Insights Gained into the Role of Topography in Modifying the Near-Storm Environments of Tornadic Storms through VORTEX-SE Observations and Numerical Simulations

The role of topography in impacting the evolution of tornadic storms has been one of the foci of the Verification of the Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) field campaign. To that end, the University of Alabama in Huntsville’s Severe Weather Institute – Radar and Lightning Laboratories (UAH-SWIRLL) has focused on the role of topography in the evolution of severe storm and tornado behavior across northeastern Alabama. The Sand Mountain and Lookout Mountain plateaus are particular focal points of interest for these studies owing to a propensity for enhanced tornado activity over this region compared to surrounding areas of eastern Alabama and southern Middle and East Tennessee. Additionally, a pattern appears to exist for tornadogenesis to be favored on the northwestern side of the Sand Mountain plateau.

UAH-SWIRLL profiling instruments were deployed to northeastern Alabama during several severe storm and tornado events from the fall of 2016 through the spring of 2018. Equipment deployed during these events has included:

1) The UAH-SWIRLL Mobile Integrated Profiling System (MIPS), which features an X-band profiling Doppler radar, a 915-MHz Doppler wind profiler, a 35-channel microwave profiling radiometer, a ceilometer, and surface instrumentation;

2) 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;

3) 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

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

Numerous additional instruments were also deployed for events during the spring 2017 VORTEX-SE field campaign. Notably, the OU/NSSL Collaborative Lower Atmospheric Mobile Profiling System 2 (CLAMPS-2), which features a Doppler wind lidar, an atmospheric emitted radiance interferometer (AERI), a 10-20 channel microwave profiling radiometer, and surface instrumentation, and the University of Massachusetts (UMass) frequency-modulated continuous-wave (FMCW) radar were deployed in the adjacent Tennessee Valley during the duration of the spring experiment, as was the MoDLS platform atop Sand Mountain, to produce long-term comparative profiling observations across the terrain interface.

The most noteworthy event during that timeframe occurred 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. Data sets were also gathered for tornado events on 22 April 2017, which featured a weak tornado atop Sand Mountain, and 19 March 2018, which featured a supercell that produced several weak EF0-EF1 tornadoes in the Tennessee Valley before producing a strong EF2 tornado atop Sand Mountain. Several null case datasets were also gathered, including data sets with non-tornadic severe convection, sub-severe convection, and no significant convection.

In addition to deploying observational profiling networks, simulations using the Advanced Research Weather Research and Forecasting (WRF-ARW) model have been performed in order to attempt to 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 select northeastern Alabama events. Observed variations between the environments atop the plateaus versus in the upstream Tennessee Valley will be displayed, including 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 ambient convergence and vertical vorticity generation along the northwestern edge of Sand Mountain. These observations will be compared and contrasted to 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. Sources of disagreement between numerical simulation results and observed environments will be explored. Finally, the possible 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.