THUNDERSTORM DOWNBURSTS: MONITORING, MODELLING, SIMULATION AND LOADING OF STRUCTURES

Thunderstorms are frequent phenomena that cause wind speeds and wind-induced damage often greater than those due to extra-tropical cyclones. This new paradigm of wind engineering gave rise to a recent burst of research despite which there is not yet a model of thunderstorm outflows and their loading of structures like that developed for cyclones in the early 1960s and still in use in the engineering practice. In a planetary phase of climatic evolutions, this shortcoming is exalting the excessive safety and cost of high-rise buildings with respect to the apparent unsafety of low- and medium-rise structures in thunderstorm days. This paper describes the research carried out at the University of Genova on this problem, the results it is producing and their perspectives

Education in wind engineering

The author maintains that an adequate education in a discipline as vast and articulated as wind engineering cannot fail to recognize first its origins, developments, fundamentals, advancements and prospects. In this context, an analysis of the actual state of education should identify its limits, gaps and strengths, as well as the hard core of knowledge that should constitute the common basis for a mature and modern discipline capable of generating figures endowed with broad views and specialized skills. This vision may enhance new strategies to exploit technological evolution and communication systems, rationalize co-operative educational programs on wide territorial scales, involve different skills as in-depth as they are varied in the wind engineering community, reach those who need to establish and consolidate this culture, but do not have the opportunity to personally experience the manifold initiatives in the discipline, seize the interest of people towards wind engineering, spread its culture and know-how into everyday life and into society

Modelling of thunderstorm outflows by means of the evolutionary power spectral density

This study proposes a model of time-modulating function for the evolutionary power spectral density (EPSD) of thunderstorm outflows, based on the slowly-varying wind speed and standard deviation records extracted from a database of 137 10-minutes thunderstorm time-histories. The analyses have been carried out under the hypothesis that the residual turbulent fluctuations can be considered an uniformly modulated process. Three different models have been proposed and provided with parameters characteristic of both the thunderstorm event and the background wind. Said models have been adopted to fit the ensemble value and its variation with the standard deviation of the dimensionless functions of both the slow-varying mean velocity and standard deviation records. Successively the relations between parameters have been provided and the procedure for the derivation of the EPSD have been outlined. The aim is to adopt this model for the derivation of the dynamic response of structures to thunderstorms outflows

Dynamic response of structures to thunderstorm outflows: Response spectrum technique vs time-domain analysis

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 \u201cWind and Ports\u201d and \u201cWind, Ports and Sea\u201d 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

Monitoring, cataloguing, and weather scenarios of thunderstorm outflows in the northern Mediterranean

High sampling rate (10Hz) anemometric measurements of the "Wind, Ports, and Sea" monitoring network in the northern Tyrrhenian Sea have been analysed to extract the thunderstorm-related signals and catalogue them into three families according to the different time-scale of each event, subdivided among 10min, 1, and 10h events. Their characteristics in terms of direction of motion and seasonality/daily occurrence have been analysed: The results showed that most of the selected events come from the sea and occur from 12:00 to 00:00UTC during the winter season. In terms of peak wind speed, the strongest events all belonged to the 10min family, but no systematic correlation was found between event duration and peaks. Three events, each one representative of the corresponding class of duration, have been analysed from the meteorological point of view, in order to investigate their physical nature. According to this analysis, which was mainly based on satellite images, meteorological fields obtained from GFS analyses related to convection in the atmosphere, and lightning activity, the thunderstorm-related nature of the 10min and 1h events was confirmed. The 10h event turned out to be a synoptic event, related to extra-tropical cyclone activity

directional response of structures to thunderstorm outflows

Due to the combined effect of the background wind, the translation of the downburst and the radial nature of the downdraft after the touchdown, a peculiar characteristic of a thunderstorm outflow as detected by an anemometer or experienced by a structure is the potential variation of the flow direction. The study of the wind-excited response of structures has habitually neglected this issue because, dealing traditionally with synoptic events, their direction is endowed with a regular pattern. This paper proposes two families of methods to take into account directional changes in the evaluation of the wind-excited response of structures. The first one generalizes the method usually applied to downbursts, which by its nature implicitly assumes a non-directional response. The second one is based on a new directional decomposition strategy of the wind speed that represents a generalization to thunderstorm outflows of what is historically done for synoptic events. The conceptual aspects involved in and the results provided by these methods are critically discussed and compared both to each other and with regard to the traditional non-directional structural analysis of thunderstorm outflows

Thunderstorm outflow characteristics in the Beijing urban area

The study of thunderstorm outflows and their effects on structures is a key topic in modern wind engineering. Based on the in-situ monitoring network in the northern Mediterranean carried out for the European \u201cWind and Ports\u201d and \u201cWind, Ports and Sea\u201d projects, the key characteristics of thunderstorm outflows in this area relevant to wind engineering with special concern for the wind loading of structures were analyzed in previous researches. Recently this work has been repeated and extended to the dataset detected by 9 anemometers installed at different levels on the Beijing 325 m high meteorological tower. This provides a unique opportunity to study the characteristics of thunderstorms in the Beijing urban area, to compare these with northern Mediterranean ones and to understand if thunderstorms in different areas have similar properties. Herein, the extraction and the cataloguing of thunderstorm outflows are illustrated firstly and then the directional signal decomposition strategy, in which their horizontal resultant velocity is decomposed along time-varying longitudinal and lateral directions, are presented. These analyses are preliminary for the further study and comparison of their characteristics