Multi-Level In-Situ Measurements During a Direct Hit by a Significant Tornado

The Enhanced Fujita (EF) Scale was developed specifically for the estimation of wind speeds based on damage caused by tornado winds. Recently, the question has arisen as to whether or not the EF Scale can be reliably used in reverse to predict the amount of wind damage based on measured or estimated hurricane wind speeds, particularly when wind action is co-mingled with severe storm-surge action and evidence of the exact level of wind damage is obliterated. In considering such use of the EF Scale, we examine separately its two major components: (1) Degrees of Damage and (2) ranges of tornado wind speeds associated with these damage levels. Our experience suggests that, in general, the EF Scale properly describes the overall progression of damage with increasing wind speeds in both tornadoes and hurricanes; however, the wind speeds associated with various degrees of damage are generally expected to be different for structures exposed to hurricanes and tornadoes. Use of the EF Scale to predict hurricane damage based on peak wind speeds is therefore discouraged. DEVELOPMENT AND USE OF THE EF SCALE The original Fujita Scale (Fujita, 1971) was designed as a wind speed scale to provide relative ratings of tornado intensity (McDonald et al., 2009). The scale divided intensity into multiple wind speed ranges and attempted to describe the type of damage one might expect for each wind speed range. This original Fujita Scale did not account for differences in the resistance of various structures. Prior to the 1970s, there was little understanding of the true magnitudes of maximum wind speeds in tornadoes due to a scarcity of tornado wind speed measurements. Engineeringoriented investigations of tornado damage subsequently highlighted the importance of

Development of a simple equivalent tornado wind profile for structural design and evaluation

The U.S. tornado record is subject to inhomogeneities that are due to inconsistent practices in counting tornadoes, assessing their damage, and measuring pathlength and path width. Efforts to improve the modern tornado record (1950–2012) have focused on the following: 1) the rationale for removing the years 1950–52, 2) identification of inconsistencies in F0, F1, and F2 counts based on implementation of the Fujita scale (F scale) and Doppler radar, 3) overestimation of backward-extrapolated F-scale intensity, and 4) a change in path-width reporting from mean width (1953–94) to maximum width (1995–2012). Unique adjustments to these inconsistencies are made by analyzing trends in tornado counts, comparing with previous studies, and making an upward adjustment of tornadoes classified by mean width to coincide with those classified by maximum width. Such refinements offer a more homogeneous tornado record and provide the opportunity to better evaluate climatological trends in significant (F/EF2–F/EF5) tornado activity. The median EF-scale (enhanced Fujita scale) wind speeds Vmed have been adopted for all significant tornadoes from 1953 to 2012, including an adjustment for overestimated intensities from 1953 to 1973. These values are used to calculate annual mean kinetic energy, which shows no apparent trend. The annual mean maximum path width from 1953 to 2012 (adjusted upward from 1953 to 1994 to obtain a common lower threshold), however, displays an increasing trend. Also, the EF-scale median wind speeds are highly correlated with . The quantity (Vmed × PWmax)2 is proposed as a tornado destruction index, and, when calculated as an annual cumulative value, the three largest years are 2007, 2008, and 2011.

Supercell thunderstorms are simulated using an idealized numerical model to analyze the effects of modifications to the environmental low-level wind profile on near-surface rotation. Specifically, the orientation, magnitude, and depth of the low-level vertical wind shear are modified in several suites of experiments and compared to control simulations with no vertical wind shear in the prescribed layer. The overall morphology of the simulated supercells is highly sensitive to even shallow changes in the low-level wind profile. Moreover, maximum near-surface vertical vorticity varies as the low-level wind profile is modified. The results suggest this is principally a consequence of the degree to which favorable dynamic forcing of negatively buoyant outflow is superimposed upon the near-surface circulation maximum. Simulations with easterly shear and weaker storm-relative winds over the depth of the gust front promote forward-surging outflow and smaller separation between the near-surface circulation maximum and the mesocyclone aloft compared with other hodograph shapes. This promotes near-surface vertical vorticity intensification in these simulations. Similar trends in near-surface vertical vorticity as a function of low-level shear orientation are observed for varying shear-layer depths and bulk-shear magnitudes over the shear layer. The degree to which specific hodograph shapes promote strong near-surface rotation may vary with different deep-layer wind profiles or thermodynamic environments from those simulated here; however, this study concludes that favorable positioning of the near-surface circulation maximum and mesocyclone aloft are a necessary condition for supercell tornadogenesis and this positioning may be modulated by the low-level wind profile.

بررسی پارامترسازی عمق لایه پایدار شبانه و تاثیر آن در آلودگی هوای یک منطقه شهری با توپوگرافی پیچیده (تهران)

This study examines the spatial and temporal distributions of short-term heavy rain (SHR) in the middle Yangtze River basin (MYRB) in the summers of the past decade. SHR events are most frequent during the annual Meiyu periods, significantly contributing to total precipitation. Additionally, these events generally last longer and tend to peak at night. The occurrence of SHR events decrease from southeast to northwest, influenced by the monsoonal flow and the small-scale terrain. Moisture convergence prior to Meiyu SHR events is predominantly influenced by both southerly and easterly winds below 700 hPa. Frequent low-level jets and quasi-steady cyclonic circulation lead to strong southerly winds prevailing over the eastern MYRB, while weaker easterly winds dominate in the west. Wind profiles derived from wind profile radar products illustrate the preceding changes in wind speed, wind directions, and vertical wind shear below 4 km above ground level (AGL), as well as the timing of these changes. In the plain area of southeastern MYRB, accelerated southwesterlies are observed 3 to 4 hours before SHR events, accompanied by an intensification of southerly winds near the boundary layer top 2 hours prior. Within the hour leading up to the SHR events, wind speeds sharply rise to their peak. In front of the mountains in west MYRB, southwesterlies strengthen 5 hours in advance but then weaken as they shift to northerlies. Just before the SHR events, however, reinforced northerlies occur near the surface. In the mountainous region of western MYRB, while changes in wind speed are minimal due to topographic blocking, the frequency of southeasterly components below 2 km AGL significantly increases 4 hours before SHR events. The preceding timing of significant vertical wind shear coincides with the increase in wind speed and the change in wind direction. Understanding the detailed characteristics of wind profiles preceding the SHR events during the Meiyu seasons can provide valuable insights for localized severe weather early warning systems. 

Performance-based engineering (PBE) is a methodology that requires specification on a range of performances or target reliabilities for structures of different importance. Information on these ‘performance levels’ require a probabilistic assessment of the potential factors that may influence a design, including information on the hazard, load, resistance, loss estimates, expert opinion and public perception. This paper describes one such probabilistic assessment in the development of empirically-based fragility functions for tornadoes using damage assessment data and a tornado wind field model for the 22 May 2011 Joplin, MO tornado. The damage assessment data was collected during field surveys of more than 1,240 structures in the aftermath of the tornado, using provisions of the Enhanced Fujita (EF) Scale to assess the damage. The wind field model was developed from the tree-fall patterns noted in the damage path of the tornado. Fragility functions were developed for the Degrees of Damage (DOD) associated with One- and Two-Family Residences in the EF Scale. The empiricallyderived fragility functions were progressive in nature, with median wind speeds varying from 33.6 m/s for initiation of visible damage to 85.2 m/s for complete destruction. These functions were compared to existing fragility functions for straightline winds to evaluate potential differences in failure mechanisms for structures exposed to tornadoes. Wind speeds associated with the median failure probability were used to estimate load factors, defined as the square of the ratio of the straightline wind speed to the tornado wind speed. Structures tended to fail at lower wind speeds in tornadoes than in straightline winds, with load factors between 1.32 and 1.51. A fragility assessment in the context of PBE naturally requires attribution and quantification of all uncertainties. Uncertainties in the both the damage and wind speed estimation in the development of fragilities are quantified and assessed using Monte Carlo methods. Preliminary results show variance in fragility parameters is higher for higher damage states but all damage states have relatively low coefficients of variation.

Performance-based engineering (PBE) is a methodology that requires specification on a range of performances or target reliabilities for structures of different importance. Information on these ‘performance levels’ require a probabilistic assessment of the potential factors that may influence a design, including information on the hazard, load, resistance, loss estimates, expert opinion and public perception. This paper describes one such probabilistic assessment in the development of empirically-based fragility functions for tornadoes using damage assessment data and a tornado wind field model for the 22 May 2011 Joplin, MO tornado. The damage assessment data was collected during field surveys of more than 1,240 structures in the aftermath of the tornado, using provisions of the Enhanced Fujita (EF) Scale to assess the damage. The wind field model was developed from the tree-fall patterns noted in the damage path of the tornado. Fragility functions were developed for the Degrees of Damage (DOD) associated with One- and Two-Family Residences in the EF Scale. The empiricallyderived fragility functions were progressive in nature, with median wind speeds varying from 33.6 m/s for initiation of visible damage to 85.2 m/s for complete destruction. These functions were compared to existing fragility functions for straightline winds to evaluate potential differences in failure mechanisms for structures exposed to tornadoes. Wind speeds associated with the median failure probability were used to estimate load factors, defined as the square of the ratio of the straightline wind speed to the tornado wind speed. Structures tended to fail at lower wind speeds in tornadoes than in straightline winds, with load factors between 1.32 and 1.51. A fragility assessment in the context of PBE naturally requires attribution and quantification of all uncertainties. Uncertainties in the both the damage and wind speed estimation in the development of fragilities are quantified and assessed using Monte Carlo methods. Preliminary results show variance in fragility parameters is higher for higher damage states but all damage states have relatively low coefficients of variation.

Assessment of wind resources in two parts of Northeast Brazil with the use of numerical models

In this study, the WRF (Weather Research and Forecasting) model was used to simulate and investigate diurnal and annual variations of wind speed and wind power density over Southern Vietnam at 2‐km horizontal resolution for two years (2016 and 2017). The model initial and boundary conditions are from the National Centers for Environmental Prediction (NCEP) Final Analyses (FNL). Observation data for two years at 20 m height at Bac Lieu station were used for model bias correction and investigating diurnal and annual variation of wind speeds. The results show that the WRF model overestimates wind speeds. After bias correction, the model reasonably well simulates wind speeds over the research area. Wind speed and wind power density show much higher values at levels of 50–200 m above ground levels than near ground (20 m) level and significantly higher near the coastal regions than inland. Wind speed has significant annual and diurnal cycles. Both annual and diurnal cycles of wind speeds were well simulated by the model. Wind speed is much stronger during daytime than at nighttime. Low-level wind speed reaches the maximum at about 14 LT to 15 LT when the vertical momentum mixing is highly active. Wind speeds over the eastern coastal region of Southern Vietnam are much stronger in winter than in summer due to two main reasons, including (1) stronger large-scale wind speed in winter than in summer and (2) funnel effect creating a local maximum wind speed over the nearshore ocean which then transports high-momentum air inland in winter.

: Mean low-level temperature and wind profiles were constructed for 44 cases of free convection using the data for O'Neill, Nebraska, during July and August 1956. Based upon the expression for the normalized logarithmic wind shear first suggested by Ellison and later refined by Panofsky, a theoretical formula for the Deacon profile number as a function of the Richardson number was derived, and values of the Deacon profile number were computed. One of the parameters entering into this theoretical formula is the ratio of the eddy diffusivities for heat and momentum. This parameter was, in turn, computed from Priestley's expression for the dimensionless heat flux for free-convective cases. In using observed wind data from the mean profile in order to verify the theoretical computations of beta (Deacon profile number), some marked discrepancies occurred above the 100 cm level. These were due to inconsistent wind speed readings, and it was necessary to employ control data based on neutral profiles to correct the wind speeds. When this was done, the theoretical and observed Deacon profile numbers were in very good agreement.

A 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.

The ASCE Standardized Reference Evapotranspiration Equation expects the weather station wind speed data to represent that occurring at a height of 2 m over and downwind of a smooth measurement surface such as clipped grass. The Task Committee on the Standardized Equation provided guidance for adjusting wind speed measured at height other than 2 m, or, for situations when the wind speed is measured over and downwind of 0.5 m alfalfa. The latter adjustment attempts to account for the effects of both grass and alfalfa crop characteristics (height, roughness) on the wind profile. A more physically representative approach to adjust wind speeds at various heights and various weather measurement surface conditions to equivalent wind speed at 2 m height over clipped grass is tested. Wind speeds were simultaneously measured during the 2008 growing season at 2-m and 3-m heights above ground surface over variable height alfalfa at two Colorado Agricultural Meteorological Network (CoAgMet) electronic weather stations and at the research alfalfa lysimeter installation at the Colorado State University Arkansas Valley Research Center. These wind speed measurements were adjusted to estimated wind speed at 2 m over grass, and compared.

Profiles of mean wind speed obtained from a 1420-ft tower are analyzed on the basis of similarity theory to determine the relationship of profile shape to lapse rate structure. A total of 274 profiles representing four observation times (0600, 1000, 1400, and 1800 CST) are used in the analysis. Thirty minute averages of the wind speed are taken at eleven levels on the tower at these observation times. The wind speed values are normalized by means of the friction velocity as well as a reference height velocity computed at the lowest observation level, and profiles for these wind speeds are grouped according to profile shape characteristics. For each group, an average profile is computed and the vertical variation of mean wind speed is compared to a logarithmic or power law profile form. Results of the study indicate that the mean wind profiles are dependent on the lapse rate structure and can be divided into “non-inversion” (lapse rates greater than isothermal) and “inversion” (lapse rates less than isothermal) profiles. For near adiabatic or slightly unstable lapse rates (mid-day non-inversion profiles), the logarithmic wind law represents the data well to a height of 300 to 400 ft. Above this height the wind speed is nearly constant. The more stable non-inversion wind profiles (lapse rates varying from about 2F to 5F per 1000 ft) generally show greater wind shears. These profiles are represented with a power law. The wind profiles associated with lapse rates containing inversions are highly variable and depend on the nature of the inversion present. There appears to be an inter-dependence of vertical shear and inversion lapse rates for these profiles.

Composite profiles of temperature, humidity and wind are constructed for severe thunderstorms that formed over central Alberta, Canada, during the period 1967–2000. Storms were divided into three categories consisting of 13 non-tornadic storms which produced hail ≥ 3 cm in diameter but no reported tornadoes, 61 weak tornadoes (F0 and F1) and 13 significant tornadoes (F2–F4). All three composites showed potential instability through most of the sounding. However, thermodynamic parameters did not discriminate among the three categories. In contrast, however, composite hodographs did show noticeable differences among the three groups. The hodograph for the significant tornado composite exhibited strong low-level veering winds having 0–3 km helicity of 65 m2 s−2 and southwest winds of about 20 m s−1 in the mid- to upper levels. The hodograph for the weak tornado composite showed weak low-level veering winds (0–3 km helicity of 17 m2 s−2) and much lighter winds overall compared with the other two composites. The non-tornado composite hodograph had nearly unidirectional southwest winds of about 15 m s−1 in the mid- to upper levels. The significant tornadic storms were similar to typical mid-latitude supercell hodographs.