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Environmental Control of Cloud-To-Ground Lightning

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Environmental Control of Cloud-To-Ground Lightning ENVIRONMENTAL CONTROL OF CLOUD-TO-GROUND LIGHTNING POLARITY IN SEVERE STORMS A Thesis by KURT MATTHEW BUFFALO Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE December 2007 Major Subject: Atmospheric Sciences ENVIRONMENTAL CONTROL OF CLOUD-TO-GROUND LIGHTNING POLARITY IN SEVERE STORMS A Thesis by KURT MATTHEW BUFFALO Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved by: Chair of Committee, Lawrence D. Carey Committee Members, Richard E. Orville Anthony T. Cahill Head of Department, Richard E. Orville December 2007 Major Subject: Atmospheric Sciences iii ABSTRACT Environmental Control of Cloud-to-Ground Lightning Polarity in Severe Storms. (December 2007) Kurt Matthew Buffalo, B.S., University of Northern Colorado Chair of Advisory Committee: Dr. Lawrence D. Carey In this study, it is hypothesized that the mesoscale environment can indirectly control the cloud-to-ground (CG) lightning polarity of severe storms by directly affecting their structural, dynamical, and microphysical properties, which in turn directly control cloud electrification and CG flash polarity. A more specific hypothesis, which has been supported by past observational and laboratory charging studies, suggests that broad, strong updrafts and associated large liquid water contents in severe storms lead to enhanced positive charging of graupel and hail via the noninductive charging mechanism, the generation of an inverted charge structure, and increased positive CG lightning production. The corollary is that environmental conditions favoring these kinematic and microphysical characteristics should support severe storms generating an anomalously high (> 25%) percentage of positive CG lightning (i.e., positive storms), while environmental conditions relatively less favorable should sustain storms characterized by a typical (≤ 25%) percentage of positive CG lightning (i.e., negative storms). Forty-eight inflow proximity soundings were analyzed to characterize the environments of nine distinct mesoscale regions of severe storms (four positive and five negative) on six days during May – June 2002 over the central United States. This iv analysis clearly demonstrated significant and systematic differences in the mesoscale environments of positive and negative storms, which were consistent with the stated hypothesis. When compared to negative storms, positive storms occurred in environments associated with a drier low to midtroposphere, higher cloud base height, smaller warm cloud depth, stronger conditional instability, larger 0-3 km AGL wind shear, stronger 0-2 km AGL storm-relative wind speed, and larger buoyancy in the mixed-phase zone, at a statistically significant level. Differences in the warm cloud depth of positive and negative storms were by far the most dramatic, suggesting an important role for this parameter in controlling CG lightning polarity. Subjective visual inspection of radar imagery revealed no strong relationship between convective mode and CG lightning polarity, and also illustrated that positive and negative severe storms can be equally intense. v ACKNOWLEDGMENTS First and foremost, I would like to sincerely thank my advisor, Dr. Larry Carey, for his endless help and support. His passion for atmospheric science and learning in general is contagious, and his dedication and work ethic truly inspiring. I owe him special thanks for his patience and continued encouragement while trying to complete my thesis remotely. I am very appreciative for the genuine care he has for his students and their development, and for the fact that he is never too busy to promptly answer a question or respond to any concern. I would also like to thank my committee members, Dr. Richard Orville and Dr. Tony Cahill, for their generous assistance and support of my thesis work, and guidance in general during my time as a graduate student. I thank Scott Steiger and Brandon Ely for their valuable assistance and mentoring with IDL code. I also owe a big thank you to Bill Fortune, Meteorologist-in-Charge of the National Weather Service Forecast Office in Pueblo, CO for his support as I worked to complete my thesis remotely while employed as a Student Career Experience Program (SCEP) student at the Pueblo Weather Forecast Office. Next, I would like to thank all of my family, and in particular my wife Julie, parents Joe and Laura, brother Kevin, and sister Kristi for their continual support, patience, and encouragement, especially during the most discouraging times. You are all core pieces to any success I have enjoyed, and without you I never would have survived graduate school. I would also like to thank Kent, Jill, and PJ Shillings for their kind hospitality during my stay in Texas. Finally, I would like to acknowledge the countless friends I made in College Station, who made my stay at Aggieland some of the best and vi most memorable years of my life. These lifelong friends are too many to name, but I would like to acknowledge a few in particular – Chrissy and Jeff Powell, Abby and Kevin Walter, and Dan Hawblitzel, who grew into family during my years at A&M. vii TABLE OF CONTENTS Page ABSTRACT........................................................................................................ iii ACKNOWLEDGMENTS................................................................................... v TABLE OF CONTENTS.................................................................................... vii LIST OF FIGURES............................................................................................. x LIST OF TABLES .............................................................................................. xix CHAPTER I INTRODUCTION.......................................................................... 1 II MOTIVATION .............................................................................. 7 a. Frequency and preferred location of positive severe storm occurrence............................................................................ 7 b. Effects of the local mesoscale environment on CG lightning behavior................................................................. 10 c. Common characteristics of positive severe storms............... 17 d. Hypothesis............................................................................ 21 III BACKGROUND............................................................................ 23 a. Thunderstorm electrification ................................................ 23 1) Charging mechanisms and typical charge structure ... 23 2) Cloud-to-ground lightning flash................................. 27 3) Charge structure associated with positive CG lightning..................................................................... 30 b. Effects of updraft intensity on cloud thermodynamics and microphysics......................................................................... 38 c. Updraft intensity................................................................... 41 1) Parcel theory and its deficiencies ............................... 42 2) Observed effects of updraft intensity on CG lightning polarity........................................................ 55 d. Warm cloud depth ................................................................ 56 e. Aerosol effects...................................................................... 58 viii CHAPTER Page IV DATA AND METHODOLOGY................................................... 60 a. Cloud-to-ground lightning data ............................................ 60 b. Meteorological data.............................................................. 62 c. Analysis of storm structure, morphology, and intensity....... 63 d. Characterization of the mesoscale environment with sounding data........................................................................ 66 1) Sounding platforms .................................................... 66 2) Proximity soundings................................................... 68 3) Calculation of meteorological parameters.................. 71 4) Assessment of the mesoscale environment using sounding data............................................................. 74 5) Statistical analysis of environmental parameters ....... 79 6) Analysis of dropsonde runs ........................................ 82 V RESULTS....................................................................................... 86 a. Cloud-to-ground lightning characteristics............................ 86 b. Meteorological scenarios...................................................... 100 1) 23 May 2002............................................................... 100 2) 24 May 2002............................................................... 103 3) 4 June 2002................................................................. 107 4) 12 June 2002............................................................... 112 5) 15 June 2002............................................................... 115 6) 19 June 2002............................................................... 123 c. Storm structure, morphology, and intensity ......................... 128 1) 23 May 2002............................................................... 128 2) 24 May 2002............................................................... 131 3) 4 June 2002................................................................
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