Analyzing the Effects of Complex Terrain in Northeastern Alabama Severe Weather Events Using Multiple Profiling Systems, Doppler Radar, and In Situ Measurements during the VORTEX-SE 2017 Field Campaign

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, particularly for enhanced tornadogenesis on the northwestern side of the plateau.

During several severe weather events from Fall 2016-Spring 2017, both prior to and during the VORTEX-SE field campaign, UAH-SWIRLL, the National Severe Storms Laboratory (NSSL), and the University of Oklahoma (OU) dispatched multiple instrument platforms to Sand Mountain and the adjacent Tennessee River valley. These platforms include:

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, 35-channel microwave profiling radiometer, and surface instrumentation;

3) The UAH-SWIRLL Mobile Integrated Profiling System (MIPS), which features a vertically-pointing X-band Doppler radar with 6 Hz sampling, a 915-MHz Doppler wind profiler, a 12-channel microwave profiling radiometer, a ceilometer, and surface instrumentation;

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

5) The OU/NSSL Collaborative Lower Atmospheric Mobile Profiling System 1 (CLAMPS-1), which features a Doppler wind lidar, an atmospheric emitted radiance interferometer (AERI), a 10-20 channel microwave profiling radiometer, a flux tower, and surface instrumentation.

In this presentation, we detail the environments observed by the UAH-SWIRLL and OU/NSSL CLAMPS-1 instrument platforms during a number of severe weather events prior to and during the VORTEX-SE 2017 field campaign. These events may include but not be limited to the following:

1) The 29-30 November 2016 significant supercellular tornado outbreak, which produced two tornadoes atop the Sand Mountain plateau, including an EF3 tornado that was responsible for 4 fatalities;

2) A nontornadic supercell event on 8 February 2017;

3) A complex pseudo-dryline event on 5 April 2017 during IOP 3b of VORTEX-SE, which included a complex mixed-mode multicellular and supercellular evolution, significant convective enhancement along the northwestern edge of Sand Mountain, and a weak EF0 tornado produced by a circulation near the northern tip of the Sand Mountain plateau; and

4) A weak supercellular tornado event on 22 April 2017 during IOP 3.5b of VORTEX-SE, which featured two EF0 tornadoes across northeastern Alabama, including one atop Sand Mountain.

The behaviors of the storms as observed by scanning radar and ground and aerial survey analysis are compared to the environmental observations to refine preliminary hypotheses of the roles of terrain in severe storm and tornado evolution in northeastern Alabama. We characterize noted changes in the environments atop Sand Mountain versus within the Tennessee Valley, particularly changes in the wind profiles observed at these locations, and discuss how these environmental changes may be related to observed changes in storm characteristics. Particular interest is paid to changes in storm-relative helicity (SRH) atop the Sand Mountain plateau and changes in flow that may imply enhancement of environmental low-level convergence and vertical vorticity along the northwestern edge of Sand Mountain. These preliminary results and refined hypotheses are placed in the broader context of the VORTEX-SE 2017 dataset to define a path forward in expanding the knowledge of how terrain and general boundary layer heterogeneity impact severe storm evolution.