Characteristics, weather scenario and statistics of thunderstorm outflows based on measured data

Wind is the most destructive natural phenomenon: 70% of damage and death caused by nature in the world comes from wind. And extreme winds such as typhoons, tornadoes and thunderstorms are crucial for structural damage. Inside, during a thunderstorm, the transient downdraft that impinges on the ground produces radial outflows that can produce strong transient wind, which is the main cause of the collapse of tall structures such as transmission tower, besides one of the threat to human life and property security, for instance, the accident of transmission line towers in Ontario, Canada in August 2006 and "Oriental star" sinking in China in June, 2015 and so on. The study of thunderstorm outflows and their loading and response of structures already become a key topic in modern wind engineering. Despite this, the understanding, the representation and the modeling of thunderstorm outflows are still full of uncertainties and problems to be clarified. This happens because the complexity of the thunderstorm outflows makes it difficult to establish physically realistic and simple engineering schemes, their short duration and small size means few data are available, and a large gap exists between wind engineering and atmospheric science. It follows that the wind loading of structures is still evaluated by the Davenport?s model for extra-tropical cyclones without any concern for the real nature and the properties of the meteorological event that causes the loading. This is nonsense because extra-tropical cyclones and thunderstorm outflows are different phenomena that need separate assessments. To overcome these limits, this PhD thesis carries out a deep research mainly on the characteristics of thunderstorm outflow according to wind field measurement and meteorological data based on a thunderstorm catalogue created extracted from a mixed climate, which mainly contains the proposal of more reasonable directional decomposed approach of thunderstorm outflow signal, the properties of thunderstorm related to wind loading on structures, the comprehensive analysis of field measurements and weather scenarios related to thunderstorms, the extreme wind speed distribution in a mixed wind climate and the preliminary study of the crucial question if thunderstorms in different areas have similar properties. The major contents and achievements are summarized as follows: Firstly, measurements for up to 6 years related to 14 high-sampling rate anemometers of the monitoring network in the Northern Mediterranean ports are analyzed. Three intense phenomena, namely extra-tropical cyclones, thunderstorm outflow, and intermediate events are separated successfully by a semi-automatic procedure. The results lead to a wide dataset of 277 wind velocity records characterized by strong transient properties and labeled by thunderstorm outflow, which are catalogued into three families, named 10 minutes, 1 hour and 10 hours, according to the different time-scale of the gust front passage, and fundamental for the subsequent study. Analyses are then executed to extract the parameters of major interest for evaluating the wind loading effects of structures. And a novel directional decomposition strategy is formulated here, which makes it possible to analyse quantitatively the directional shift of thunderstorm outflows, makes the study of thunderstorm outflows and synoptic winds fully coherent and is strategic to perform directional analyses of the dynamic behaviour of structures in terms of alongwind and crosswind response. Then this strategy is applied to thunderstorm records comparing with the classical decomposition approach and furnishing a comprehensive statistical characterization. While the general analysis in wind engineering has a shortcoming that it misses the knowledge of the weather scenarios that occur during events classified as thunderstorms, without recognizing their actual meteorological nature. In order to take the first step towards filling this gap, a typical thunderstorm downburst and three events, each one representative of the corresponding class of duration, detected by our network are investigated from the meteorological point of view to represent a first step and a pilot attempt in this direction. The results obtained bring new insights into a thunderstorm?s onset and detection in the Mediterranean, its evolution at the local scale, and possible connections to specific synoptic-scale weather conditions. Design wind speeds based on the statistical analysis of conventional extreme mean wind speed 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 continuous records registered in two Port areas of the monitoring network, a preliminary but representative analysis of the extreme wind speed distribution is carried out in this mixed wind climate area frequently struck by thunderstorms. Results show that wind speeds with high return period are always related to thunderstorm outflows. It proves that gathering the ensemble of all extreme values into a single set and the analyses of the local wind climate ignoring thunderstorms may lead to underestimating the extreme wind speed. At the end of the research, this analysis procedure is applied to the 5-year data from the 9 anemometers installed at different heights on Beijing 325m high meteorological tower to study the characteristics of thunderstorms in the Beijing urban area, to compare these with northern Mediterranean ones and to understand if thunder-storms in different areas have similar properties. In addition, the property of the mean wind speed profile and coherent function of thunderstorms are described, which provides a reference for the simulation of thunderstorm signal. Hope this thesis could make some contributions to step further research on thunderstorm-resistant design for building structures.

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