Between 2011 and 2019, we conducted experiments with the Atmospheric Imaging Radar (AIR; Isom et al. 2013; Kurdzo et al. 2017) each spring to collect rapid-scan observations of severe thunderstorms. The AIR is a mobile phased array radar (PAR) that uses digital beamforming to collect observations simultaneously in a vertical column. Thus, by only mechanically scanning in azimuth, the AIR can collect volume scans every 5 to 10 s without having to steer the beam in the vertical direction.

Using this unique system, we collected 46 supercell cases and over 10 tornado cases. We are analyzing these data to study rapidly evolving processes during all phases of the tornado's life cycle, including genesis, intensification, and dissipation. An example of AIR data during tornado dissipation is shown on the right for the 27 May 2015 tornado (Griffin et al. 2019). Dissipation began with a weakening at mid-level (~1.25 to 1.5 km) and then proceeded in a downward fashion. During the final minute or two of the tornado, it remained more intense between 500 and 1000 m compared to below and the tornado fully dissipates first at the surface and then upward in time over ~30 s (around 22:08 UTC).

Using the archive of tornadic and nontornadic supercell cases, we are exploring differences leading to tornado formation or tornadogenesis failure. One of our recent REU students, Kyle Pittman, examined tornadogenesis failure from an AIR case collected near Denver, CO in 2014 (REU Paper).

The AIR is being replaced by a dual-polarization C-band radar called the Polarimetric Atmospheric Imaging Radar (PAIR; Salazar et al. 2019) as well as an S-band dual-polarized PAR called HORUS. We will be using these systems once they are completed to continue studies of severe convective storms using phased array radars.

The importance of friction to tornado dynamics has been noted for decades with a seemingly paradoxical result that friction intensifies tornadoes. Yet, it serves as a critical mechanism necessary for generating the tornado's intense corner flow and can allow angular momentum to reach closer to the tornado's central axis than would be possible in its absence.

The surface beneath tornadoes is often modeled or assumed to be homogeneous, yet we know from observations that tornadoes encounter a rich variety of terrain and manmade structures along their paths. To date, few studies have used high-resolution numerical models to understand how terrain or manmade structures impact tornado dynamics and how these resulting changes affect societal risk to tornadoes. Complex terrain is particularly common in the Southeast United States where a rich variety of small mountain ranges, plateaus are present across this often heavily forested region.

To model the impact of terrain and buildings on tornado dynamics, we use a high-resolution numerical model (Large-Eddy Simulation model) which provides 1 to 2 m horizontal and vertical resolution near the surface. An immersed boundary method (IBM) is used to implement different types of terrain and manmade structures. The IBM applies forces along the boundary surface to impose the proper boundary condition along a non-uniform 2D surface.

Terrain: As part of a VORTEX-SE experiment, we examined the sensitivity of tornado dynamics to terrain by varying the height of the terrain, distance between terrain features, and type of terrain (hill, valley, ridge, etc.). This work was led by M.S. student Martin Satrio and the results are published in the Journal of Atmospheric Sciences (Satrio et al. 2020). Some of the key conclusions from this work include:

• Complex terrain increases the variability of near-surface winds in tornadoes, but by a relatively small magnitude for the horizontal component (<10%)
• Near-surface horizontal wind speeds are highest in valleys and low-lying areas
• Small terrain features are found to slightly enhance near-surface horizontal wind speeds whereas large terrain features become disruptive and reduce near-surface wind speeds
• Terrain increases near-surface vertical motions and increases in terrain height produce larger vertical velocities

Buildings: Considerable variability in damage is often evident in residential areas, which stems from differences in the quality of construction as well as differences in wind and debris loading. The wind and debris loading experienced by one residence depends on how the near-surface winds are modified by nearby structures as well as what debris is generated. To better understand tornadoes interacting with residential areas, we started a new VORTEX-SE project aimed at understanding how 1) the geometry of residences and residential areas impacts the tornado's three-dimensional wind structure, and 2) what wind and debris forces impinge upon residential structures. The study will also model the damage process to emulate observed damage paths, and the simulations will be compared to tornadoes in the Southeast US.