Examining the Sensitivity of Tornadic Supercell Behavior on 27 April 2011 to the Background Environment Using Observation-Driven Simulations and MYRORSS Azimuthal Shear Analyses

Abstract

The prolific 27–28 April 2011 tornado outbreak produced multiple rounds of tornadic supercells in four spatiotemporal regions. All of these regions exhibited extremely favorable environments, with small differences in thermodynamic and kinematic profiles and varying proximities to boundaries. This study uses multiple radar-based and environmental observations to inform high-resolution, 25-member numerical ensembles of five select supercells that formed in these regions. Storm Prediction Center surface objective analysis (SFCOA) profiles, RUC profiles blended with surface observations, are used in a base-state substitution technique to continuously nudge the simulated environments towards observations. A more gradual tendency nudging convection initiation technique is used to attempt to capture the mesocyclone development noted in MRMS-based azimuthal shear (AzShear) analyses. Individual ensemble members vary in the magnitude and duration of tendency nudging. This study provides insight into the effects of small changes in tendency nudging on simulated storms across different, albeit all extreme, environments. The resulting simulations are compared to AzShear analyses from the MYRORSS dataset and observed reflectivity fields to judge the performance of our combined tendency nudging and base-state substitution approach, evaluate how much of the observed storm evolutions may have been dependent on the background mesoscale environment, and assess the resulting predictability of the observed storm evolutions. The ensemble suites exhibit a diversity in storm outcomes. The successful simulations mature comparatively quickly, and represent observed storms that initiated at higher latitudes where low-level and deep-layer shear were maximized. The degree to which they emulate their observed counterparts varies, with the most realistic simulated supercells corresponding to the northernmost-initiating storms in the highest-shear environments. The unsuccessful suites, in which ensembles could not sustain organized convection after the cessation of the heating tendencies, have less humid mid- and upper-tropospheres. Several external, mesoscale features absent from the simulations are identified as playing a critical role in maintaining observed storms in environments that failed to sustain organized, simulated convection. These findings highlight a reliance of tornadic supercells on inhomogeneous influences (terrain, cell interactions, etc) during the 27 April 2011 super outbreak, and the limited predictability of similar high-impact events in the future.