Tornado Intensity Research Articles

Investigating the Surrey tornado of 21 December 2019

On 21 December 2019, a tornado hit several towns and villages in Surrey, UK. This article details the findings of a damage site investigation by the Tornado and Storm Research Organisation and discusses the synoptic and mesoscale settings of the event. The tornado lasted for approximately 13min, reached a strength of T3 on the International Tornado Intensity Scale, and produced a damage track of length ~11.35km and maximum width ~60m.

Seamless Detection of Cutoff Lows and Preexisting Troughs

AbstractWe propose a new scheme based on geopotential height fields to detect cutoff lows starting in the preexisting trough stage. The intensity and scale derived from the proposed scheme will allow for a better understanding of the cutoff low life cycle. These cutoff lows often accompany mesoscale disturbances, causing adverse weather-related events, such as intense torrential rainfall and/or tornadoes. The proposed scheme quantifies the geometric features of a depression from its horizontal height profile. The height slope of a line intersecting the depression bottom and the nearest tangential point (optimal slope) locally indicates the intensity and scale of an isolated depression.The strength of the proposed scheme is that, by removing a local background height slope from a geopotential height field, the cutoff low and its preexisting trough are seamlessly detected as an identical depression. The distribution maps for the detected cutoff lows and preexisting troughs are illustrated along with their intensities, sizes, and local background flows estimated from snapshot height fields. We conducted climatological comparisons of cutoff lows to determine the utility of the proposed scheme.

High-resolution observations of microscale influences on a tornado track using Unpiloted Aerial Systems (UAS)

AbstractTopography can have a significant influence on tornado intensity and direction by altering the near-surface inflow. However, past research involving topographic influence on tornadoes has shown significant variety in investigative approaches and conclusions. This study uses Unpiloted Aerial Systems (UAS)-based high-resolution imagery, UAS-based 3D-modeling products, and correlation analyses to examine topographical influences on a portion of the 01 May 2018 Tescott, Kansas EF3 tornado. Two new metrics, Visible Difference Vegetative Index (VDVI) gap and (VDVI) aspect ratio, are introduced to quantify damage severity using UAS-based imagery and elevation information retrieved from a UAS-based digital surface model (DSM). Areas of enhanced scour are seen along the track in areas of local elevation maxima. Correlation analysis shows that damage severity, as measured by VDVI gap and VDVI aspect ratio, are both well correlated with increasing elevation. VDVI gap is only weakly correlated with slope, while VDVI aspect ratio is not correlated with slope. These findings are statistically significant at p < 0.05. As the tornado weakened in intensity, the path became non-linear, traversing between two local elevation maxima. It is hypothesized that fast-moving intense flow formed and weakened as elevation increased over the short spatial distance. This research shows topography and surface conditions are two of many important variables that should be considered when performing tornado-damage site investigations. It also illustrates the importance of UASs in detailed tornado analysis. VDVI gap and VDVI aspect ratio can provide insight into damage severity as a function of topography.

Controls of Quasi-Linear Convective System Tornado Intensity

AbstractAlthough tornadoes produced by quasi-linear convective systems (QLCSs) generally are weak and short lived, they have high societal impact due to their proclivity to develop over short time scales, within the cool season, and during nighttime hours. Precisely why they are weak and short lived is not well understood, although recent work suggests that QLCS updraft width may act as a limitation to tornado intensity. Herein, idealized simulations of tornadic QLCSs are performed with variations in hodograph shape and length as well as initiation mechanism to determine the controls of tornado intensity. Generally, the addition of hodograph curvature in these experiments results in stronger, longer-lived tornadic-like vortices (TLVs). A strong correlation between low-level mesocyclone width and TLV intensity is identified (R2 = 0.61), with a weaker correlation in the low-level updraft intensity (R2 = 0.41). The tilt and depth of the updraft are found to have little correlation to tornado intensity. Comparing QLCS and isolated supercell updrafts within these simulations, the QLCS updrafts are less persistent, with the standard deviations of low-level vertical velocity and updraft helicity approximately 48% and 117% greater, respectively. A forcing decomposition reveals that the QLCS cold pool plays a direct role in the development of the low-level updraft, providing the benefit of additional forcing for ascent while also having potentially deleterious effects on both the low-level updraft and near-surface rotation. The negative impact of the cold pool ultimately serves to limit the persistence of rotating updraft cores within the QLCS.

Смерчи в российских регионах

New data are presented on tornadoes over land in Russia for the period of 1900–2018 based on various sources. In total, information on 1763 tornadoes was collected, including 993 tornadoes from eyewitness reports on tornado passage and/or associated impacts and 770 tornadoes from satellite data on tornado-induced windthrows. Both single tornadoes and tornado outbreaks, the cases of formation of several tornadoes within one meso- or synoptic-scale system, were reported. On average for 2009– 2018, more than 100 tornadoes are observed in Russia per year, including 15 significant tornadoes (with a wind speed >50 m/s) and one intense tornado (with a wind speed >70 m/s). In some years, these rates can be significantly higher and reach 342, 52, and three tornadoes per year, respectively. Tornadoes are observed on about 41 days per year, up to 68 days per annum in some years. The frequency of occurrence of tornadoes of different categories and the probability of their passage over a point on the ground were estimated. These estimates can be further used to assess a risk of tornado-hazardous situations. The general underestimation of the number of tornadic events in routine meteorological observations and existent statistics is discussed.

Synoptic characteristics of an extreme weather event: The tornadic waterspout in Tivat (Montenegro), on June 9, 2018

Recently Montenegro has often been faced with extreme weather events. The aim of this paper is to provide a detailed synoptic analysis of a severe weather event, a waterspout, and to confirm an indication that in most cases such events could potentially be forecasted, which is of great practical significance, since human lives and property can be saved. The paper presents the research results of synoptic and mesoscale weather conditions which created a favourable meteorological environment for a waterspout development in Tivat (Montenegrin coast) on June 9, 2018, around 01 UTC (03 CET). Based on field survey analysis, the rating of tornado intensity by the Fujita scale (F-scale) has been done by assessing the damage. The synoptic type for this situation was CLOSED-SW and was determined by a detailed examination of atmospheric circulation. The results presented in the manuscript can help decision makers in Montenegro to take certain adaptation measures (above all, in tourism and construction) in order to mitig te the negative consequences of weather extremes.

WSR-88D Tornado Intensity Estimates. Part II: Real-Time Applications to Tornado Warning Time Scales

AbstractA sample of damage-surveyed tornadoes in the contiguous United States (2009–17), containing specific wind speed estimates from damage indicators (DIs) within the Damage Assessment Toolkit dataset, were linked to radar-observed circulations using the nearest WSR-88D data in Part I of this work. The maximum wind speed associated with the highest-rated DI for each radar scan, corresponding 0.5° tilt angle rotational velocity Vrot, significant tornado parameter (STP), and National Weather Service (NWS) convective impact-based warning (IBW) type, are analyzed herein for the sample of cases in Part I and an independent case sample from parts of 2019–20. As Vrot and STP both increase, peak DI-estimated wind speeds and IBW warning type also tend to increase. Different combinations of Vrot, STP, and population density—related to ranges of peak DI wind speed—exhibited a strong ability to discriminate across the tornado damage intensity spectrum. Furthermore, longer duration of high Vrot (i.e., ≥70 kt) in significant tornado environments (i.e., STP ≥ 6) corresponds to increasing chances that DIs will reveal the occurrence of an intense tornado (i.e., EF3+). These findings were corroborated via the independent sample from parts of 2019–20, and can be applied in a real-time operational setting to assist in determining a potential range of wind speeds. This work provides evidence-based support for creating an objective and consistent, real-time framework for assessing and differentiating tornadoes across the tornado intensity spectrum.

Environmental Controls on the Climatological Scaling of Tornado Frequency with Intensity

AbstractThe scaling of U.S. tornado frequency with enhanced Fujita (EF)-rated intensity is examined for the range EF1–EF3. Previous work has found that tornado frequency decreases exponentially with increasing EF rating and that many regions around the world show the same exponential rate of decrease despite having quite different overall tornado frequencies. This scaling is important because it relates the frequency of the most intense tornadoes to the overall tornado frequency. Here we find that U.S. tornado frequency decreases more sharply with increasing intensity during summer than during other times of the year. One implication of this finding is that, despite their rarity, when tornadoes do occur during the cool season, the relative likelihood of more intense tornadoes is higher than during summer. The environmental driver of this scaling variability is explored through new EF-dependent tornado environmental indices (TEI-EF) that are fitted to each EF class. We find that the sensitivity of TEI-EF to storm relative helicity (SRH) increases with increasing EF class. This increasing sensitivity to SRH means that TEI-EF predicts a slower decrease in frequency with increasing intensity for larger values of SRH (e.g., cool season) and a sharper decrease in tornado frequency in summer when wind shear plays a less dominant role. This explanation is also consistent with the fact that the fraction of supercell tornadoes is smaller during summer.

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

AbstractThis study presents an investigation into relationships among topographic elevation, surface land cover, and tornado intensity using rapid scan, mobile Doppler radar observations of four tornadoes from the U.S. Central Plains. High spatiotemporal resolution observations of tornadic vortex signatures from the radar’s lowest elevation angle data (in most cases ranging from ~100 to 350 m above ground level) are coupled with digital elevation model (DEM) and 2011 National Land Cover Database (NLCD) data using a geographic information system (GIS). The relationships between 1) tornado intensity and topographic elevation or surface roughness and 2) changes in tornado intensity and changes in topographic elevation or surface roughness are investigated qualitatively, and statistical relationships are quantified and analyzed using a bootstrap permutation method for individual case studies and all cases collectively. Results suggest that there are statistically significant relationships for individual cases, but the relationships defy generalization and are different on a case-by-case basis, which may imply that they are coincidental, indicating a null correlation.

What is the Intrinsic Predictability of Tornadic Supercell Thunderstorms?

AbstractA 25-member ensemble of relatively high-resolution (75-m horizontal grid spacing) numerical simulations of tornadic supercell storms is used to obtain insight on their intrinsic predictability. The storm environments contain large and directionally varying wind shear, particularly in the boundary layer, large convective available potential energy, and a low lifting condensation level. Thus, the environments are extremely favorable for tornadic supercells. Small random temperature perturbations present in the initial conditions trigger turbulence within the boundary layers. The turbulent boundary layers are given 12 h to evolve to a quasi–steady state before storms are initiated via the introduction of a warm bubble. The spatially averaged environments are identical within the ensemble; only the random number seed and/or warm bubble location is varied. All of the simulated storms are long-lived supercells with intense updrafts and strong mesocyclones extending to the lowest model level. Even the storms with the weakest near-surface rotation probably can be regarded as weakly tornadic. However, despite the statistically identical environments, there is considerable divergence in the finescale details of the simulated storms. The intensities of the tornado-like vortices that develop in the simulations range from EF0 to EF3, with large differences in formation time and duration also being exhibited. The simulation differences only can be explained by differences in how the initial warm bubbles and/or storms interact with turbulent boundary layer structures. The results suggest very limited intrinsic predictability with respect to predicting the formation time, duration, and intensity of tornadoes.

Tornadoes in Portugal: An Overview

From 2000 onwards, a systematic documentation of tornado reports was maintained at the Portuguese Meteorological Service (IPMA). The characteristics of 195 tornado events over land and at sea off the coast of mainland Portugal, Azores, and Madeira Islands, reported until 2020, were compiled into a new tornado database for Portugal. Each event was identified through the direct observation of the vortex, photography or video footage, or eyewitness descriptions, as well as by site surveys, including the interpretation of traces of the tornado found on the ground. For events after 2006, each data record was complemented with Doppler radar observations and derived products, which allowed a tornado type (TT) classification. The synoptic regimes and atmospheric environments favoring each type were identified using observations and numerical weather prediction model data. Results showed that tornadoes over Portugal were more frequent during autumn, winter, and spring. It was found that the occurrence of more than one tornado on the same day was frequent. The most intense tornadoes, classified as F3 intensity, were spawned by supercells, but a large proportion of weaker, shorter-lived, but still damaging tornadoes were found to be spawned in association with quasi linear convective systems (QLCS).

Observed Relationship between Tornado Intensity and Pretornadic Mesocyclone Characteristics

AbstractIn a previous study, idealized model simulations of supercell thunderstorms were used to demonstrate support of the hypothesis that wide, intense tornadoes should form more readily out of wide, rotating updrafts. Observational data were used herein to test the generality of this hypothesis, especially to tornado-bearing convective morphologies such as quasi-linear convective systems (QLCSs), and within environments such as those found in the southeastern United States during boreal spring and autumn. A new radar dataset was assembled that focuses explicitly on the pretornadic characteristics of the mesocyclone, such as width and differential velocity: the pretornadic focus allows us to eliminate the effects of the tornado itself on the mesocyclone characteristics. GR2Analyst was used to manually analyze 102 tornadic events during the period 27 April 2011–1 May 2019. The corresponding tornadoes had damage (EF) ratings ranging from EF0 to EF5, and all were within 100 km of a WSR-88D. A key finding is that the linear regression between the mean, pretornadic mesocyclone width and the EF rating of the corresponding tornado yields a coefficient of determination (R2) value of 0.75. This linear relationship is higher for discrete (supercell) cases (R2 = 0.82), and lower for QLCS cases (R2 = 0.37). Overall, we have found that pretornadic mesocyclone width tends to be a persistent, relatively time-invariant characteristic that is a good predictor of potential tornado intensity. In contrast, the pretornadic mesocyclone intensity (differential velocity) tends to exhibit considerable time variability, and thus would offer less reliability in anticipating tornado intensity.

Damage Analysis of Three Long-Track Tornadoes Using High-Resolution Satellite Imagery

Remote sensing of tornado damage can provide valuable observations for post-event surveys and reconstructions. The tornadoes of 3 March 2019 in the southeastern United States are an ideal opportunity to relate high-resolution satellite imagery of damage with estimated wind speeds from post-event surveys, as well as with the Rankine vortex tornado wind field model. Of the spectral metrics tested, the strongest correlations with survey-estimated wind speeds are found using a Normalized Difference Vegetation Index (NDVI, used as a proxy for vegetation health) difference image and a principal components analysis emphasizing differences in red and blue band reflectance. NDVI-differenced values across the width of the EF-4 Beauregard-Smiths Station, Alabama, tornado path resemble the pattern of maximum ground-relative wind speeds across the width of the Rankine vortex model. Maximum damage sampled using these techniques occurred within 130 m of the tornado vortex center. The findings presented herein establish the utility of widely accessible Sentinel imagery, which is shown to have sufficient spatial resolution to make inferences about the intensity and dynamics of violent tornadoes occurring in vegetated areas.

Numerical investigation on the aerodynamics and dynamics of a high-speed train passing through a tornado-like vortex

This paper investigates the interaction between a high-speed train and a tornado-like vortex numerically using Detached Eddy Simulation and modal analysis. Key parameters related with the train operation safety, including the operation path, tornado intensity, and train speed, are considered. It is found that the train is subjected to larger aerodynamic forces and moments when the train operates along with the rotating vortex flow, and the head car is more dangerous comparing to the middle and tail cars. Coefficients of derailment and overturning are calculated with vehicle dynamics, and used to assess the operation safety of the train. It is also found that derailment is the main accident that might be caused by tornado, as the resultant lateral force acts at the lower half of the train. Moreover, resonant response of the train dynamic system to the aerodynamic loads caused by the tornado is observed. For the train and the dynamic parameters studied herein, the speed range for the occurrence of resonance is between 160km∕h and 250km∕h, within which the coefficients of derailment and overturning reach their maximum values. And the resonant phenomenon is independent with tornado intensity. The numerical results obtained in the current study are of significance for the assessment of operation safety while moving objects encountering tornadoes.

The Effect of Surface Drag Strength on Mesocyclone Intensification and Tornadogenesis in Idealized Supercell Simulations

AbstractA suite of six idealized supercell simulations is performed in which the surface drag coefficient Cd is varied over a range of values from 0 to 0.05 to represent a variety of water and land surfaces. The experiments employ a new technique for enforcing a three-force balance among the pressure gradient, Coriolis, and frictional forces so that the environmental wind profile can remain unchanged throughout the simulation. The initial low-level mesocyclone lowers toward the ground, intensifies, and produces a tornado in all experiments with Cd ≥ 0.002, with the intensification occurring earlier for larger Cd. In the experiment with Cd = 0, the low-level mesocyclone remains comparatively weak throughout the simulation and does not produce a tornado. Vertical cross sections through the simulated tornadoes reveal an axial downdraft that reaches the ground only in experiments with smaller Cd, as well as stronger corner flow in experiments with larger Cd. Material circuits are initialized enclosing the low-level mesocyclone in each experiment and traced backward in time. Circulation budgets for these circuits implicate surface drag acting in the inflow sector of the supercell as having generated important positive circulation, and its relative contribution increases with Cd. However, the circulation generation is similar in magnitude for the experiments with Cd = 0.02 and 0.05, and the tornado in the latter experiment is weaker. This suggests the possible existence of an optimal range of Cd values for promoting intense tornadoes within our experimental configuration.

Using Overshooting Top Area to Discriminate Potential for Large, Intense Tornadoes

AbstractRecent work established strong links between storm updraft width and the tornado intensity, suggesting that updraft width could be used to gauge potential tornado intensity. It was also posited that overshooting top area (OTA) could be used as an analog for updraft width and, thus, as a means to assess potential tornado intensity in observed storms. The implementation of new high‐resolution GOES‐R series satellites presents a unique opportunity to investigate these findings in severe weather observations. Herein, a method using GOES‐16 longwave infrared satellite data to quantify OTA of tornadic storms is explored. A comparison between observed tornado strength and OTA yields a strong correlation (R2 = 0.54). These results show the potential of these quantifications to be used with real‐time observations of tornadic storms, irrespective of storm mode, seasonality, or geographic location, allowing forecasters to determine which storms pose the highest risk to life and property.

Tornado loss model of Oklahoma and Kansas, United States, based on the historical tornado data and Monte Carlo simulation

This paper introduces a new tornado loss assessment model for one-family and two-family wood frame residential buildings in the states of Kansas (KS) and Oklahoma (OK), United States. The model belongs to the group of Monte Carlo simulations and consists of five main modules: (1) tornado climatology; (2) tornado intensity and tornado track geometry; (3) exposure map; (4) fragility and vulnerability functions; and (5) financial module that is used to calculate the resulting damages. In addition, two analytical formulations of radial distribution of tangential velocity in tornado vortex were implemented in the tornado intensity module—Rankine Vortex (RV) and Modified Rankine Vortex (MRV) models. Using the MRV tornado model, the resulting mean aggregated annual expected losses for the return period of 1000 years are US$ 392,080,526.8 and US$ 641,281,336.0 for KS and OK, respectively. Although house prices in KS are higher than in OK, the higher occurrence of tornadoes in OK combined with the larger population of OK (i.e., larger density of houses) result in the increased tornado losses for OK. The losses derived using the MRV module are higher than the losses from the RV module due to the RV module underestimating the tornado tangential velocity. Furthermore, the losses predicted by the current model using the MRV formulation are less than the estimated losses from the previous version of the model (Romanic et al., 2016). The model is also tested for the case of a single tornado track positioned over suburban area of Oklahoma City, OK.

Fragility analysis of the roof structure of low-rise buildings subjected to tornado vortices

The fragility curves of roof failures for wood-frame low-rise building were developed in this study with the focus on the roof sheathing panels and roof-to-wall connections. Two different nail spacings for roof sheathing panels and three types of roof-to-wall connections were considered. This study focused on the first failure of roof sheathing panels without considering the sequential loss of the sheathing panels, and the unit failure of effective wind zones containing different numbers of roof-to-wall connections. The tornado simulator-based dataset was used for determination of the statistics of wind loads, without considering the translational motion effects of tornado vortices. The results show that the failure probability of roof is dependent on the proximity of a tornado to a building and the opening azimuth on walls. Additionally, the combined effects of pressure drop and aerodynamic interactions dominate the failure modes of roof system. The obtained fragility curves in this study can be used to estimate the maximum tangential wind speed of the tornado vortices and the local oncoming tangential wind speed at roof height. The estimation of tornado intensity using the present results is likely to approach the lower-bound of wind speed estimation in WSEC (2006) report.

Tornado-Resolving Ensemble and Probabilistic Predictions of the 20 May 2013 Newcastle–Moore EF5 Tornado

AbstractAn ensemble of 10 forecasts is produced for the 20 May 2013 Newcastle–Moore EF5 tornado and its parent supercell using a horizontal grid spacing of 50 m, nested within ensemble forecasts with 500-m horizontal grid spacing initialized via ensemble Kalman filter data assimilation of surface and radar observations. Tornadic circulations are predicted in all members, though the intensity, track, and longevity of the predicted tornado vary substantially among members. Overall, tornadoes in the ensemble forecasts persisted longer and moved to the northeast faster than the observed tornado. In total, 8 of the 10 ensemble members produce tornadoes with winds corresponding to EF2 intensity or greater, with maximum instantaneous near-surface horizontal wind speeds of up to 130 m s−1 and pressure drops of up to 120 hPa; values similar to those reported in observational studies of intense tornadoes. The predicted intense tornadoes all acquire well-defined two-cell vortex structure, and exhibit features common in observed tornadic storms, including a weak-echo notch and low reflectivity within the mesocyclone. Ensemble-based probabilistic tornado forecasts based upon near-surface wind and/or vorticity fields at 10 m above the surface produce skillful forecasts of the tornado in terms of area under the relative operating characteristic curve, with probability swaths extending along and to the northeast of the observed tornado path. When probabilistic swaths of 0–3- and 2–5-km updraft helicity are compared to the swath of wind at 10 m above the surface exceeding 29 m s−1, a slight northwestward bias is present, although the pathlength, orientation, and the placement of minima and maxima show very strong agreement.

High-Temporal Resolution Observations of the 27 May 2015 Canadian, Texas, Tornado Using the Atmospheric Imaging Radar

Abstract On 27 May 2015, the Atmospheric Imaging Radar (AIR) collected high-temporal resolution radar observations of an EF-2 tornado near Canadian, Texas. The AIR is a mobile, X-band, imaging radar that uses digital beamforming to collect simultaneous RHI scans while steering mechanically in azimuth to obtain rapid-update weather data. During this deployment, 20°-by-80° (elevation × azimuth) sector volumes were collected every 5.5 s at ranges as close as 6 km. The AIR captured the late-mature and decaying stages of the tornado. Early in the deployment, the tornado had a radius of maximum winds (RMW) of 500 m and exhibited maximum Doppler velocities near 65 m s−1. This study documents the rapid changes associated with the dissipation stages of the tornado. A 10-s resolution time–height investigation of vortex tilt and differential velocity is presented and illustrates an instance of upward vortex intensification as well as downward tornado decay. Changes in tornado intensity over periods of less than 30 s coincided with rapid changes in tornado diameter. At least two small-scale vortices were observed being shed from the tornado during a brief weakening period. A persistent layer of vortex tilt was observed near the level of free convection, which separated two layers with contrasting modes of tornado decay. Finally, the vertical cross correlation of vortex intensity reveals that apart from the brief instances of upward vortex intensification and downward decay, tornado intensity was highly correlated throughout the observation period.