Using Observations and Numerical Simulations to Assess the Effects of Topography on the 29-30 November 2016 Tornado Outbreak in Northeastern Alabama

As part of the Verification of the Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) field campaign, the University of Alabama in Huntsville’s Severe Weather Institute – Radar and Lightning Laboratories (UAH-SWIRLL) has sought to investigate the role of topography in the evolution of severe storm and tornado behavior. The Sand Mountain plateau in northeastern Alabama is a particular focal point of interest for these studies owing to a propensity for enhanced tornado activity over this region compared to surrounding areas. Additionally, a pattern appears to exist for tornadogenesis to be favored on the northwestern side of the plateau.

UAH-SWIRLL profiling instruments were deployed to northeastern Alabama on the night of 29-30 November 2016, during a significant regional tornado outbreak. The event produced two tornadoes atop the Sand Mountain plateau, including an EF3 tornado that was responsible for 4 fatalities. Equipment deployed during the event included:

1) The UAH-SWIRLL Rapidly-Deployable Atmospheric Profiling System (RaDAPS), which features a 915-MHz Doppler wind profiler, a 35-channel microwave profiling radiometer, a ceilometer, and surface instrumentation;

2) The UAH-SWIRLL Mobile Doppler Lidar and Sounding system (MoDLS), which features a Doppler wind lidar, 15-channel microwave profiling radiometer, and surface instrumentation; and

3) The UAH-SWIRLL Mobile Alabama X-band (MAX) polarimetric radar.

In addition to deploying an observational profiling network, simulations using the Advanced Research Weather Research and Forecasting (WRF-ARW) model have been performed in order to attempt to simulate the evolution of the tornado outbreak and further assess the potential role of topography in the evolution of the near-storm environment and of the storms in northeastern Alabama.

This presentation highlights results of the observational and numerical analysis performed for the 29-30 November 2016 outbreak. Observed changes in the environment atop Sand Mountain during the outbreak will be highlighted. These observations include a systematic, substantial increase in low-level wind shear, with 0-1 km storm-relative helicity (SRH) values atop Sand Mountain persistently twice those observed in the Tennessee Valley, as well as evidence of possible persistent low-level convergence and positive vertical vorticity. These observations will be compared and contrasted to the WRF-ARW results, which limited low-level shear enhancement to the immediate vicinity of the northwestern slope of Sand Mountain but also featured persistent low-level convergence along the northwestern slope and lower lifted condensation level (LCL) and level of free convection (LFC) heights across Sand Mountain versus over the Tennessee Valley. Potential physical causes for the changes in the near-storm environment will be emphasized, including the potential for flow acceleration over the plateau and weak downslope wind storm development on the northwestern (leeward) slope. Finally, the role that environmental changes linked to the underlying topography may play in the evolution of severe storms and tornadoes in northeastern Alabama will be discussed.