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Downbursts are cold descending winds that develop from thunderstorm clouds and, after impingement on the ground, produce an intense low-level horizontal front characterized by an axisymmetric toroidal vortex structure. Surface roughness is a key factor in the characterization of mean and turbulent wind speed features of synoptic-scale stationary atmospheric boundary layer winds. The goal of the present research is to physically assess whether the same can apply to the surface layer produced during thunderstorms, which are non-stationary, highly time-transient, and spatially limited phenomena. Downburst-like flows were produced through the impinging jet technique at the WindEEE Dome, at Western University in Canada. Three different surfaces were tested, and an equivalent full-scale roughness length ( z 0 , eq ) was determined. Experimental records are made publicly available. The large geometric and kinematic scales produced high Reynolds numbers, which enabled us to classify the flow as "fully turbulent" and therefore representative of full-scale downbursts. Results indicate a weak dependency on the Reynolds number, which suggests no relevant flaws in extending the results to the natural environment. The overall wind speed maxima weakly depend on z 0 , whereas a sharp velocity decrease is observed beyond the radial position of the maxima with increasing z 0 . Surface roughness enhances the boundary layer separation and consequently elevates the height of maximum wind speed above the surface. Vertical profiles of the horizontal velocity return a quite clear nose shape. Turbulence intensity shows a C-like shape with maxima in the near proximity of the ground that increase with z 0 . [ABSTRACT FROM AUTHOR]

Downbursts are strong downdrafts that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. This study is part of the comprehensive experimental research on downburst outflows produced as large-scale impinging jets in the WindEEE Dome simulator at Western University, Canada. The 2800 tests carried out form the largest database of experimental measurements on downburst winds developed thus far, which is made available to the public in its whole and described in detail in a complementary study. Therefore, the current manuscript merely focuses on the data post-processing outcomes and interpretation of results from a selected subset of measurements. Impinging jets are here simulated as transient phenomena in which velocity time series are characterized by a sudden ramp-up of velocity, followed by the velocity peak, a short statistically stationary region, and the final velocity slowdown, as it is expected to occur in the actual downbursts. A dominant velocity peak that was systematically observed in all velocity records is associated with the radial advection of the primary vortex in the outflow. Depending on the radial distance from the downdraft, the primary vortex was sometimes preceded by a secondary, much smaller, vortex close to the surface. Vertical profiles of mean velocity and turbulence intensity are for the first time characterized through the extent of a downburst-like event in the spatiotemporal domain. Particularly, these profiles rapidly change in relation to the passage of the primary vortex and consequent variation of the surface layer thickness. This study lays out a foundation for an experimental model of non-stationary downburst outflows to come. Article highlights: Large experimental campaign and public database of measurements on transient downburst-like impinging jets. Characterization of the spatiotemporal evolution of wind speed and turbulence profiles. Measuring the effects of the primary and secondary vortices in the outflow on velocity and turbulence profiles. [ABSTRACT FROM AUTHOR]

This paper describes the dataset of measurements collected and published in the context of the comprehensive experimental campaign on downburst-like outflows that was performed at the WindEEE Dome at Western University, Canada. Downbursts are strong downdrafts of air that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. Downbursts are here simulated as transient phenomena produced by large-scale impinging jet. Two jet velocities were adopted in the experiments. The three-component velocity measurements were recorded using 7 Cobra probes mounted on a vertical stiff mast and displaced at 10 radial positions in respect to the downdraft centerline. For every radial position, each experiment with the same initial condition was repeated 20 times to inspect the deterministic features of the signal. Overall, the total of 2800 tests (2 jet velocities × 20 repetitions × 10 radial positions × 7 heights) represent one of the largest experimental campaigns on downburst winds carried out in a wind tunnel facility thus far. Measurement(s) atmospheric wind speed Technology Type(s) Cobra Probe Factor Type(s) wind velocity Sample Characteristic - Organism wind flow Sample Characteristic - Environment wind tunnel [ABSTRACT FROM AUTHOR]

Downburst winds are strong downdrafts of cold air that embed into the atmospheric boundary layer (ABL) and produce intense horizontal outflow upon impingement on the ground. They are highly transient and three-dimensional extreme wind phenomena with a limited spatiotemporal structure that often makes the anemometric measurements in nature inadequate for reconstructing their complex flow fields. In the framework of the project THUNDERR, an experimental campaign on downburst outflows has been carried out at the WindEEE Dome at Western University, Canada. The present study analyzes the three-dimensional interaction between downburst (DB) outflows produced as large-scale impinging jets and ABL winds. Most experimental, numerical and analytical models in the literature neglect this flow interplay or treat it in an oversimplistic manner through a vector superposition. We found that the generated near-surface outflow is asymmetric, and a high-intensity wind zone develops at the interface between DB and ABL winds. The time variability of the leading edge of the outflow was investigated by synchronizing all wind measurements across the testing chamber. The three-dimensional flow structure was studied using a refined grid of Cobra probes that sampled the flow at high frequencies. The passage of the primary vortex produced a significant decrease in the height of maximum radial wind speed, predominantly in the ABL-streamwise direction. The turbulence intensity was the highest in the region where DB propagates into oppositely directed ABL winds. [ABSTRACT FROM AUTHOR]

Thunderstorm winds are cold descending gravity currents whose impingement on the ground creates strong radial outflows with maximum wind speeds in the near-ground region. They represent one of the greatest hazards for natural and built environment as well as one of the deadliest phenomena all over the world. This study carries on the post-processing analyses of the downburst experimental campaign performed at the WindEEE Dome, at Western University in Canada, in the context of the ERC project THUNDERR. While a former study presented the interaction between downburst and atmospheric boundary layer (ABL) winds, here the focus is on the influence exerted by the thunderstorm cloud translation. This was experimentally replicated at large scale by means of impinging jet technique where the jet axis was inclined to a non-zero angle with respect to the vertical. Finally, the inclusion of background ABL wind allowed to reconstruct the complete three-dimensional and non-stationary nature of the phenomenon. The outflow radial symmetry is lost in case of inclined jet axis. This leads to an intensification of the front-wind side and weakening of the rear-wind side, where the entrainment of the counter-directed ABL wind, and consequent flow speed-up, are not as pronounced as in the vertical-axis case. The non-linearity of the complex interaction between downburst, ABL flow and cloud translation is proven and quantified. Vertical profiles of mean wind speed and turbulence intensity are discussed in relation to the mutual interaction among flow components. [ABSTRACT FROM AUTHOR]

This paper focuses on analyzing the ice and wind conditions for a high voltage transmission line system going over mountainous terrain in British Columbia, Canada. After analyzing sixteen surrounding weather stations, an Artificial Neural Network (ANN) is performed for wind speed predictions, the Inverse Distance Weighted Interpolation method (IDW) is used for temperature estimations, and the K Nearest Neighbor Imputation (KNNI) is performed for precipitation rate on the unsampled site of interest. In-cloud icing is estimated using a simple empirical equation proposed by Makkonen & Ahti (1995), in addition to the ice accretion model proposed by Makkonen (2000). The weather data from the described procedure is compared with data from a Weather Research and Forecast model (WRF) reported in previous studies. The wind speed and ice accretion data are fitted to the Weibull and Generalized Pareto distributions, respectively, and five hundred years of data pairs are simulated in addition to the historical data based on those distributions. Finally, hazard contours are drawn, and the results are compared with the combined wind and ice values proposed in the National Standard of Canada (CSA-C22.3) for different reliability levels. • Provides weather analysis of 16 weather stations for a specific site of interest located in mountainous terrain. • Analyzes ice accretion on transmission lines. • Characterizes joint wind and ice hazard. • Provides a comparison with the standard CSA-C22.3 for three different hazard levels. [ABSTRACT FROM AUTHOR]