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Soil Moisture Effects on Supercellular Convective Initiation and Atmospheric Moisture

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Soil Moisture Effects on Supercellular Convective Initiation and Atmospheric Moisture Soil Moisture Effects on Supercellular Convective Initiation and Atmospheric Moisture in the Midwestern United States 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 Doug E. Schuster August 2016 © 2016 Doug E. Schuster. All Rights Reserved. 2 This thesis titled Soil Moisture Effects on Supercellular Convective Initiation and Atmospheric Moisture in the Midwest United States by DOUG E. SCHUSTER has been approved for the Department of Geography and the College of Arts and Sciences by Jana Houser Assistant Professor of Geography Robert Frank Dean, College of Arts and Sciences 3 ABSTRACT SCHUSTER, DOUG E., M.S., August 2016, Geography Soil Moisture Effects on Supercellular Convective Initiation and Atmospheric Moisture in the Midwest United States Director of Thesis: Jana Houser With the increased availability of soil moisture measurements in recent years, literature linking soil moisture to convection has begun to appear. These studies primarily link soil moisture to ordinary convection or rainfall. This study focuses specifically on the relationship between the soil moisture and the location of convective initiation (CI) of supercellular storms. The relationship is established using two separate methods: 1) Soil moisture anomalies are compared with supercell CI success and failure through use of chi-square analyses, and 2) soil moisture anomalies are linked to atmospheric moisture (specifically relative humidity and dewpoint) by use of the Student’s t-test. The former method suggests that low soil moisture is more favorable for CI, though this relationship is only significant after diurnal heating has occurred. The latter suggests a positive relationship between relative humidity and soil moisture. It is hypothesized that the lower soil and relative humidity values make supercells more likely by raising the lifted condensation level and slowing the rate at which convective inhibition is removed. 4 ACKNOWLEDGMENTS Graduate school has been one of the most difficult tasks that I have had to face in my life. There is no question that without the support of those close to me, I would not have been able to complete this difficult journey. First and foremost, I want to thank my advisor Dr. Jana Houser for her undying patience and encouragement, as well as insightful input to help me build this project from the ground up. Even in a time where you are incredibly busy, both academically and personally, you find the time to support my endeavors, and I cannot thank you enough. I also want to extend my gratitude to the committee, Dr. Ryan Fogt and Dr. Dorothy Sack, for their constructive criticism and all the work they have put into the classes that have shaped me into the scholar I am now. Additionally, to my close friends Ethan Bottone and Megan Sympson, thank you for giving me an escape from the stress of graduate school and acting as my family away from home. Finally, I want to give a special thanks to those that have been close to me over the years. To Mom and Dad, you always know how to keep my spirits high and push me to get everything out of the opportunities that are presented to me. To my sister Megan, you bring lighthearted encouragement to all of my difficult situations, making any problem look that much less intimidating. And to my loving girlfriend Kara, for my whole college career, you have kept my spirits bright even when I felt like all I could do is give up. Thank you for always being there for me! 5 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Acknowledgments ............................................................................................................... 4 List of Tables ...................................................................................................................... 7 List of Figures ..................................................................................................................... 8 Chapter 1: Introduction ..................................................................................................... 10 Chapter 2: Literature Review ............................................................................................ 13 2.1 Conditions for CI .................................................................................................... 13 2.1.1 Dewpoints and Instability ................................................................................ 15 2.1.2 Supercells ......................................................................................................... 16 2.2 Types of Forcing Mechanisms ................................................................................ 19 2.3 Soil Moisture Effects .............................................................................................. 24 Chapter 3: Data Employed ................................................................................................ 27 3.1 Soil Moisture ........................................................................................................... 28 3.1.1 ERA-Interim Soil Moisture .............................................................................. 32 3.2 Forcing Mechanisms ............................................................................................... 35 3.3 Radar ....................................................................................................................... 37 3.4 Atmospheric Moisture Parameters .......................................................................... 38 3.5 Instability ................................................................................................................ 39 Chapter 4: Methods ........................................................................................................... 41 4.1 Soil Moisture Impacts on Supercells ...................................................................... 41 4.2 Soil Moisture Impacts on Dewpoints ...................................................................... 44 4.3 Statistics .................................................................................................................. 45 4.3.1 Linear Relationship .......................................................................................... 45 4.3.2 Chi-square Test of Independence ..................................................................... 47 Chapter 5: Results ............................................................................................................. 51 5.1 Soil Moisture and Supercell CI ............................................................................... 51 5.1.1 Cold Front Cases .............................................................................................. 54 5.1.2 Warm Front Cases ............................................................................................ 56 5.1.3 Max Heating Hour Cases ................................................................................. 57 6 5.1.4 Non-Max Heating Hour Cases ......................................................................... 59 5.2 Soil vs. Atmospheric Moisture ............................................................................... 64 5.2.1 Soil Moisture vs Dewpoint .............................................................................. 66 5.2.2 Soil Moisture vs RH ......................................................................................... 68 Chapter 6: Discussion ....................................................................................................... 79 6.1 Limitations .............................................................................................................. 87 Chapter 7: Conclusions ..................................................................................................... 91 References ......................................................................................................................... 95 7 LIST OF TABLES Page Table 4.1: An example of a contingency table for this study, using soil moisture and CI outcome ..............................................................................................................49 Table 5.1: The chi-square analysis for all observed cases of supercell CI success and failure, set in the same format as described in Table 4.1. Soil moisture is divided into positive (wet)and negative (dry) categories based on the sign of the anomaly. Regular-type numbers are the observed frequency, while the italic-type numbers are the expected distribution based on the totals for each column. Chi-square value and p value are given at the bottom. ..........................................................52 Table 5.2: As in Table 5.1, but only for the outer quartile of all cases. ..........................53 Table 5.3: As in Table 5.1, except for cold front cases only. .........................................55 Table 5.4: As in Table 5.1, except for warm front cases only. .......................................57 Table 5.5: As in Table 5.1, except for MHH cases only. ................................................59 Table 5.6: As in Table 5.1, except for NMHH cases only. .............................................61 Table 5.7: As in Table 5.1, except for PMHH cases only. .............................................63 Table 5.8: Summary of the results for comparisons
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