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We study the occurrence of relativistic microbursts observed by the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) satellite. An algorithm is used to identify 193,694 relativistic microbursts in the > 1.05 MeV electron fluxes occurring across the time period 23 August 1996 to 11 August 2007, nearly a full solar cycle. Our observations are normalized to provide the change in absolute occurrence rates with various parameters. We find that relativistic microbursts are mostly confined to the outer radiation belt, from L = 3-8, occurring primarily on the morningside, between 0 and 13 magnetic local time (MLT). This L and MLT distribution is consistent with the L and MLT distribution of whistler mode chorus amplitude. Thus, our observations favor whistler mode chorus wave activity as a driver of relativistic microbursts. Relativistic microbursts become more frequent as the geomagnetic activity level increases and are more frequent during equinoxes than during the solstices. The peak occurrence frequency of the relativistic microbursts moves to lower L as the geomagnetic activity increases, reaching a peak occurrence rate of one microburst every 10.4 s (on average) at L = 4 for 6.6 ≤ K p ≤ 8.7. Microbursts primarily occur outside of the plasmapause and track the inward movement of the plasmapause with increasing geomagnetic activity. The L and MLT distribution of the relativistic microbursts exhibits a peak occurrence of one microburst every 8.6 (98.0) s during active (disturbed) conditions, with the peak located at L = 5 ( L = 5.5) and 08 (08) MLT. [ABSTRACT FROM AUTHOR]

A major thunderstorm asthma epidemic struck Melbourne and surrounding Victoria, Australia, on 21 November 2016, which led to multiple deaths, a flood of residents seeking medical attention for respiratory problems, and an overwhelmed emergency management system. This case day had all the classic ingredients for an epidemic, including high rye grass pollen concentrations, a strong multicellular thunderstorm system moving across the region, and a large population of several million people in the vicinity of Melbourne. A particular characteristic of this event was the strong, gusty winds that likely spread the pollen grains and/or allergenic contents widely across the region to increase population exposure. This exploratory case study is the first to examine the usefulness of low-to-middle-atmospheric thermodynamic information for anticipating epidemic thunderstorm asthma outbreaks by allowing the forecast of strong downdraft winds. The authors investigated the utility of several mesoscale products derived from atmospheric soundings such as downdraft convective available potential energy (DCAPE) and indices for predicting surface wind gusts such as microburst wind speed potential index (MWPI) and a wind gust index (GUSTEX). These results indicate that DCAPE levels reached 'high' to 'very high' thresholds for strong downdraft winds in the lead-up to the thunderstorm, and the MWPI and GUSTEX indices accurately predicted the high maximum surface wind observations. This information may be useful for diagnostic and prognostic assessment of epidemic thunderstorm asthma and in providing an early warning to health practitioners, emergency management officials, and residents in affected areas. [ABSTRACT FROM AUTHOR]

The properties of a cylindrical-shaped magnetic-field-aligned channel of electrical conductivity produced by the precipitation of relativistic-electrons into the atmosphere during a spatially localized magnetospheric microburst are estimated. The conducting channel connects the middle atmosphere (~50 km) to the ionosphere. A channel diameter of ~ 8 km with an electric conductivity of 1.2×10 −9 Ω −1 m −1 near the bottom and 1.8×10 −7 Ω −1 m −1 higher up is found. In the fair-weather electric field, the higher-conductivity portions of the channel can carry substantial electrical currents. [ABSTRACT FROM AUTHOR]

In past years, many electrical transmission lines have failed during downbursts. This study is part of a research program aimed at understanding the behavior of transmission lines under localized winds. Most downbursts need to be modeled using computational fluid dynamic (CFD) models; this is not an easy task for engineers in general. This paper focuses on replacing the complicated velocities obtained from CFD models with simplified loads that represent, in a more practical fashion, the critical downburst configurations causing the guyed transmission towers to fail. Different options are considered in terms of the location and nature of the design velocity associated with different configurations. The profile of the horizontal velocity is developed. A practical simplified load corresponding to each of the critical configurations is developed. Detailed steps of the conductors' reaction calculations are provided. A simplified procedure is developed for designing downburst-loaded guyed transmission towers. [ABSTRACT FROM AUTHOR]

Despite most weather-related failures of high-voltage transmission lines (HVTLs) being attributed to the downbursts accompanied by heavy rainfall, research works mainly focused on the behaviors of the high-voltage transmission tower-line structures under dry downburst winds. This paper thus presents a preliminary study to discuss the characteristics of rainfall in the downbursts and their effects on responses of HVTLs. Based on Vicroy model, the velocities of raindrops and their loads and pressure ratios of downburst wind-driven rain and only downburst wind on the surface of HVTLs per unit length are obtained. A downburst wind-rain induced vibration model is established to calculate the effects of the rainfall intensity and wind velocities on the motions of HVTLs. To verify the feasibility and accuracy of the model, the model is applied to evaluate responses of HVTLs with measured aerodynamic coefficients. The responses of HVTLs from the evaluated (the model) and the field observation results are compared. The results indicated that the model is feasible and can capture main features of the rainfall acting on HVTLs in the downbursts. Furthermore, the effects of rainfall cannot be neglected, and more attention should be paid to the wet downbursts and their effects on aerodynamic property of HVTLs. [ABSTRACT FROM AUTHOR]

Downbursts wind are believed to be responsible for most of weather-related transmission line failures worldwide. Based on a theoretical model of a hinged single-span transmission line structure under moving downbursts, this paper studies downburst -induced vibration of a multi-span transmission line-insulator system (MSTLIS). Firstly, a nonlinear analytical framework for quasi-static responses of the MSTLIS under moving downbursts is established, and linearized equations of the MSTLIS under moving downbursts with time-varying mean wind speeds are derived. Then, the influence line functions of the insulator tension and displacement of the MSTLIS are derived, based on which the quasi-static responses of the MSTLIS under fluctuating wind is calculated. A case study on a six-span transmission line-insulator subsystem is conducted. The proposed nonlinear analytical model and the approximate linearization approach is verified by finite element model (FEM) results. The proposed analytical framework with linear approximation is accurate and more efficient than FEM, and hence has a great potential for real application. • An analytical framework for quasi-static responses of the transmission lines under moving downbursts is established. • Approximate linearized longitudinal stiffness is used to develop a more efficient tension under time-varying mean wind. • The linearized influence line functions of the insulator tension and displacement under fluctuating wind are derived. • The proposed analytical framework with linear approximation is accurate and more efficient than finite element analysis. [ABSTRACT FROM AUTHOR]

B/JP-10 suspension fuel is a novel energetic blended fuel with high application prospects, but it suffers from incomplete energy release in the actual combustion process. In this study, B/EtOH/JP-10 suspension fuels with different EtOH contents were prepared by a two-step method. The ignition and combustion characteristics were investigated using a CO 2 laser ignition test system. The focus is on the effect of EtOH addition on the fragmentation process and energy release of boron agglomerates. The results show that the addition of EtOH changes the ignition pattern of B/JP-10 suspension fuel and thus shortens its ignition delay, and the ignition delay of fuel droplets decreases with increasing EtOH content when the EtOH content is >31.2 wt%. During the combustion process, on the one hand, EtOH will strengthen the droplet's own "microburst" in the early stage of combustion, transporting JP-10 vapor and active boron to the oxygen-rich zone, thus improving the utilization of oxygen around the droplet; on the other hand, after the agglomerates are formed, EtOH will gradually break the agglomerates and transport active boron outward, promoting the contact between active boron and oxygen and finally improving the overall energy release efficiency of the fuel. • First improved energy release characteristics of boron-based suspension fuels from the base fluid perspective • "Microburst" behavior during two-phase combustion process is critical to the fuel system energy release • EtOH can break boron agglomerates from inside to promote the energy release of boron particles [ABSTRACT FROM AUTHOR]

• The Hilbert spectrum-based method and the two-dimensional (2D) indicial response function was introduced in the time domain buffeting analysis of a long-span bridge under nonsynoptic events. • The downburst wind presents significantly nonstationarity characterized by time-varying mean and spectrum, while slightly nonstationary nature was observed in the typhoon events. • The downburst-induced transient effects on the indicial response function, aeroelastic loads and buffeting response were highlighted, while negligible transient effects were observed under typhoon events. Inherent nonstationarity of the wind speed records is frequently captured during downburst and typhoon events, however, the significance of nonstationary effects on the bridge aerodynamics has not been widely investigated yet. In this study, the nonstationary wind-induced effects on aerodynamics under downburst and typhoon events were discussed. More specifically, the wind field input was generated using the instantaneous information embedded in the Hilbert spectrum, which was extracted from full-scale measurements of downburst and typhoon wind. The two-dimensional (2D) indicial response function was employed in the time domain buffeting analysis of a long-span bridge, and the transient effects on the bridge aerodynamics under downburst and typhoon were discussed. The results from the present study highlighted the significant transient nature of the downburst wind and its transient effects on the indicial response function, aeroelastic loads and buffeting response, while negligible transient effects under typhoon events. [ABSTRACT FROM AUTHOR]

Extreme gust observations near the ground, which are critical in assessing the impact of wind on structures, are a disjoint mixture of events created by different meteorological mechanisms which require separation before assessment. Classification by visual inspection of the meteorological data becomes impractical for very large datasets. In this study recent automated approaches that use statistics, pattern recognition and/or neural networks, were calibrated against 9014 visually classified gust events of ≥40kn from 25 locations across the USA over 22 years. The visual classification distinguished between five classes of valid gust event: synoptic scale storms, deep convection, the forward flank and the rear flank of gust fronts, and downbursts from isolated thunderstorms; and between two classes of artefact. The ensemble-averaged timeseries of each class formed a distinctive hierarchy. The misclassification rate against the visual classification varied between 24% for the poorest method to 10% for the best method, with most differences between adjacent classes. Extending the most promising method to include contemporaneous temperature and pressure observations reduced the misclassification rate to 2.3%. When applied to >107 gust events ≥20kn from 450 locations across the USA, the class hierarchy remained stable. Implementation is by open-source R scripts. • Extreme wind gust observations in the USA are compromised by artefacts. • Visual classification distinguishes five Classes of gust events from artefacts. • Existing automated methods have a classification error rate around 20%. • A neural network using speed, temperature and pressure has an error rate of 2.4%. • The neural network also identifies artefacts, even at very unreliable stations. [ABSTRACT FROM AUTHOR]

Thunderstorm is a severe phenomenon which can produce downburst winds capable for causing significant damage or even the collapse of low-rise structures. Experimental tests and numerical simulations are widely used by the Wind Engineering (WE) community for gaining increased understanding of complex winds like downbursts. In this paper, the accuracy of the Unsteady Reynolds-averaged Navier-Stokes (URANS) and Scale-Adaptive Simulation (SAS) to reproduce a vertical downburst wind - previously tested in the WindEEE Dome wind-simulator facility - was investigated. CFD results were first validated with the experiments and then used to study the dynamics of the phenomenon. The most suitable CFD setting (i.e. SAS case based on the k-ω SST turbulence model) was able to reproduce the development of the primary vortex ring, shedding of secondary rings induced by Kelvin-Helmholtz (KH) instabilities and the periodic detachments of smaller secondary vortices ahead the primary vortex. The superposition of these three contributions has shown to determine the temporal variation of the characteristic nose-shaped vertical profile which was found in the experiments. The performed simulations were also compared to field measurements from the NIMROD project showing a high level of agreement. • URANS and SAS simulation can recreate WindEEE Dome produced downburst flow features. • The bell-shaped inflow nozzle impacts the development of ring vortices. • Secondary vortices (SV) ahead of the primary vortex (PV) are detached by PV rotation. • Downburst wind field is determined by PV, trailing ring vortices, and detached SVs. • Characteristic nose-shaped velocity profiles are both space and time-dependent. [ABSTRACT FROM AUTHOR]

Affected by the large-scale boundary layer wind, thunderstorm clouds are often accompanied by a horizontal movement while producing downdrafts. However, previous studies mainly focused on the stationary downburst. Hence, the understanding of the wind field characteristics of moving downbursts is still insufficient. To study the influence of a storm's movement on the downburst wind field, based on the impinging jet test device with a movable nozzle, the stationary and moving downburst are simulated. Firstly, the moving average method is used to process the wind speed time history, and according to various existing criteria, a reasonable moving average time is determined. Afterwards, the vertical, radial, and lateral wind profiles are respectively given, and the influence of the parameter η – which represents the ratio of the storm translating speed (V tr) to the jet velocity (V jet) – on the wind profile is studied. Moreover, the applicability of the vector sum method is verified. Finally, turbulence properties including turbulence intensity, power spectral density function (PSD), and probability density distribution function (PDF) are furthermore analyzed. • The characteristics of stationary and moving downburst wind profiles are comprehensively analyzed. • The effect of the ratio of moving velocity to jet velocity on the wind field is studied. • Verified the applicability of the vector sum method. • The turbulence properties of stationary and moving downbursts are compared. [ABSTRACT FROM AUTHOR]

Many failure incidents for transmission line structures have been observed in various locations around the globe as a result of downburst wind events. The localized nature of such downbursts imposes a load case on transmission towers that does not exist under large-scale events such as hurricanes. This load case happens when the downburst location leads to an uneven and unequal distribution of wind velocity along the conductor spans located at opposite sides of a tower. Hence, a difference in the tension forces will develop between the two spans of the conductors attached to the left and right sides of the tower. This difference in tension forces will lead to a net longitudinal force acting on the tower cross arm, which is believed to be the reason for the failure of a number of towers during downbursts. The estimation of this force requires conducting a nonlinear analysis of the conductors under the variable loading conditions resulting from the most critical downburst location, while taking into account a number of key parameters including the flexibility of the insulators and the conductor’s pretension force. The objective of this paper is to develop a simple procedure that can be used by practitioners to estimate such a force. A study is first conducted to determine the most critical downburst configuration expected to produce the highest values of the longitudinal forces. A parametric study is then conducted to assess the variations in the longitudinal forces with various geometric and material parameters influencing the behaviour of the conductors. Results of this parametric study are then used to develop a set of charts that can be used to estimate the longitudinal forces using three-dimensional linear interpolation. An example is provided to illustrate the application of the developed procedure. [ABSTRACT FROM AUTHOR]

The paper focuses on the origins of the wind action model of the CENELEC standard dedicated to the design of electrical overhead lines (EOL). One of the main purposes is to conclude on the full consistence of the CENELEC model with the Eurocode model proposed in EN 1991-1-4 (2005) . A successful comparison with experimental wind results is given for a practical validation. A complete calculation of the reliability index regarding design wind loads applied to suspension towers is detailed. When the CENELEC minimum reference reliability level is used in the EOL design, it is shown that the theoretical yearly probability of failure is of at least 10 −4 for only a handful of towers per line, which may be charged at their limit conditions. Even if the CENELEC model deals with stationary wind events as winter storms, the resistance of EOL regarding other high-intensity localized wind events like tornadoes or downbursts is also discussed. This background document is written to give an overall understanding of the CENELEC wind load model to EOL engineers. [ABSTRACT FROM AUTHOR]

This paper reports a high-speed volumetric observation of a wet microburst event using X-band phased array weather radar (PAWR) in Japan. On 10 September 2014, PAWR observed the three-dimensional structure of a convection cell, which had a vertical extent of 5-6 km and a horizontal dimension of 2-10 km, moving toward the east-northeast. At 2310 Japan standard time (JST), a precipitation core with a radar reflectivity of >40 dB Z appeared at 3-5 km above ground level. The core then increased in size and intensity and rapidly descended to the ground. During this time, a reflectivity notch associated with midlevel inflow was initially formed near the top of the precipitation core and, subsequently, at lower altitudes. A strong low-level outflow with a radial divergence of >4 × 10−3 s−1 appeared just below the notch at around 2321 JST. The outflow lasted for approximately 13 min and eventually disappeared after 2333 JST along with dissipation of the causative storm cell. These results suggest that, in addition to hydrometeor loading, evaporative cooling due to the entrainment of midlevel relatively dry air played an additional role in driving a strong downdraft. The preceding signatures including descending precipitation core, reflectivity notch, and midlevel convergence observed by PAWR are useful precursors to forecast the occurrence of low-level wind shear 5-10 min ahead, which is important for safe air traffic operation. [ABSTRACT FROM AUTHOR]

Electrical transmission tower-line systems transport the needed electrical energy. These systems are spatially distributed and subjected to extreme natural environmental events such as downbursts. The capacity curve of a single tower has been reported under the downbursts but neglecting the turbulent wind component and potential dynamic tower-wire interaction. This study is focused on the assessment of capacity curve of a tower within the tower-line system considering the turbulent winds and the tower-wire interaction. The incremental dynamic analysis and the nonlinear static pushover analysis are employed to estimate the capacity curve. Moreover, comparison of such a capacity curve to that of a single tower is also presented. It was shown that the turbulent winds introduce variability in the capacity curve. The estimated capacity curve by using the nonlinear static pushover analysis is in close agreement to the average capacity curve obtained based on the curves estimated using the incremental dynamic analysis. Most importantly, it is shown that the capacity curve of a single tower represents adequately that of a tower within a tower-line system, and the effect of the dynamic interaction between the tower and the wires on the capacity curve is negligible. [ABSTRACT FROM AUTHOR]

We analyze the nature and physical conditions in the generation regions of decimeter microbursts (MBs), which were discovered with the radiotelescope of the Russian Academy of Sciences (RATAN-600). One of the main peculiarities of MBs is an almost constant upper-frequency limit of about 1.1 GHz, which has not been explained in previously studied generation models. Here it is shown that this spectral peculiarity can be explained by the generation of the upper-hybrid waves at the double plasma resonance (DPR) and a subsequent transformation into low-frequency plasma waves considering free-free and cyclotron absorption. Model calculations show that MBs occur in the active regions where the magnetic-field strength is close to 100 G. MBs are most probably generated in the transition region of the solar atmosphere between main magnetic fields with opposite polarities. [ABSTRACT FROM AUTHOR]

To simulate and analyze a large aircraft with severe wind shear, nonlinear equations for a rigid aircraft under various wind conditions were designed. In this paper, a double-vortex-ring method is adopted to generate a three-dimensional microburst wind field. This model incorporates a flight dynamics module and three flight simulations based on various forms of nonlinear equations. In addition, four primary wind gradients are considered to compute the aerodynamic coefficients. In simulation experiments with a China C series large aircraft, equations using air-relative velocities and inertial velocities are compared and analyzed. Although the nonlinear equations derived using the air-relative velocities should be identical to those using the inertial velocities, the results show that the latter are more accurate in practice because the former bring error into the simulation when estimating the acceleration induced by the wind field. [ABSTRACT FROM AUTHOR]

A significant, convectively induced windstorm known as a derecho occurred over parts of Utah, Wyoming, Idaho, and Colorado on 31 May 1994. The event was unusual in that it occurred not only in an environment of relatively limited moisture, but also one with a thermodynamic profile favorable for dry microbursts in the presence of moderate midtropospheric flow. The development and evolution of the severe wind-producing convective system is described, with emphasis on the synoptic and mesoscale features that may have contributed to its strength and maintenance. A very similar derecho that affected much the same region on 1 June 2002 is more briefly introduced. Questions are raised regarding the unique nature of these events and their potential utility in achieving an increased understanding of the mechanics of derecho-producing convective systems in more moisture-rich environments. [ABSTRACT FROM AUTHOR]

On 14 June 2011, an intense multicell thunderstorm produced one nonsevere and three severe downbursts within 35 km of the rapid-update, S-band phased array radar (PAR) at the National Weather Radar Testbed in Norman, Oklahoma, and the nearby polarimetric research Weather Surveillance Radar 1988-Doppler (KOUN). Data collected from these radars provided the opportunity to conduct a quantitative analysis of downburst precursor signature evolution depicted by 1-min PAR data and the associated evolution of differential reflectivity ZDR depicted by 5-min KOUN data. Precursors analyzed included descent of the reflectivity core, evolution of the magnitude and size of midlevel convergence (i.e., number of bins), and descending 'troughs' of ZDR. The four downbursts exhibited midlevel convergence that rapidly increased to peak magnitude as the reflectivity core (65-dB Z isosurface) bottom and top descended. The ZDR troughs seen in the 5-min KOUN data appeared to descend along with the core bottom. Midlevel convergence size increased to a peak value and decreased as the reflectivity core descended in the three severe downbursts. In contrast, midlevel convergence size exhibited little change in the nonsevere downburst. The time scale of trends seen in the PAR data was 11 min or less and happened several minutes prior to each downburst's maximum intensity. These results point to the importance of 1-min volumetric data in effectively resolving the evolution of downburst precursors, which could be beneficial to forecast operations. [ABSTRACT FROM AUTHOR]

A model is proposed to represent nonstationary fluctuating winds based on the concept of instantaneous power spectrum representation and amplitude modulation to model the fluctuating winds with time-varying mean wind speed. This results in the nonstationary fluctuating winds at a point to be characterized by a power spectral density function with a frequency modulation; it is proposed and shown that the frequency modulation can be assigned based on a time transformation defined through a dimensionless distance traveled by the winds that is calculated using the mean wind speed. In essence, nonstationary fluctuating winds are obtained by mapping the stationary fluctuating winds through a nonlinear time transformation and by applying an amplitude modulation. The proposed model is employed to simulate nonstationary winds for parametric investigation of the peak displacement of linear and nonlinear inelastic single-degree-of-freedom systems. It is shown that a structure designed by neglecting ductility capacity and/or for stationary winds of 10 to 60 min has an unaccounted capacity to sustain nonstationary winds with a maximum mean wind speed that can be much greater than that for stationary winds. The use of the proposed model to represent the incoherent nonstationary fluctuating wind fields is also explored. An illustrative example focused on the fluctuating downburst winds at a few points within the downburst wind field is presented. [ABSTRACT FROM AUTHOR]

Based on observational analyses and on-site ground and aerial damage surveys, this work aims to reveal the weather phenomena-especially the wind situation-when Oriental Star capsized in the Yangtze River on June 1, 2015. Results demonstrate that the cruise ship capsized when it encountered strong winds at speeds of at least 31 m s near the apex of a bow echo embedded in a squall line. As suggested by the fallen trees within a 2-km radius around the wreck location, such strong winds were likely caused by microburst straight-line wind and/or embedded small vortices, rather than tornadoes. [ABSTRACT FROM AUTHOR]

A numerical investigation on the microburst-like wind characteristics in block array configurations has been performed using Computational Fluid Dynamics (CFD). The CFD modelling of impinging jet mimics a microburst wind shear. Effects of plan and frontal area densities on the drag and lift force acting on the arrays are studied by investigating the wall shear stress and pressure distributions. A semi-empirical model based on Poreh et al. (1967) is derived to estimate the spatially-averaged wall shear stress of the finite urban array located near the microburst storm centre. Moreover, the pressure and viscous drag force acting on obstacles in the arrays with different plan and frontal area densities are discussed and compared with the published results regarding the arrays placed in a neutrally stratified Atmospheric Boundary Layer (ABL) flow. The present results show that the viscous drag is insignificant relative to the total drag force for all the cases with different frontal and plan area densities (i.e. roughness packing densities). The mean vertical lift force acting on the arrays for various packing densities is discussed, and the lift force is compared with drag and resultant forces. The averaged lift force acting on a block in the array is 0.3–0.6 times of the magnitude of the resultant force. Therefore, it should be taken into account for the design and maintenance of high-rise buildings in cities. [ABSTRACT FROM AUTHOR]

In several locations around the globe, many failures of transmission line structures have been observed during high intensity wind events in the form of tornadoes and downbursts. There is a lack of procedures in the design codes and manuals of practice related to the estimation of tornado forces on transmission line systems. As such, the purpose of this paper is to present load cases that simulate the critical effect of F2 tornadoes on tangent lattice transmission line structures. The current study builds on extensive research previously conducted at the University of Western Ontario, Canada on this subject. A main challenge in this application of localized wind events is that the forces acting on the structure vary significantly based on the location of the tornado and a large parametric study involving varying the tornado location has to be conducted for each system to determine critical cases. Critical load cases are determined in this study based on parametric studies carried out in previous investigations as well as others conducted in the current study. The vertical profile of three velocity components associated with each critical load case as well as the horizontal profile of the associated transverse velocity along the lines are provided in this paper. A procedure that will allow practicing engineers to use those profiles for analyzing lattice transmission lines under F2 tornadoes is described. Validation of the developed procedure is conducted by considering two independent transmission line systems. The results indicate that the developed load cases estimates peak internal forces that are either slightly higher or 5% less than the values predicted by the detailed parametric studies. [ABSTRACT FROM AUTHOR]

The purpose of this study is to apply a stochastic optimal control method to the guidance of an aircraft. The aircraft is given optimal control to make a final approach through a microburst, a hazardous weather phenomenon for lowflying aircraft that may cause accidents during landing. As the determination of the size and the strength of a microburst always involves uncertainty, its precise detection is difficult. To minimize the risk of accidents, we must account for system uncertainty, a difficult task using conventional optimization techniques, which cannot handle random variables. This paper aims to solve this problem by employing a stochastic optimization algorithm that incorporates the generalized polynomial chaos method into a conventional direct method. The generalized polynomial chaos method is a numerical technique for solving stochastic differential equations. To demonstrate the effectiveness of the proposed algorithm, we conduct a numerical simulation in which an aircraft flies through a microburst and attempts to land. The results of the simulation successfully verify its effectiveness. [ABSTRACT FROM AUTHOR]

• Development of a global optimization procedure to estimate the main downbursts parameters. • The performance efficiency of two global metaheuristic optimization algorithm is compared. • Ten different real scale thunderstorm downburst outflows are simulated. Strong localized downbursts generated in thunderstorms can produce surface winds very dangerous for civil structures and infrastructures. Modelling and simulating such severe wind systems is therefore extremely important for structural safety and design wind speed evaluation. This paper deals with the downburst wind field simulation by means of an optimization algorithm that uses a downburst analytical model, previously developed by the authors, and two metaheuristic algorithms, namely the Differential Evolution (DE) and the Teaching-Learning-Based Optimization (TLBO), for the downburst kinematic and geometrical parameters evaluation. The optimization problem minimizes the relative error between recorded and simulated wind speed and direction time histories. A comparison is made between the performance of two algorithms for ten thunderstorm events measured in north-western Italy between October 2011 and October 2015. Both algorithms provide solutions which are coherent with the downburst parameters values present in literature. TLBO outperforms DE since it has a faster convergence rate to the optimal solution. [ABSTRACT FROM AUTHOR]

SUMMARY During winter, at Shonai Airport located at the northern part of Japan, some low-level turbulence caused by tornadoes and downbursts often interrupts takeoffs and landings. This is a primary source of economic loss. It is well-known that a weather radar system is one of the most useful devices for observing various weather phenomena. However, it is difficult for conventional radar systems with a range resolution of several hundred meters and a temporal resolution of about 5 min to detect small-scale phenomena with a horizontal scale of several hundred meters and duration of several minutes. We have been developing a meteorological Ku-band broadband radar (BBR) that can detect small-scale phenomena with high resolution (a range resolution of several meters and a temporal resolution of about 1 min). We constructed an advisory system with the BBR in order to support aircraft operation at this airport, and performed a comparison between BBR data and flight data recorder (FDR) data. [ABSTRACT FROM AUTHOR]

The National Environmental Satellite, Data, and Information Service (NESDIS) Center for Satellite Applications and Research (STAR) has developed and evaluated a suite of products that assess convective storm-generated downburst potential derived from Geostationary Operational Environmental Satellite-13-15 ( GOES-13- 15). The existing suite of downburst prediction algorithms employs the GOES sounder to calculate risk based on conceptual models of favorable environmental thermodynamic profiles for downburst occurrence. A diagnostic nowcasting product, the Microburst Windspeed Potential Index (MWPI), is designed to identify attributes of a favorable downburst environment: 1) the presence of large CAPE and 2) the presence of a surface-based or elevated mixed layer with a large temperature lapse rate. This paper provides an updated assessment of the MWPI algorithm, presents case studies demonstrating effective operational use of the MWPI product, and presents validation results for the Great Plains and mid-Atlantic coastal region of the United States. MWPI data were collected for downburst events that occurred during the convective seasons of 2007-13 and were validated against surface observations of convective wind gusts as recorded by wind sensors in high quality mesonetworks over the southern Great Plains and the Chesapeake Bay region. Favorable validation results include a correlation greater than 0.6 and low mean error [<0.1 knot (kt; where 1 kt = 0.51 m s−1)] between MWPI values and measured confirmed downburst wind speeds over contrasting climate regions of the continental United States. Case studies over the mid-Atlantic region and northern Florida highlight the adaptability of the MWPI algorithm to severe convective storm forecasting and warning operations. [ABSTRACT FROM AUTHOR]

Nonstationary extreme winds are responsible for many structural damages. However, empirical models are not available for these winds, especially for fluctuations, mainly due to the difficulties in mathematical treatments. In this study, nonstationary wind characteristics are studied and analytic models are also proposed based on field measurements of wind speeds. Firstly, the discrete wavelet transform and kernel regression method are used to infer the time-varying mean and variance of the nonstationary extreme wind, respectively. Then, based on the estimated evolutionary power spectral density (EPSD), transient features of nonstationary winds are examined. Results show that spectral variations in nonstationary wind fluctuations including concerned downbursts and typhoons are relatively weak. This means that these nonstationary fluctuations can be modeled as uniformly modulated processes. Also, the validity of nonstationary wind spectrum models directly extended from current stationary wind spectra is evaluated. The study demonstrates that this extension is not appropriate. Furthermore, two analytical models are suggested to characterize nonstationary wind fluctuations, including a fully nonstationary process model and a simplified uniformly modulated process model. Both models have physical meaning and provide satisfactory fitting for the estimated EPSD. They will be helpful in the Monte Carlo simulation and structural dynamic analysis. An evaluation of these models is conducted based on the structural dynamic analysis using a series of tall buildings. Results show that these two models have good performance in structural response prediction [ABSTRACT FROM AUTHOR]

The instantaneous turbulent velocity fields induced by the breaking of plunging regular waves on a 1 in 40 plane slope were measured using a volumetric three-component velocimetry (V3V) system. The measurement volume was located in the outer surf zone to capture the plunger vortex generated at incipient breaking and the splash-up vortex generated at the second plunge point. The measurement volume extended from a few millimeters above the bottom to just below the wave trough level. The aerated water was seeded with fluorescent particles and long-pass filters were used to block scattered light from entrained air bubbles. The volumetric velocity measurements revealed the three-dimensional (3D) structure of breaking-wave-generated vortices previously captured in two-dimensional (2D) planes using a particle image velocimetry (PIV) system. The evolution of the vortices was also captured. The most common turbulent coherent structure observed was a vortex loop with counter-rotating vorticity; two longitudinal vortices oriented obliquely upward in the direction of wave propagation are connected at the base by a transverse vortex. The vortex loop was carried in a downburst of turbulent fluid, which impinged on the bottom around the instant of maximum positive (onshore) wave-induced velocity. Downbursts carried large amount of turbulent kinetic energy and induced large apparent shear stress over the bottom. The distributions of vorticity, turbulent kinetic energy, turbulent kinetic energy flux and turbulent shear stress in the transverse segment of a vortex loop were examined. In a separate experiment, water particle velocities associated with non-breaking waves were measured at different heights above a plane slope using an acoustic Doppler velocimeter (ADV) to verify the V3V measurements. [ABSTRACT FROM AUTHOR]

In this study, a closed form solution to calculate the reactions of transmission line conductors subjected to downburst loads is derived. The derivation is conducted using a simplified multi-spanned conductor–insulator system, where the insulators to the right and to the left of the tower of interest are modeled using a combination of roller supports and linear springs. The considered system allows for the interaction between the spans without the need of solving a set of non-linear equations to avoid the complexity. The closed form solution is derived to be applicable for downbursts with arbitrary size and location relative to the line. The solution is then focused on a critical downburst case that is previously recommended for the line design. Accuracy of the derived solution is assessed under both downbursts with arbitrary size and location and downbursts corresponding to the critical case. The assessment is carried out by comparing the conductor reactions obtained using the proposed solution with those from the Finite Element Analyses (FEAs). It is found that the predicted reactions by the proposed solution are in a very good agreement with those from the FEA in both the transverse and the longitudinal directions. For example, the highest differences for the critical case are found equal to 2% and 8% for the reactions in the transverse and the longitudinal directions, respectively. This indicates that the solution is reasonably accurate given the complexity of both the downburst wind field and the structure behavior of the conductor system. The proposed solution represents a practical and useful tool for practitioner engineers involved in the design of transmission lines, particularly under downburst winds. [ABSTRACT FROM AUTHOR]

Thunderstorms are transient phenomena characterized by short duration. Engineering is usual to evaluate the structural response to such phenomena, most notably to earthquakes, by the response spectrum technique, a method extensively applied in both the scientific and technical fields. This paper represents the first stage of a research project aimed at introducing a “new” method that generalizes the “old” response spectrum technique from earthquakes to thunderstorms. In such a framework, it establishes the fundamentals of the structural response of ideal point-like Single-Degree-Of-Freedom systems subjected to thunderstorm wind actions perfectly coherent over the exposed structural surface. A general definition of thunderstorm response spectrum is introduced, which allows a straightforward evaluation of the maximum structural response and the equivalent static force. A new definition of thunderstorm base response spectrum is also provided. Several forms of representation and parameterization of these response spectra are studied, showing the merits and the shortcomings of each of these. The general properties of the response spectra are investigated, pointing out some relevant analogies with the seismic case and the classical analysis of synoptic events. In particular, while wind engineering is usual to express the wind loading starting from wind velocity models, the method proposed herein, similarly to seismic engineering, involves the use of real thunderstorm records. The records adopted in this paper to illustrate the new method come from the wind monitoring campaign conducted for the European project “Wind and Ports”. Despite the application to this case study, the methods and the definitions presented are fully general and may be applied in any context. [ABSTRACT FROM AUTHOR]

Transient winds, such as thunderstorm downbursts, are the cause of design-load wind speeds in many countries. An understanding of the loading experienced by buildings during a downburst is therefore important to allow well designed and engineered buildings to be constructed. In contrast to boundary layer winds, the maximum wind speed in thunderstorm downbursts occurs as low as z m =30 m above the ground, within the range of heights of man-made structures, suggesting that the wind loading will be dependent on the building eaves height relative to z m . In a novel set of experiments, the University of Birmingham Transient Wind Simulator (a 1 m diameter impinging jet with aperture control) has been used to simulate a downburst striking buildings of different heights, ranging from below to above z m . Two forms of building have been used – a square-plan, flat-roofed structure, and a rectangular, portal-frame – at three angles (0°, 45° and 90°) relative to the radial wind direction. Pressure coefficients have been calculated (using eaves height velocity) over the roofs of these buildings, and are shown to be of greatest magnitude when the roof is above the region of maximum outflow velocity, with the exception of windward edges perpendicular to the flow, when they are generally greatest for the lowest building heights. [ABSTRACT FROM AUTHOR]

An experimental investigation is performed to assess the characteristics of the fluid–structure interactions and microburst-induced wind loads acting on a wind turbine model sited in microburst-liked winds. The experiment study was conducted with a scaled wind turbine model placed in microburst-like winds generated by using an impinging-jet-typed microburst simulator. In addition to quantifying complex flow features of microburst-like winds, the resultant wind loads acting on the turbine model were measured by using a high-sensitive force–moment sensor as the turbine model was mounted at different radial locations and with different orientation angles with respect to the oncoming microburst-like winds. The measurement results reveal clearly that, the microburst-induced wind loads acting on the turbine model were distinctly different from those in a conventional atmospheric boundary layer (ABL) wind. With the scales of the wind turbine model and the microburst-like wind used in the present study, the dynamic wind loadings acting on the turbine model were found to be significantly higher (i.e., up to 4 times higher for the mean loads, and up to 10 times higher for the fluctuation amplitudes) than those with the same turbine model sited in ABL winds. Both the mean values and fluctuation amplitudes of the microburst-induced wind loads were found to vary significantly with the changes of the mounted site of the turbine model, the operating status (i.e., with the turbine blades stationary or freely rotating), and the orientation angle of the turbine model with respect to the oncoming microburst-like wind. The dynamic wind load measurements were correlated to the flow characteristics of the microburst-like winds to elucidate underlying physics. The findings of the present study are helpful to gain further insight into the potential damage caused by the violent microbursts to wind turbines to ensure safer and more efficient operation of the wind turbines in thunderstorm-prone areas. [ABSTRACT FROM AUTHOR]

Non-stationary extreme winds cause significant damages to buildings and other structures worldwide. Accurate modeling of these winds is crucial to the evaluation of structural safety. However, the derivation of a reasonable time-varying mean for these extreme winds appears to be not straightforward due to the non-stationarity, which is different from the stationary boundary layer winds. Currently, a variety of techniques have been developed to derive the time-varying mean for non-stationary winds, such as moving average, kernel regression (KR), discrete wavelet transform (DWT) and empirical mode decomposition (EMD). However, these approaches with different parameters may lead to inconsistent time-varying means and ensuing fluctuations. The evaluation of these approaches and corresponding non-stationary wind effects on structures has not been sufficiently addressed in previous research. In this study, two sets of full-scale non-stationary downburst wind records are used as examples to evaluate the performance of three approaches including KR, DWT and ensemble EMD with different time window sizes in deriving the time-varying mean. Based on these evaluations, the recommendations about the selection of the appropriate approach and time window size to derive a reasonable time-varying mean are provided. [ABSTRACT FROM AUTHOR]

The simulation of thunderstorm downbursts and associated loads on a utility-scale wind turbine is the focus of this study. Using a deterministic-stochastic hybrid model, downburst-related wind fields are generated separately from non-turbulent and turbulent parts. The non-turbulent part builds on available analytical models developed from field data that include recorded downburst events; the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions adjusted by information on parameters such as the downburst's translation velocity. Key thunderstorm downburst-related parameters include the maximum radial velocity, the height and radial distance to the maximum radial velocity, the downburst intensity, the downburst translation velocity and the downburst translation direction. In addition, the streamwise ambient (environmental) velocity and the downburst touchdown location relative to the wind turbine are also important in turbine load computation. A utility-scale 5-MW wind turbine model is selected, and loads are generated using stochastic simulation of the aeroelastic response. Information available in the literature on recorded downbursts is used to define the cases studied. A single downburst simulation and associated turbine response simulation is first discussed to illustrate loads computation and highlight downburst-related parameters of interest. Next, a Monte Carlo simulation study is performed to investigate the influence of touchdown locations and translation direction on turbine extreme loads. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

How climate change might affect tornadoes remains an open scientific question. Climatological studies are often contested due to inconsistencies in the available data. Statistical methods are used to overcome some of the data limitations. A few of these methods including using the proportion of tornadoes occurring on big tornado days, estimating tornado energy from the damage path, and modeling counts spatially are described here. The methods move beyond analyses of occurrences by damage ratings and spatial smoothing. Applications of these and related methods will help grow the nascent field of tornado climatology. [ABSTRACT FROM AUTHOR]

The authors consider a thunderstorm event in 2011 during a music festival in Belgium that produced a short-lived downburst of a diameter of less than 100 m. This is far too small to be resolved by the kilometric resolutions of today's operational numerical weather prediction models. Operational forecast models will not run at hectometric resolutions in the foreseeable future. The storm caused five casualties and raised strong societal questions regarding the predictability of such a traumatic weather event. In this paper it is investigated whether the downdrafts of a parameterization scheme of deep convection can be used as proxies for the unresolved downbursts. To this end the operational model ALARO [a version of the Action de Recherche Petite Echelle Grande Echelle-Aire Limitée Adaptation Dynamique Développement International (ARPEGE-ALADIN) operational limited area model with a revised and modular structure of the physical parameterizations] of the Royal Meteorological Institute of Belgium is used. While the model in its operational configuration at the time of the event did not give a clear hint of a downburst event, it has been found that (i) the use of unsaturated downdrafts and (ii) some adaptations of the features of this downdraft parameterization scheme, specifically the sensitivity to the entrainment and friction, can make the downdrafts sensitive enough to the surrounding resolved-scale conditions to make them useful as indicators of the possibility of such downbursts. [ABSTRACT FROM AUTHOR]

Having been identified as the cause of design load winds in many parts of the world, transient winds such as gust fronts and thunderstorm downbursts have been increasingly researched over recent years. The difficulties in simulating the flow structure of downbursts in the laboratory, particularly their rapid radial acceleration and associated ring vortices, have complicated measuring wind loads on structures subject to these conditions. The University of Birmingham Transient Wind Simulator (UoB-TWS, a 1 m diameter impinging jet with aperture control) has been used to simulate the transient aspects of downburst-like flow, allowing the pressure distributions they create over cube and portal framed structures to be measured for the first time, at model-scale (1:1600). Analysis of the velocity and pressure fields show that the simulator is capable of creating velocity fields which are similar to those observed in nature. Development of the ring vortex is demonstrated through phase-plot analysis. Two methods of calculating the turbulence intensity of the unsteady flow field have been used, giving mean values of between 3% and 10% depending on the method. Force coefficient time series have been estimated with the buildings angled at 0°, 45° and 90° to the radial wind direction. These are presented along with the instantaneous pressure coefficient distribution at the time of maximum roof suction. This novel research also highlights the difficulties of undertaking transient flow at model scale and drawing conclusions which are applicable to full-scale, i.e., where no two events are the same. [ABSTRACT FROM AUTHOR]

Winds generated by non-synoptic events such as those from tornadoes, microbursts or gust fronts, that are non-stationary or transient in nature and extreme in magnitude, can cause major damage to flexible structures. In this paper, a time-domain method is used to simulate the response of a long-span bridge subject to winds generated by a particular type of non-synoptic wind phenomenon such as a microburst to assess the vulnerability of the bridge to such winds. The self-excited or motion-induced and buffeting or turbulence-induced wind loads on the structure were modeled and simulated by Rational Functions and buffeting indicial functions, respectively. Wind from a translating microburst was simulated using empirical relationships that were derived from measurements of a laboratory-simulated microburst and the bridge response calculated to compare it with those induced by an equivalent straight-line wind that is used for structural design. It is shown that microburst induced structural vibration could be larger or smaller than the vibration induced by straight-line wind of equivalent magnitude depending on the relative size of the microburst with respect to the bridge span. [ABSTRACT FROM AUTHOR]

Loads associated with downbursts represent a significant vulnerability on various structures. Designing the structures to withstand such loads requires the knowledge about the turbulent characteristics of downbursts, which are the focus of the current study. To this effect, large eddy simulations (LES) of downbursts impinging over four different exposures namely open, countryside, suburban and urban, are performed. Ground surface roughness is simulated using fractal surfaces generated by random Fourier modes (RFM) and scaled to match a targeted aerodynamic roughness z 0 . Simulated wind velocities are averaged spatially and temporally to extract the mean and turbulent components. Properties of both the mean and the turbulent components are discussed. Turbulence length scales, which govern the wide band correlations of the turbulence, are determined in the circumferential, the vertical and the longitudinal directions. It is found that the length scales in the circumferential direction are larger than those in the vertical direction by at least an order of magnitude, indicating that downburst turbulence is more correlated in the circumferential direction. This has a particular importance for long horizontal structures such as transmission lines and long span bridges. Narrow band correlations and the turbulent spectra, which have a particular importance for flexible structures, are also discussed. Applicability of using the resulting turbulent characteristics to estimate the peak forces on structures, e.g. transmission lines, is deduced by employing the gust factor approach. [ABSTRACT FROM AUTHOR]

A quasi-linear convective line with a trailing stratiform region developed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) while being sampled by two airborne Doppler radars. The finescale reflectivity and Doppler velocities recorded by the radars documented the evolution of the convective line. Bands of positive and negative vertical vorticity oriented parallel to the convective line were resolved in the analysis. This type of structure has rarely been reported in the literature and appears to be a result of the tilting and subsequent stretching of ambient horizontal vorticity produced by the low-level wind shear vector with a significant along-line component. The radar analysis also documented the evolution of an embedded bow echo within the convective line. Embedded bow echoes have been documented for a number of years; however, a detailed analysis of their kinematic structure has not been previously reported in the literature. The counterrotating circulation patterns that are characteristic of bow echoes appeared to be a result of tilting and stretching of the horizontal vorticity produced in the cold pool. The analysis suggests that the location along the convective line where embedded bow echoes form depends on the local depth of the cold pool. The rear-inflow jet is largely driven by the combined effects of the counterrotating vortices and the upshear-tilted updraft. [ABSTRACT FROM AUTHOR]

In the aviation industry, microburst wind shear is considered to be a major safety threat, especially in some crucial phases of flight like takeoff and landing. Recently, the air speed sensor called Doppler LIDAR (Light Detection and Ranging) which measures the speed of air disturbance even Clear Air Turbulences (CAT) in front of the aircraft is developed. In order to utilize measurement data of Doppler LIDAR, we design the model predictive flight controller to avoid accidents caused by microburst wind shear in the landing configuration assuming the prior information would be obtained by Doppler LIDAR. The effectiveness of the proposed controller, numerical simulations are carried out, in which the plant uncertainties in the aircraft states and the measurement errors in the prior disturbance information are considered. [ABSTRACT FROM AUTHOR]