The Effects of Land Cover Type on Tornado Intensity in the Southeastern U.S. A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science Kelly M. Butler August 2017 © 2017 Kelly M. Butler. All Rights Reserved. 2 This thesis titled The Effects of Land Cover Type on Tornado Intensity in the Southeastern U. S. by KELLY M. BUTLER has been approved for the Department of Geography and the College of Arts and Sciences by Jana B. Houser Assistant Professor of Geography Robert Frank Dean, College of Arts and Sciences 3 ABSTRACT BUTLER, KELLY M., M.S., August 2017, Geography The Effects of Land Cover Type on Tornado Intensity in the Southeastern U.S. Director of Thesis: Jana B. Houser While storm-scale mechanisms are known to be associated with changes in tornado intensity, tornadoes also intensify and weaken without direct correlation to a known storm- or tornado-scale feature. One non-storm mechanism that could influence the intensity of a tornado is the land cover over which the vortex is traversing. In theory, friction disrupts the cyclostrophic balance achieved between the pressure gradient force and the centrifugal force. While conserving angular momentum, a new balance must be achieved which ultimately alters the rotational velocities. The purpose of this research is to determine if there are statistical relationships between land cover (a proxy for friction) and tornado intensity (as quantified by the difference between the maximum inbound and the maximum outbound Doppler velocities, ΔVmax). Historical storm report data and level II WSR-88D radar data are acquired from archival resources for 30 tornadic storms within the domain and land cover data are extracted from the National Land Cover Database. Tornado locations are approximated from radar observations and a radius of influence extends out from the central point of the tornado, representing area directly affecting the tornadic flow field. Tornado intensity is spatially analyzed against land cover types within the tornadic flow fields using a GIS framework, and only at locations along the tornado path where radar data are observed. Linear and non-parametric statistics are utilized in this preliminary study. Conclusions 4 reveal there are statistical relationships between land cover and tornado intensity. It is found that both the magnitude and direction of surface roughness change are associated with unique changes in tornado intensities. 5 DEDICATION This work is dedicated to my loving mother, Doreen Butler. Thank you for always pushing me to follow my dreams and for always believing in me, even on the days I did not believe in myself. “Believe it, and you will achieve it”. 6 ACKNOWLEDGMENTS Primarily, I would like to thank my academic advisor, Dr. Jana Houser. Jana, you are the reason I specifically sought out Ohio University to obtain my Master of Science degree. You have inspired me in many different capacities. Your depth of knowledge on severe storms and tornadoes is unparalleled, as is your ability to effectively communicate the science. Thank you for your patience, your advice, and your willingness to listen during the many meetings that we had; and especially for the countless hours that were spent reading drafts of this thesis. Thank you for being a great female role model personally and professionally. I appreciate all the real-world wisdom and life lessons that you have passed on to me. To my other committee members Dr. Ryan Fogt and Dr. Gaurav Sinha, thank you for your professional critiques of this thesis. I would also like to thank you both for the invaluable lessons that were taught in your classes. Chad Goergens, thank you for the motivation and the distractions. You are one of the hardest workers I know and inspired me many of times to be productive on days when I felt doing like anything but thesis work. You were always there to lighten the mood whenever the stress load was high. Even if destressing meant making fun of an imaginary chicken, Clarence. While we are going separate ways, I hope that we can keep in touch, and I wish you and your wife the best of luck on your future endeavors. I would like to say thank you to Nate McGinnis for the numerous conversations that we had discussing our theses and for pioneering some of the methods used herein. 7 Also I would like to thank Dr. Kevin Farrell for all the slight nudges I needed to apply for graduate school after taking time off from school. To Kayla Flynn, thank you for helping me with Matlab and for your generosity in helping me make my code run efficiently and working through the bugs in my script. I can’t forget to thank you for saving my thesis document when all my equations decided to turn into question marks and I thought my life was over. You always were the calm and collected one out of the two of us! Thank you for your continued encouragement and motivation, and most of all, for being the best friend a woman could ask for. Finally, I need to thank my mother, Doreen Butler. You have shown me more love and support than anyone else these last couple of years. You were always there for a quick video chat to tell me to keep smiling and were always replacing my doubt with hope. You have always taught me to work hard for things in life, so I hope you can see that I have continued to do so. 8 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Dedication ........................................................................................................................... 5 Acknowledgments............................................................................................................... 6 List of Tables .................................................................................................................... 10 List of Figures ................................................................................................................... 11 Chapter 1: Introduction and Motivation ........................................................................... 14 1.1 Introduction ..................................................................................................... 14 1.2 Motivation ....................................................................................................... 16 Chapter 2: Literature Review ............................................................................................ 19 2.1 Understanding Supercells ............................................................................... 19 2.2 Tornadogenesis ............................................................................................... 19 2.2.1 Stages of Genesis ................................................................................... 20 2.2.2 Genesis Failure....................................................................................... 24 2.3 Storm Scale Mechanisms ................................................................................ 25 2.3.1 The Rear Flank Downdraft and its Internal Momentum Surge ............. 26 2.3.2 Descending Reflectivity Cores .............................................................. 28 2.3.3 Summary ................................................................................................ 30 2.4 Friction (Surface Roughness) ......................................................................... 30 2.4.1 Numerical and Lab Simulations............................................................. 30 2.4.2 Observational Studies ............................................................................ 37 Chapter 3: Data and Methods ........................................................................................... 39 3.1 Event Acquisition............................................................................................ 39 3.3 Radar Interpretation and Analysis .................................................................. 43 3.4 GIS Analysis ................................................................................................... 46 3.5 Statistical Analyses ......................................................................................... 56 3.5.1 Parametric Analysis ............................................................................... 56 3.5.2 Non-Parametric Analysis Theory .......................................................... 59 3.5.3 Non-Parametric Analysis Applications.................................................. 62 Chapter 4: Results ............................................................................................................. 67 4.1 Parametric Results .......................................................................................... 67 9 4.2 Non-Parametric Results .................................................................................. 73 4.2.1 Results of Zw and ΔV at Lag 0 .................................................................... 73 4.2.2 Results of Zw and ΔV at Lag 1 .................................................................... 82 4.2.3 Results of dZw and dΔV at Lag 0 ................................................................ 87 4.2.4 Results of dZw and dΔV at Lag 1 ................................................................ 94 Chapter 5: Discussion and Conclusions .......................................................................... 101 5.1 Discussion ....................................................................................................