Mesoscale and stormscale ingredients of tornadic supercells producing long-track tornadoes in the 2011 Alabama super outbreak

This study focuses on the environmental and stormscale dynamics of supercells that produce long-track tornadoes, with modeling emphasis on the central Alabama storms from April 27, 2011 - part of the 2011 Super Outbreak. While most of the 204 tornadoes produced on this day were weaker and short-lived, this outbreak produced 5 tornadoes in Alabama alone whose path-length exceeded 50 documented miles. The results of numerical simulations have been inspected for both environmental and stormscale contributions that make possible the formation and maintenance of such long–track tornadoes. A two-pronged approach has been undertaken, utilizing both ideal and case study simulations with the Weather Research and Forecasting [WRF] Model. Ideal simulations are designed to isolate the role of the local storm environment, such as instability and shear, to long-track tornadic storm structure. Properties of simulated soundings, for instance hodograph length and curvature, 0-1km storm relative helicity [SRH], 0-3km SRH and convective available potential energy [CAPE] properties are compared to idealized soundings described by Adlerman and Droegemeier (2005) in an effort to identify properties conducive to storms with non-cycling (sustained) mesocyclones. 200m horizontal grid spacing simulations have been initialized with the 18 UTC Birmingham/Alabaster, Alabama [KBMX] soundings, taken from an area directly hit by the day’s storms. The simulation yielded a right-moving storm that exhibited non-occluding cyclic mesocyclogenesis, as described by Adlerman and Droegemeier (2002). Analysis scripts tracked vertical vorticity maxima, quantifying mesocyclone cycle strength, duration, updraft speed, and other associated properties. Case-study simulations have been used to understand the mesoscale forcing and environmental changes along storm tracks. A strong, sustained (duration > 90 minutes) mesocyclone has been successfully modeled at high resolution for the 27 April case, allowing detailed analysis of the long-track storm evolution and its environment. Simulations with horizontal grid spacing of 9km, 3km, 1km and 333m have been investigated for mechanisms contributing to the sustenance of the aforementioned storm. Model soundings have also been examined from the inflow of simulated storms in order to diagnose and understand favorable environments in which the storms may have developed and been sustained, with specific concentration on SRH, CAPE, and hodograph length and curvature. Unsteadiness was found in key environmental parameters – including CAPE, LCL, and vapor mixing ratio – indicating that storm inflow does not require steady state thermodynamic characteristics in order to sustain storm rotation. Additional analysis focuses on investigating the importance of surface moisture fluxes on influencing storm morphology. Three experimental simulations modifying various surface conditions were conducted, altering friction, surface fluxes and longwave radiation emission. Inflow soundings from these storms also showed fluctuations in thermodynamic fields, as seen in the full surface physics control simulation. However, further analysis revealed that storm inflow kinetic fields, such as 0-1km and 0-3km storm relative helicity varied from those in the Control. Storms simulated under full surface physics conditions demonstrated the ability to modify the wind fields in their inflow regions both to a greater degree and geographical extent than those simulated under modified surface physics. Additionally, Control storms boasted lifetimes roughly twice as long as those seen in the experimental surface simulations. A theory and its operational applications explaining this behavior are presented. Future work and continuing questions are also discussed