Ensemble Sensitivity Analysis of Controls on Updraft Rotation for Two Southeastern U.S. Tornado Events

Sensitivity analysis is a useful technique for diagnosing how small perturbations in initial conditions can influence the forecast of the future state. Recently, ensemble sensitivity analysis (ESA) began to be used for such purpose, where the linear component of the relationship between a forecast metric (e.g., updraft helicity, maximum reflectivity) and the initial conditions of the simulation could be quantified.  With such knowledge, identified areas of sensitivity can be adaptively sampled with surface and upper-air observations to reduce the forecast error variance. 

Much of the existing work using ESA has involved larger time and space scales, where the assumptions of linearity inherent to ESA are relatively appropriate (e.g., Ancell and Hakim 2007; Torn and Hakim 2008).  Ongoing work has successfully applied this technique to the mesoscale to assess how heterogeneities in initial conditions influence the development of deep convection along the Southern Plains dryline (e.g., Hill et al. 2016). 

This paper will explore the utility of ESA on the mesoscale and storm scale, to understand the specific environmental and storm-generated controls on low-level vertical vorticity in simulations of storms from two cases over the southeastern United States: the 27 April 2011 tornado outbreak across Mississippi and Alabama, and the 31 March 2016 VORTEX-SE case over northern Alabama and southern Tennessee, the latter sampled with a mesonetwork of 16 Texas Tech StickNet surface stations.  Retrospective work with this first case has demonstrated the skill of a 36-member WRF-ARW ensemble in producing forecast probability swaths of storm rotation for individual storms in the outbreak (Yussouf et al. 2015).  Now, we extend this work to higher resolution (1-km horizontal grid spacing for the “free” forecasts) and apply the ESA technique to identify controls on storm rotation.

Findings from these simulations will be reported as well as some thoughts on the utility of ESA-based techniques in storm-scale simulations and for future observation targeting experiments aimed at improving short-term prediction of severe weather hazards.