomalies During Tropical Cyclogenesis 9 2.1 Introduction . 9 2.2 Numerical Model and Methods . 12 2.3 Analysis of LCVAs . 15 2.3.1 The Early LCVA . 15 2.3.2 The Genesis LCVA . 20 2.4 Shear and Curvature Vorticity . 24 2.5 Vortex Merger Analysis . 30 2.6 Discussion . 33 2.7 Conclusions . 36 3 The Role of Radiation In Accelerating Tropical Cyclogenesis in Idealized Simulations 39 3.1 Introduction . 39 viii 3.2 Methods . 43 3.2.1 Numerical Model and Experimental Design . 43 3.2.2 Computations . 46 3.3 Results . 46 3.3.1 Sensitivity to Initial Vortex Strength . 46 3.3.2 Sensitivity to Radiation . 50 3.3.3 Sensitivity to Moisture . 54 3.4 Conclusions . 58 4 The Role of Radiation in the Genesis of Atlantic Hurricane Matthew (2016) 62 4.1 Introduction . 62 4.1.1 Synoptic Overview of Hurricane Matthew . 63 4.2 Methods . 64 4.2.1 Numerical Model . 64 4.2.2 Determining Track and Intensity . 66 4.3 Results . 67 4.4 Conclusions . 69 5 Relevant Contributions to Coauthor Publications 71 5.1 Idealized VHT Simulation . 71 5.2 Idealized LCVA Simulation . 77 5.3 Sensitivity to Radiation in RAMS . 80 6 Conclusions 85 6.1 Objectives . 85 6.2 Key Findings . 86 6.3 Avenues for Future Work . 88 ix Bibliography 89 Appendix A LCVA Hydrostatic and Cyclostrophic Balance (Chapter 2) 98 B Surface-Based CAPE (Chapter 2) 101 C Estimation of Friction for the Curvature Vorticity Budget (Chapter 2) 103 D Modification of Initial Vortex Winds in Idealized WRF Simulations (Chapter 3) 104 E Changes to Initial Sounding for MOIST runs (Chapter 3) 106 F Ensemble Simulations (Chapter 3) 107 G Acronyms 109 x Tables Table 3.1 Physics schemes and initial vortex strengths of the six core idealized WRF simulations . 45 3.2 List of times it took to attain tropical depression, tropical storm, and hurricane status for the idealized WRF simulations . 47 E.1 Water vapor mixing ratio profiles for the three soundings used in the idealized WRF simulations . 106 F.1 A list of the time it takes (in hours) for each idealized WRF ensemble member to undergo TCG. 108 xi Figures Figure 2.1 Tracks and intensity time series of the early and genesis LCVAs . 16 2.2 Near-surface horizontal cross sections from the full LCVA simulation at 43 hours and 40 minutes. Analysis of vorticity generation around a cold pool . 17 2.3 Horizontal and vertical cross sections from the full LCVA simulation at 46 hours. Structure of the early LCVA . 18 2.4 Horizontal and vertical cross sections from the full LCVA simulation at 49 hours and 40 minutes. Structure of the early LCVA . 19 2.5 Near-surface horizontal cross sections from the full LCVA simulation at 82 hours and 40 minutes, 83 hours and 15 minutes and 83 hours and 50 minutes. Birth of the genesis LCVA from cold pool outflows . 21 2.6 Vertical cross sections from the full LCVA simulation at 82 hours and 40 minutes, 83 hours and 20 minutes, 86 hours, and 88 hours and 40 minutes. Evolution of convection near the genesis LCVA . 22 2.7 Vertical cross sections from the full LCVA simulation at 86 hours and 20 minutes. Local shear near the genesis LCVA . 23 2.8 Horizontal cross sections from the full LCVA simulation at 85 hours and 40 minutes and 86 hours. Pressure depression near the genesis LCVA . 25 2.9 Horizontal and vertical cross sections from the full LCVA simulation at 90 hours. Structure of the genesis LCVA as TCG occurs . 26 xii 2.10 Near-surface horizontal cross sections from the full LCVA simulation at 83 hours and 22 minutes. Analysis of shear and curvature vorticity near the genesis LCVA . 28 2.11 Near-surface horizontal cross sections from the full LCVA simulation at 83 hours and 34 minutes. Analysis of shear and curvature vorticity near the genesis LCVA . 29 2.12 Azimuthally averaged cross sections from the full LCVA simulation at 48 hours. Tangential momentum budget performed on the early LCVA . 33 2.13 Azimuthally averaged cross sections from the full LCVA simulation at 86 hours and 42 minutes. Tangential momentum budget performed on the genesis LCVA 34 3.1 Time series of maximum wind speeds from the idealized WRF simulations . 48 3.2 Azimuthally averaged tangential wind fields from the idealized WRF simula- tions at 42 hours .
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