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Numerical Simulation of Intense Multi-Scale Vortices Generated by Supercell Thunderstorms

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Numerical Simulation of Intense Multi-Scale Vortices Generated by Supercell Thunderstorms Numerical Simulation of Intense Multi-Scale Vortices Generated by Supercell Thunderstorms by Catherine A. Finley PI: Roger A. Pielke Department of Atmospheric Science Colorado State University Fort Collins, Colorado NSF ATM-9306754 and ATM-9420045 NUMERICAL SIMULATION OF INTENSE MULTI-SCALE VORTICES GENERATED BY SUPERCELL THUNDERSTORMS by Catherine A. Finley Department of Atmospheric Science Colorado State University Fort Collins, CO 80523 Research Supported by National Science Foundation under Grants ATM-9306754 and ATM-9420045 December 19, 1997 Atmospheric Science Paper No. 640 ABSTRACT NUMERICAL SIMULATION OF INTENSE MULTI-SCALE VORTICES GENERATED BY SUPERCELL THUNDERSTORMS A nested grid primitive equation model (RAMS version 3b) is used to study various aspects of tornadoes and the thunderstorms that produce them. A unique aspect of these simulations is that the model was initialized with synoptic data, and telescoping grids allow atmospheric flows ranging from the synoptic-scale down to sub-tornado-scale vortices to be represented in the model. Two different case studies were simulated in this study: June 30, 1993, and May 15, 1991. The June 30, 1993, simulation produced a classical supercell storm which developed at the intersection between a stationary front and an outflow boundary generated by previ­ ous convection. As the simulation progressed, additional storms developed west of the main storm along the stationary front. One of these storms interacted with the main storm to pro­ duce a single supercell storm. This storm had many characteristics of a high-precipitation (HP) supercell, and eventually evolved into a bow-echo. The transition of the storm into a bow-echo is discussed and possible physical processes responsible for the transition are presented. The June 30, 1993, simulated supercell produced two weak tornadoes. The first tor­ nado developed along the flanking line of the storm to the southeast of the mesocyclone. The second tornado developed along a strong horizontal shear zone beneath the rotating comma-head structure of the HP supercell. Neither tornado was clearly linked to the meso­ cyclone in the parent storm, and both tornadoes formed first near the surface and then developed upward with time. Circulation and vorticity analyses were used to investigate the tornadogenesis process in this case. Results from these analyses indicated that the circulation associated with both tornadoes was already present at low-levels in the storm environment 15-20 minutes before the tornadoes developed. Although the baroclinic term associated with the downdraft air made a negligible contribution to the circulation in this case, the downdraft played an important role in tilting horizontal vorticity into the verti- cal just above the surface in the near tornado environment where horizontal convergence could then act to amplify it. A comparison with the proposed tornadogenesis process( es) in classical supercells is also presented. The May 15, 1991, simulation produced a classical supercell which developed along the dryline in the Texas panhandle. This supercell in turn produced a tornado which lasted for 50 minutes in the simulation. During a ten minute period toward the end of the simulation, six secondary vortices developed within the main tornado vortex. The simulated secondary vortices had many features in common with multiple-vortex tornadoes and secondary vortices produced in laboratory vortices. The evolution and structure of the simulated secondary vortices is presented, and physical mechanisms responsible for their development and dissipation are discussed. Catherine A. Finley Department of Atmospheric Science Colorado State University Fort Collins, Colorado 80523 Spring 1998 11 ACKNOWLEDGEMENTS There are many people who have contributed greatly to my graduate experience here at CSU. First I would like to thank my advisor, Roger Pielke, for allowing me to pursue my own path through my graduate studies. Bill Cotton, Sonia Kreidenweis and Bob Meroney are thanked for taking the time to serve on my committee. Special thanks are given to Bill Cotton who also acted in the unofficial role of co-advisor. Without his support, completion of this work would not have been possible. John Lee is thanked for 'showing me the ropes' with the RAMS model. His willingness to share his knowledge of numerical modeling benefited me greatly throughout the course of this work. I would also like to thank Bob Walko for patiently answering questions about the model, and for his help and advice when model problems arose or when code modifications were needed. Louie Grasso and Bob Walko are thanked for many thought-provoking discussions on tornadoes, tornadogenesis and related topics, from which I benefited greatly. Ligia Bernardet and Jason Nachamkin are thanked for their moral support, and for many en­ lightening discussions on the dynamical and modeling aspects of convective systems. My office mates Tom Chase, Jerry Harrington and Mark Sellers are thanked for providing comic relief in times of extreme stress, and for insightful discussions on science, philosophy, and the MRHLF. I would also like to thank the following people who have provided moral and/or technical support during the course of my graduate work: Donna Chester, James Cizek, Jeff Copeland, Brian Gaudet, Louie and Karen Grasso, Richard and Margaret Finley, Abby Hodges, Tara Jensen, Dallas McDonald, Brenda Thompson, Bill Thorson and Jane Wilkins. Finally I would like to acknowledge the people who have had the greatest impact on my life. I would like to thank my parents, Emil and Mary Hamann, for supporting and encouraging me to 'take the road less traveled' and pursue a career in science. Lastly and iii most importantly, I would like to thank my husband Steve for his unwaivering support and love during this long and arduous journey. lV TABLE OF CONTENTS 1 Introduction 1 2 Background 4 2.1 Environmental Factors. 4 2.2 Supercell Thunderstorms 6 2.2.1 The Supercell Spectrum 8 2.2.2 Origins of Updraft Rotation. 14 2.3 Tornadoes and Tornadogenesis 21 2.3.1 Observations ......... 22 2.3.2 Modelling Studies ...... 27 2.4 Secondary Vortices and Vortex Stability 35 2.4.1 Observations . 35 2.4.2 Laboratory Studies. 37 2.4.3 Analytical/Numerical Modeling Studies 44 3 Case Overviews 62 3.1 June 30, 1993 Case. 62 3.2 May 15, 1991 Case . 63 4 Model Description and Configuration 74 4.1 PE Model ............ 74 4.2 Data Sets and Initial Conditions 76 4.3 Model Configuration . 78 4.3.1 June 30, 1993 Case. 78 4.3.2 May 15, 1991 case . 79 5 Simulated June 30, 1993 HP Supercell 84 5.1 Storm Evolution and Structure ..... 84 5.2 Further Analysis of Bow-echo Transition .. 93 5.2.1 Gravity Waves . 94 5.2.2 Cold Pool Dynamics .. 100 5.3 Summary and Discussion 104 6 June 30, 1993 Simulated Tornadoes 146 6.1 Development and Evolution of T1 148 6.2 Development and Evolution of T2 153 6.3 Tornadogenesis . 158 6.3.1 Circulation and Vorticity 158 6.3.2 June 30-T1 . 162 6.3.3 June 30-T2 ....... 171 v 6.4 Comparison of Tornadogenesis Processes with Previous Studies 180 7 Simulated Secondary Vortices-May 15, 1991 Case 224 7.1 Overview of May 15,1991 Simulation ....................... 225 7.2 Evolution and Structure of the Secondary Vortices . .. 227 7.3 Comparison of Model Results with Observations, Laboratory Studies and Pre- vious Modeling Work . 233 7.4 Summary and Discussion .............................. 237 8 Summary, Conclusions, and Future Work 270 8.1 June 30, 1993, HP Supercell Simulation .. 270 8.2 June 30, 1993 Tornado Simulation .. 271 8.3 Simulated Secondary Vortices-May 15, 1991 Case. 272 8.4 Suggestions for Future Research .......... 272 vi LIST OF ABBREVIATIONS API Antecedent Precipitation Index BLV Boundary Layer Vortices BRN Convective Bulk Richardson Number BWER Bounded Weak Echo Region CAPE Convective Available Potential Energy CCOPE Cooperative Convective Precipitation Experiment DPE Dynamic Pipe Effect FFD Forward Flank Downdraft HP High Precipitation HSI Horizontal Shear Instability LCL Lifting Condensation Level LFC Level of Free Convection LP Low Precipitation MCC Mesoscale Convective Complex MCS Mesoscale Convective System NCEP National Center for Environmental Prediction NST Non-Supercell Tornado PBL Planetary Boundary Layer PV Parent Vortex RAMS Regional Atmospheric Modeling System Re Reynolds Number RFD Rear Flank Downdraft SREH Storm-Relative Environmental Helicity SST Sea Surface Temperature SV Secondary Vortex TVS Tornado Vortex Signature UTC Universal Time Coordinate VORTEX Verifications of the Origins of Rotation in Tornadoes EXperiment vii Chapter 1 INTRODUCTION We were among the first to arrive at the scene of the disaster; and our pen fails entirely to depict the sight which met our view. We found the town, as the messenger had reported, literally blown to pieces, and destruction and death scattered everywhere with the sweep of destruction. The first pile that met our eye was the ruins of the Millard House, occupied by H.G. Sessions, formerly of Erie, Pa. This was a three-story brick hotel; and it could not have been more effectively destroyed had a barrel of gunpowder exploded within its walls. The inmates were all more or less hurt. From this we proceeded to look about the town, and we found that hardly a house was left uninjured, and that many of them were swept entirely away. -Press report printed in the Lyons City Advocate (Iowa), June 4, 1860 The above quotation was taken from a press report describing the scene in the town of Camanche, Iowa, following the passage of a tornado late in the day on June 3, 1860 (Stanford 1987). Tornadoes are among the most violent storms on earth. Winds associated with tornadoes can reach in excess of 100ms-1 (225 mph) leaving damage paths 3km wide in extreme cases (Davies-Jones 1983). Fortunately, less than 10% of all reported thunderstorms produce severe weather (Doswell 1985) and few of these actually produce tornadoes.
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