Search

Thunderstorm outflows are usually analysed by decomposing their horizontal velocity into a slowly-varying mean part and a residual fluctuation; in this framework the wind direction is often examined qualitatively but disregarded quantitatively. This is incoherent with the traditional analysis of synoptic wind speeds, where the mean velocity and direction are first determined, then the fluctuations are decomposed in terms of longitudinal and lateral turbulence components. Aiming to overcome these shortcomings, a novel directional decomposition strategy is formulated here, which makes the study of thunderstorm outflows and synoptic winds fully coherent. The results provided by the classic and novel decomposition rules are first compared and elucidated with reference to a couple of real thunderstorm and synoptic wind velocity records. Later on, they are applied to a broad class of thunderstorm outflow records whose statistical properties are investigated. The novel approach is strategic to carry out directional analyses of the dynamic behaviour of structures in terms of alongwind and crosswind response. • The classical decomposition rule of thunderstorm outflows disregards wind direction. • Classical synoptic wind and thunderstorm outflow decomposition rules are incoherent. • A novel directional rule is formulated that overcomes the above shortcomings. • The novel rule is strategic to carry out directional analyses of the structural response. [ABSTRACT FROM AUTHOR]

Highlights • Design wind velocity and wind-induced damage are often related to thunderstorms. • Research on thunderstorm loading of structures is still fragmentary and uncertain. • Calibration and advance are provided of response spectrum technique and time-domain analysis. • The response spectrum technique is a suitable tool for engineering calculations. • Time-domain analysis is highly efficient with a limited computational burden. Abstract Thunderstorms are transient events. Design wind velocity and wind-induced damage are often related to them. Despite this, research on thunderstorm loading of structures is still fragmentary and uncertain due to their complexity, short duration and small size. These issues make it difficult to set physically realistic and simple models as well as to gather real data. This favoured the implementation of refined methods based on limited measurements. The European Projects "Wind and Ports" and "Wind, Ports and Sea" realised an extensive monitoring network from which many thunderstorm outflow records were extracted. They were analysed to inspect their characteristics and to formulate methods coherent with measurements. Firstly, the response spectrum technique conceived for earthquakes was extended to thunderstorms. Then, a hybrid simulation strategy was proposed and time-domain integrations of the structural response were applied. This paper provides a joint calibration and advancement of these two methods, leading to results that substantially agree, especially faced with their conceptual and operative diversities. This confirms the potential of the response spectrum technique to become a suitable tool for calculating the thunderstorm loading of structures and the efficiency of hybrid simulations and time-domain analyses to investigate, with a limited computational burden, advanced structural issues. [ABSTRACT FROM AUTHOR]

The study of thunderstorm outflows and their loading and response of structures is a key topic in modern wind engineering. This paper provides a new contribution to this research by analyzing a wide dataset of 277 wind velocity records characterized by strong transient properties and labeled by thunderstorm outflow. These records have been detected for up to 6 years by 14 anemometers belonging to an extensive in-site monitoring network distributed in the Northern Mediterranean ports. Analyses are carried out in order to extract the parameters of major interest for evaluating the wind loading effects of structures and furnishing a comprehensive statistical characterization of the huge amount of data recorded. Results lead to a novel classification of thunderstorm outflows with reference to the time scale of the gust front passage and their intensity; a refined interpretation of the differences involved by the turbulence intensity, the integral length scale and the gust factor of mesoscale downbursts and synoptic low-pressure systems; a confirmation of the substantial independence of these quantities with respect to the ratio between the height above ground of the sensor and the roughness length of the terrain, together with their correlation with the wind velocity; a new parameterization of the harmonic content of the turbulent fluctuations. [ABSTRACT FROM AUTHOR]

The meteorological services of mid-latitude countries record wind speeds averaged over 10 min or 1 h periods and peak wind speeds for the same averaging period or a full day. Design wind speeds based on the statistical analysis of this data in a mixed wind climate may prove to be imprecise and unsafe due to the occurrence of intense, small and rapid extreme wind events such as thunderstorm outflows. Considering the 6 year continuous high-frequency records registered in two Port areas of the Upper Tyrrhenian Sea, a preliminary but representative analysis of the extreme wind speed distribution has been carried out in a mixed wind climate area frequently struck by thunderstorms. Results show that wind speeds with a high return period are always related to thunderstorm outflows. The mixed extreme distribution asymptotically overlaps with that for thunderstorms for high return periods and always provides the highest wind speeds. Gathering the ensemble of all extreme values into a single set leads to underestimating of the extreme wind speed. The Italian code provides conservative estimates of the extreme wind speed that protect designers from thunderstorms as well. However, refined analyses of the local wind climate that ignore thunderstorm events may lead to severe underestimations of the design wind velocity. [ABSTRACT FROM AUTHOR]

This paper presents a new approach for modelling the evolutionary power spectral density (EPSD) of thunderstorm outflows based on a database of 129 real-scale thunderstorm time-histories. Two equivalent methods of decomposition of the wind velocity are investigated, outlining the derivation of the EPSD. The analyses are based on the hypothesis that the residual turbulent fluctuations can be treated as a uniformly modulated process, whose reliability is verified studying the steadiness of its up-crossing rate. The assumption of considering a constant turbulence intensity is checked through the evaluation of the dynamic response of a set of SDOF systems in time-domain and through the response spectrum technique. Two models for the time-modulating function of the EPSD are proposed that include the parameters characteristic of both the thunderstorm event and the background wind. Finally, the steps for the derivation of the EPSD are outlined pointing out the limits of the representation of the PSD of the reduced turbulent fluctuations through spectral models commonly adopted in wind engineering. The final aim of this research is finalising a triad of complementary methods for evaluating the dynamic response of structures to thunderstorm outflows - EPSD, time-domain and response spectrum - consistent with real-scale time-history records. • The non-stationarity of turbulent fluctuations is analysed through their up-crossing rate. • The residual turbulent fluctuations can be dealt with a uniformly modulated process. • Assuming steady turbulence intensity causes overestimations in the dynamic response. • The models proposed for the time-modulating functions encase a physical meaning. • Two different ways of modelling the evolutionary power spectral density are outlined. [ABSTRACT FROM AUTHOR]

This paper discusses different strategies for the extraction of the time-varying mean from wind speed time histories. Due to the advantage of allowing analytical evaluations, the attention is focused on kernel regression techniques, considering different weighting functions, namely a constant, a Gaussian and a cardinal sine weighting function. The problem is firstly treated analytically, and the frequency-domain properties of the filter associated to different kinds of weighting functions in the definition of the slowly varying mean through kernel regression are analysed. Then, different weighting functions are adopted for the analysis of digitally-simulated stationary wind speed time histories and for the time histories of thunderstorm outflows recorded by a tri-axial anemometer. The consequences of the adoption of different weighting functions on the harmonic content and statistical properties of turbulence are studied. The same features are found also for thunderstorm outflow records. • Kernel regression techniques for the time-varying mean extraction are studied. • The properties of the filter associated to various weighting functions are analysed. • The statistical moments of the reduced turbulent fluctuation are estimated. • Applications to stationary and nonstationary wind speed time histories are provided. [ABSTRACT FROM AUTHOR]

Wind actions are crucial for the safety and sustainability of structures. The wind climate of Europe and many other parts of the world is dominated by synoptic extra-tropical cyclones and mesoscale thunderstorms. Thunderstorms are frequent events that cause wind speeds and damage often greater than those of cyclones. This paper describes a wide research activity including an extensive monitoring network, an unprecedented database of transient wind speed recordings, a broad spectrum of numerical tools for elaborating data and extracting their statistical properties relevant to the wind loading of structures, the development of wind tunnel tests and CFD simulations, investigations on weather scenarios and damage surveys aiming to complete the information provided by field measurements. Based on these resources, a new generation of wind loading models is being developed – response spectrum technique, time-domain integration, evolutionary spectral density - robustly coherent with measured data and intrinsically coherent with each other. These studies are being carried out in the framework of the project THUNDERR, funded by the European Research Council (ERC) with an Advanced Grant 2016 in order to produce outcomes physically correct, transferable to design and standards, suitable to modify the current wind loading format and make constructions wind-safer and cost-efficient. • THUNDERR project studies thunderstorm outflows and their impact on structures. • Outflows are studied by real measurements, CFD simulations and wind tunnel tests. • New wind loading models are being developed to account for thunderstorm effects. • Design practices will benefit of THUNDERR outcomes to make constructions wind-safer. [ABSTRACT FROM AUTHOR]

The study of thunderstorm outflows and their actions on structures is a dominant topic of wind engineering. This paper contributes to knowledge in this field through a systematic analysis of the thunderstorm outflows recorded by 9 anemometers installed at different heights along the 325 m high Beijing Meteorological Tower. Analyses are carried out through a directional decomposition strategy. The characteristics of a wide dataset of thunderstorm signals are analysed in a statistical environment coherent with the traditional one for synoptic winds; in this stage of the research, estimations of the mean wind speed and direction, the turbulence parameters and gust factor are carried out considering the records as disjoint from each other. Based on this choice, the results are compared with those obtained in the northern Mediterranean, providing a preliminary answer to the crucial question whether thunderstorms have similar properties in different areas, or, better, what of their properties are similar and what depend on the area itself. Besides to previous researches, the time evolution of the wind speed and direction profile is investigated with reference to a thunderstorm test case event. The systematic analysis of the full dataset of thunderstorm outflow fields is postponed to future researches. • A series of thunderstorm outflow records are detected in Beijing urban area. • A novel directional decomposition strategy recently formulated is implemented. • The characteristics of a wide dataset of thunderstorm signals are analysed in a statistical environment. • The results are compared with those obtained in the northern Mediterranean preliminarily. • The time evolution of the wind speed and direction profile is investigated condidering a test case. [ABSTRACT FROM AUTHOR]