OBSERVATIONAL ANALYSIS OF THE INTERACTION BETWEEN A BAROCLINIC BOUNDARY AND SUPERCELL STORMS ON 27 APRIL 2011 by ADAM THOMAS SHERRER A THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in The Department of Atmospheric Science to The School of Graduate Studies of The University of Alabama in Huntsville HUNTSVILLE, ALABAMA 2014 ABSTRACT The School of Graduate Studies The University of Alabama in Huntsville Degree Master of Science College/Dept. Science/Atmospheric Science Name of Candidate Adam Sherrer Title Observational Analysis of the Interaction Between a Baroclinic Boundary and Supercell Storms on 27 April 2011 A thermal boundary developed during the morning to early afternoon hours on 27 April as a result of rainfall evaporation and shading from reoccurring deep convection. This boundary propagated to the north during the late afternoon to evening hours. The presence of the boundary produced an area more conducive for the formation of strong violent tornadoes through several processes. These processes included the production of horizontally generated baroclinic vorticity, increased values in storm- relative helicity, and decreasing lifting condensation level heights. Five supercell storms formed near and/or propagated alongside this boundary. Supercells that interacted with this boundary typically produced significant tornadic damage over long distances. Two of these supercells formed to the south (warm) side of the boundary and produced a tornado prior to crossing to the north (cool) side of the boundary. These two storms exhibited changes in appearance, intensity, and structure. Two other supercells formed well south of the boundary. These two storms remained relatively weak until they interacted with the boundary. These storms then rapidly intensified and produced tornadoes. iv Acknowledgements The completion of this thesis would not have been possible without the help and guidance of many people. I would first like to thank Dr. Knupp who has provided me hours of guidance not only with this research and graduate school but throughout my undergraduate study as well. I would also like to thank the other members of my committee Drs. Larry Carey and Donald Perkey for the assistance they have provided as well. I would also like to thank Todd Murphy and Ryan Wade for their continued guidance through this process as well. In addition, many other friends and colleagues helped in the research process. These friends include: Aaron Mayhew, Matthew Saari, Lamont Bain, Stephanie Mullins, Ryan Rogers, Dustin Phillips, and Tony Lyzaa. Finally, I need to thank my family and fiancé. If not for their unwavering support the completion of this process would have seemed impossible. This research was supported by National Science Foundation, Grant ASG- 1110622. vi TABLE OF CONTENTS Page List of Figures.....................................................................................................................ix List of Tables ...................................................................................................................xvi Chapter 1. INTRODUCTION ..................................................................................................1 2. BACKGROUND ....................................................................................................4 2.1 Supercell Thunderstorms ……....................................................................4 2.1.1 Environmental Parameters of Supercells ........................................5 2.1.2 Supercell Structure .........................................................................8 2.2 Quasi Linear Convective Systems & Bow Echoes ...................................10 2.3 Thermal Boundaries Roles in Supercell & Tornado Outbreaks ...............12 2.4 Previous Work with Thermal Boundaries and Tornadic Storms ………..15 3. DATA AND METHODOLGY ............................................................................22 3.1 Radars .......................................................................................................22 3.2 Radar Processing …………………………………….………………......24 3.2.1 Editing …………………………………………………………...24 3.2.2 EVAD Analysis …………………………....................................26 3.3 Surface Data ………………………………………….............................27 3.4 MIPS Data ……………………………………………………………….32 4. SYNOPTIC & MESOSCALE ENVIRONMENT ………………………………34 4.1 Synoptic Environment …………………………………............……......35 4.2 Mesoscale Environment …………………………………………………40 5. BOUNDARY DEVELOPMENT & CHARACTERISTICS ……………………44 vii 5.1 Boundary Development ……………………………....…………………44 5.2 Boundary Progression ………………………………………………...…54 5.2.1 Vertical Boundary Development ………………………………..63 5.3 Boundary Characteristics ……………………………………………………67 5.3.1 Temperature and Stability ……………………………………….67 5.3.2 Variation of the Vertical Winds …………………………………...70 5.3.3 Storm-Relative Helicity …………………................................…76 6. SUPERCELLS INTERACTING WITH THE BOUNDARY….………..………78 6.1 Cullman EF-4 Supercell …………………………………………………79 6.2 Hackleburg EF-5 Supercell ……………………………………………...86 6.3 Smithville EF-5 Supercell ……………………………………….............95 6.4 Jackson County EF-4 Supercell …………............................……………98 6.5 Limestone County Supercell ..……………………………….................102 6.6 Non-Tornadic or Weakly Tornadic Supercells ……………...................105 6.6.1 Supercells in Central Mississippi ………………...........…….…105 6.6.2 Northwest Alabama Supercell ………………………................105 6.7 Summary………………………………………………………………..106 7. DISCUSSION ………………………….............................................................108 8. CONCLUSION & FUTURE WORK…………..................................................115 8.1 Conclusions…………………………………………………………………115 8.2 Future Work …………………………................................................……..117 viii LIST OF FIGURES Page Figure 2.1 Figure 2.1: Tilting of horizontal vortex lines relative to storm motion. Image from Markowski and Richardson 2010, adapted from Davies-Jones 1984. ……………………7 2.2 Conceptual structure of a supercell (Lemon and Doswell 1979) solid lines indicate storm echo. Solid lines with arrows represent storm relative streamlines. The storm’s forward-flank, rear-flank downdrafts are represented as well as the updraft location…....9 2.3 Conceptual model of a QLCS denoting the locations of the pre-squall low, mesohigh, straitiform regions and wake low (Markowski and Richardson 2010, adapted from Johnson and Hamilton 1988). ……………………………………………………..12 2.4 Depiction of the production of baroclinically generated horizontal vorticity (Markowski and Richardson 2010 adapted from Klemp 1987). Solid black lines represent the horizontal vortex lines. The white arrows represent parcel trajectories into the storm. …………………………………………………………………………………………...14 2.5 Bar graphs presenting the frequency of tornadogensis in the presence of a boundary during VORTEX (top) and tornado occurrence relative to distance from boundary in VORTEX (bottom) (Markowski et al. 1998). …………………..…………17 2.6 Figure from Markowski et al. 1998 demonstrating potential storm locations and interactions relative to horizontal vorticity generated by the boundary. ………..……....18 2.7 Coneptual model of a boundary and associatied wind profiles. Wind profiles show backing of the wind at lower levels on cool side of boundary with more unidirectional flow on warm side(Maddox et al. 1980). ..........................................................................19 2.8 Chart showing measured rotational velocity as a function of boundary position (Rasmussen et al. 2000). Relative maximum occurs between 0 and 50 m s -1. …...…….21 3.1 Surface observations and organization type. Locations without a symbol denote other cities mentioned in this study. ……………………………………………….........30 4.1 : Storm Prediction Center’s 12 UTC Convective Outlook for April 27, 2011. A high risk was placed over northern Alabama bordering parts of Mississippi, Tennessee, and Georgia. ……………………………………………………………………………..34 4.2 : 1200 UTC 300 mb observations showing a deep trough with large divergence located over Mississippi, Alabama, and Tennessee. …………………………………….36 4.3 1200 UTC 850 mb observations showing strong southerly low-level winds over the southeast…………….………………………………………………………………..36 ix 4.4 1200 UTC surface observations showing a warm front in extended through Arkansas into Missouri with a cold front in southwestern Arkansas. Southerly flow resides over much of the southeast……..…………………………..……………….…...38 4.5 1200 UTC 300 mb observations displaying the negatively tilted trough axis over Arkansas Mississippi, and Alabama. Strong diffluence existed in the exit region of the Jetstream that enhanced divergence values. ……………………..………………….…..38 4.6 850 hPa analysis at 0000 UTC on 28 April demonstrating strong southerly flow east of the trough axis……………………………………………………………………39 4.7 0000 UTC surface pressure and wind at 0000 UTC on 28 April demonstrating the location of the cold front across Mississippi and Alabama. …………………………….39 4.8 SPC Mesoanalysis graphics for SBCAPE (solid contours J kg -1) and SBCIN (dashed contours with shading, top figure) and 0-1 SRH (m 2 s-2). Surface temperatures are not shown due to the presence of cold air over north Alabama which was no accurately diagnosed. ……………….................................................................................................42 4.9. 0.7° PPI from ARMOR at 2000 UTC. White dashed line denotes approximate boundary position at this time. …………………………………………………………..43