External Controls on Tornadogenesis and Evolution: Potential Significance and Current State of Knowledge

This paper summarizes controls external to deep convection (supercells or QLCSs) that may influence tornadogenesis or tornado evolution. Such controls, summarized below, are typically most prominent in the atmospheric boundary layer and represent some of the research foci of VORTEX-SE.

Interactions between storms and thermal boundaries. The impact of preexisting thermal boundaries on tornadic storms is well-established, but detailed physical processes not necessarily well understood due to paucity of detailed obserations and substantial variations from case to case. For example, early work (e.g., Rasmussen et al 2000) examined a narrow boundary zone in which thermodynamic contrasts were several kilometers in horizontal extent, while other studies (e.g., Sherrer et al. 2014) documented a much wider boundary zone having thermodynamic contrasts over a 10-30 km span that was effective in initiating or enhancing tornadoes on 27 April 2011.

Wave interactions and “Wave Reflectivity Segments” (WRS). Initial research on this topic was considered by Coleman and Knupp (2008), and later extended by Murphy et al. (2014). WRS features are particularly common during the cold season when stability at low levels is often greater. Ducted gravity waves, which represent a subset of WRS types, can produce rapid variations in low-level shear and convergence. Interactions (mergers) between supercells and weaker, less orgnaized deep convection, may be considered to be a subset of this category.

Topography has also been hypothesized to exert both direct and indirect impacts on tornado evolution. Direct impacts include any disruption around the tornado boundary layer, as recently modeled by Lewellen (2014). Indirect impacts include mesoscale variations in ABL flow, such as channeling in valleys and acceleration of airflow over topographic features. Multiple cases in the latter category have been documented by Lyza (2015). A primary challenges in this category is acquiring measurements of vertical wind profiles around topographic features that will require both scanning Doppler radars and atmospheric profiling systems.

Surface roughness (z0) and horizontal variations in z0 are also hypothesized to play an important role in tornadogenesis (Coleman et al 2014, Weigel 2016). Such variations have been shown to produce significant variations in vertical vorticity within the ABL (Asefi et al 2010). Recent work by Weigel (2016) has demonstrated systematic variations in z0 that would be expected to produce cyclone shear zones with the lower ABL. Relevant questions in this category include “What is the effective z0 in inhomogenous terrain?” and “How can we effectively measure z0 variations?”

Because most of these controls are most significant within the ABL, detailed measurements from scanning radars that sample into the surface layer, multiple profiling systems, high frequency balloon soundings, and dense surface measurements are required to provide an observational foundation required to validate numerical simulations to enhance understanding of these ABL processes.