A Background Investigation of Tornado Activity across the Southern Cumberland Plateau Terrain System of Northeastern Alabama

Abstract

The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. This study evaluates an apparent tornado activity maximum across the Sand Mountain and Lookout Mountain plateaus of northeastern Alabama. These plateaus, separated by the narrow Wills Valley, span ~5000 km2 and were impacted by 79 tornadoes from 1992 to 2016. This area represents a relative regional statistical maximum in tornadogenesis, with a particular tendency for tornadogenesis on the northwestern side of Sand Mountain. This exploratory paper investigates storm behavior and possible physical explanations for this density of tornadogenesis events and tornadoes. Long-term surface observation datasets indicate that surface winds tend to be stronger and more backed atop Sand Mountain than over the adjacent Tennessee Valley, potentially indicative of changes in the low-level wind profile supportive to storm rotation. The surface data additionally indicate potentially lower lifting condensation levels over the plateaus versus the adjacent valleys, an attribute previously shown to be favorable for tornadogenesis. Rapid Update Cycle and Rapid Refresh model output indicate that Froude numbers for the plateaus in tornadic environments are likely supportive of enhanced low-level flow over the plateaus, which further indicates the potential for favorable wind profile changes for tornado production. Examples of tornadic storms rapidly acquiring increased low-level rotation while reaching the plateaus of northeast Alabama are presented. The use of this background to inform the VORTEX-SE 2017 field campaign is discussed.

A classification scheme for identifying snowstorms affecting central New York State

“…A Getis Ord Gi* hotspot analysis was performed in ArcGIS. The analysis identified areas of significant geographic clustering of cyclogenesis for storms that produce either heavy (hot spot) or light (cold spot) snowfall totals in Central New York (Figure 3) (Getis and Ord, 1992; Lyza and Knupp, 2018; Saunders et al ., 2018). The number of storms forming in each grid cell was then calculated and a Getis Ord Gi* hotspot analysis was again used, this time to determine typical areas of cyclogenesis (Figure 3).…”

Statistical and Empirical Relationships between Tornado Intensity and Both Topography and Land Cover Using Rapid-Scan Radar Observations and a GIS

“…Rapid scan, mobile radar data from four tornadoes that occurred in the U.S. plains region are analyzed. While this area of the country generally does not have particularly high topographic relief, Lyza and Knupp (2014) determined that topographic elevation changes as small as 20 m had an impact on tornado intensity in their study, and Karstens et al (2013) found similarly that elevation changes as small as 30 m were impactful. Such conditions are commonly found throughout the central plains and in the study domain (Figs.…”

Progress toward Characterization of the Atmospheric Boundary Layer over Northern Alabama Using Observations by a Vertically Pointing, S-Band Profiling Radar during VORTEX-Southeast

“…This site was selected to align with the objectives of VORTEX-SE pertaining to terrain influences on tornadogenesis. Scottsboro is located west (nominally upstream) of the southern Cumberland terrain range (known locally as Sand Mountain), where a regional maximum in tornadogenesis events has been documented (Lyza and Knupp 2018). UMass FMCW operated at Scottsboro from 10 March to 1 May 2017, with only a few gaps in data coverage (Frasier et al 2017) (Fig.…”

“…In our implementation, at each UMass FMCW observation time, the 08C height was derived from the Atmospheric Emitted Radiance Interferometer (AERI; Knuteson et al 2004a,b) optimal-estimation (AERIoe; Turner and Löhnert 2014) product of the Collaborative Lower Atmospheric Profiling System (Wagner et al 2019). CLAMPS was collocated with UMass FMCW for both the 2016 and 2017 VORTEX-SE field campaigns (Turner , 2017Lyza et al 2018). The CLAMPS temperature profiles are derived from downwelling radiance measurements collected by the onboard AERI, and have root-mean-square errors of less than 1.0 K in clearsky conditions up to 5 km AGL when compared with radiosonde temperature profiles (Turner and Löhnert 2014).…”