The Pennsylvania State University The Graduate School Department of Meteorology and Atmospheric Science ANALYZING TROPICAL CYCLONE STRUCTURES DURING SECONDARY EYEWALL FORMATION USING AIRCRAFT IN-SITU OBSERVATIONS A Thesis in Meteorology by Katharine Wunsch 2018 Katharine Wunsch Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2018 The thesis of Katharine Wunsch was reviewed and approved* by the following: Anthony C. Didlake, Jr. Assistant Professor of Meteorology Thesis Advisor Fuqing Zhang Professor of Meteorology Director, Center for Advanced Data Assimilation and Predictability Techniques George Young Professor of Meteorology Hans Verlinde Professor of Meteorology Associate Head, Graduate Program in Meteorology *Signatures are on file in the Graduate School iii ABSTRACT In the evolution of mature tropical cyclones (TCs), intensity and structural changes can occur due to a process called an eyewall replacement cycle (ERC). Secondary eyewall formation (SEF) is the initial phase of an ERC, in which a ring of convection forms outside of the pre- existing primary eyewall of the TC. The dynamical mechanisms for SEF remain unclear, but most hypotheses rely on the early presence of persistent and widespread rainband convection outside of the primary eyewall. The evolving rainband convection has both axisymmetric and asymmetric structures that play a role in SEF processes. This project uses aircraft reconnaissance observations from the FLIGHT+ dataset to examine the evolution of these structures. We create composites from this dataset which includes USAF C-130 and NOAA P-3 aircraft observations of Atlantic basin TCs from 1999-2015. The axisymmetric structures of TCs undergoing SEF are first compared to intensifying TCs that did not experience an ERC. Tangential wind and angular momentum profiles show a broadening of the outer wind field prior to SEF, while thermodynamic observations indicate features consistent with strengthening eyewall convection. Next, the ERC TCs are analyzed in quadrants relative to the deep-layer environmental wind shear to examine the evolution of asymmetric kinematic and thermodynamic structures. We utilize a new normalization technique based on the radii of both eyewalls to isolate structures that surround the secondary eyewall before and during SEF. We found that the kinematic structures of the developing secondary eyewall were most prominent in the storm half left of the wind shear vector. The thermodynamic structures of the secondary eyewall became more axisymmetric over time during SEF, but those of the primary eyewall became more asymmetric as it began to weaken prior to being fully replaced. Analyzing observations from Hurricane Earl as a case study illustrates variations in convective coverage that are captured in the composite study. Understanding the structures observed by aircraft reconnaissance and their relation to iv mechanisms that lead to SEF will improve our ability to predict the resultant changes in TC intensity and structure. v TABLE OF CONTENTS List of Figures ........................................................................................................................ v List of Tables ......................................................................................................................... vi Acknowledgements ................................................................................................................ vii Chapter 1 Introduction ........................................................................................................... 1 Chapter 2 Data and Methodology .......................................................................................... 8 Chapter 3 Axisymmetric composite structures of ERC and non-ERC TCs ............................ 19 Axisymmetric composite-mean kinematic structures ............................................. 19 Axisymmetric composite-mean thermodynamic structures .................................... 22 Chapter 4 Asymmetric composite structures of TCs undergoing secondary eyewall formation ........................................................................................................................ 26 Quadrant-averaged kinematic structures ................................................................. 26 Quadrant-averaged thermodynamic structures........................................................ 31 Chapter 5 Case Study: Hurricane Earl (2010) ........................................................................ 37 Kinematic profiles .................................................................................................. 37 Thermodynamic profiles ......................................................................................... 41 Chapter 6 Conclusions ........................................................................................................... 44 References .............................................................................................................. 48 vi LIST OF FIGURES Figure 1. Schematic illustrating the typical organization of TC convection during an eyewall replacement cycle. From Houze (2010). ........................................................... 2 Figure 2. Normalization methdology applied to flights from Hurricane Earl.. ....................... 11 Figure 3. Box and whisker plot of eyewall and moat radii for major hurricanes and anchor sets for each ERC event.................................................................................................. 17 Figure 4. Radial distribution of data coverage for axisymmetric mean profiles and individual flight legs....................................................................................................... 17 Figure 5. Axisymmetric composite-mean radial profiles and statistical significance test results for tangential wind (m s-1), relative vorticity (s-1), and nondimensional angular momentum in major hurricanes. ........................................................................ 20 Figure 6. As in Figure 5, for temperature (°C), specific humidity (kg kg-1), relative humidity (%), and equivalent potential temperature (K). ............................................... 25 Figure 7. Quadrant mean profiles and statitical significance test results for tangential wind for all shear environments. .................................................................................... 27 Figure 8. As in Figure 7, but for shear magnitude greater than 5 m s-1 only. .......................... 29 Figure 9. Mean quadrant profiles and statistical significance test results for relative vorticity (s-1) and nondimensional absolute angular momentum in moderate and high shear environments.. ....................................................................................................... 31 Figure 10. As in Figure 9, but for temperature and specific humidity. ................................... 33 Figure 11. As in Figure 9, but for relative humidity and equivalent potential temperature .... 36 Figure 12. Quadrant distribution of tangential wind (m s-1) for selected flights from Hurricane Earl (2010). .................................................................................................... 39 Figure 13. As in Figure 12, but for nondimensional absolute angular momentum. ................ 41 Figure 14. As in Figure 12, but for temperature (°C). ............................................................ 42 Figure 15. As in Figure 11, but for relative humidity (%). ..................................................... 43 vii LIST OF TABLES Table 1. Parameters and values used in the algorithm for tangential wind speed maximum identification .................................................................................................................. 10 Table 2. Eyewall replacement cycle events in Atlantic TCs from 1999-2015 ........................ 11 Table 3. Intensifying, non-eyewall replacement cycle Atlantic TCs from 1999-2012 ........... 14 viii ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Anthony Didlake Jr., for giving me the opportunity to participate in this research and for providing guidance and assistance along the way. In addition, I want to thank my committee members, Dr. Fuqing Zhang, and Dr. George Young for providing valuable feedback in the construction of this thesis. I also want to thank Jonathan Vigh from NCAR, and his collaborators who developed the FLIGHT+ dataset, which was fundamental in the design of this study. Finally, I wish to thank my family and friends for all of their support and encouragement along this journey. 1 Chapter 1 Introduction The ability to forecast tropical cyclone (TC) intensity accurately is valuable for coastal areas susceptible to landfalling storms. However, intensity forecasts – particularly in short-term forecasts (12-48h) – have not improved significantly over the past ten years (Cangialosi and Franklin 2017). The difficulty with intensity forecasts is that a number a factors play a role in modulating the intensity. Certain large-scale environmental conditions are necessary for intensifying a storm, such as warm sea surface temperatures, high mid-level relative humidity, and low environmental wind shear (Gray 1968). TC intensity changes are also tied to the evolution of the “inner core” features – including the eye, eyewall, and inner rainbands. The inner core region of a storm is largely dynamically isolated from the environment because it is dominated