Characteristics Of Thunderstorm Outflows Relevant To Wind Engineering 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.However,due to the local,sudden,random,short and complex properties,the development of the related researches is restricted.Accurate understanding of the characteristics of thunderstorms is the premise of this research direction,and the field measurement is the most direct and reliable method.While the limited availability of high-precision thunderstorm data hinders the accurate analysis of its wind field characteristics;in addition,a large gap exists between wind engineering and atmospheric science,so the current understanding of the complex characteristics of thunderstorms in the field of wind engineering is not comprehensive;most existing methods of wind speed signal analysis of thunderstorms ignore or only show more important directional factors qualitatively,which leads to the implied assumption that the wind-induced response results are generated along the downwind direction;at the same time,most of the current codes still adopt the structural design wind speed based on the definition of the maximum average wind speed of large-scale steady wind.Obviously,in the mixed climate region where severe thunderstorms often occur,this will inevitably lead to the insecurity of building structure.To overcome these limits and provide a reliable basis for strucralwind resistance design of under the action of thunderstorm outflows,this Ph D 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 analysis of weather scenarios related to a typical thunderstorm outflows,the proposal of more reasonable directional decomposed approach of thunderstorm outflow signal,the establishment of a high precision model of thunderstorm wind field,the extreme wind speed distribution in a mixed wind climate considering thunderstorm outflows.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 according to their properties.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.In order to take the first step towards filling the gap between wind engineering and meteorologist and learning the knowledge of the weather scenarios that occur during events classified as thunderstorms,a typical thunderstorm downburst detected by our network is 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.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,furnishing a comprehensive statistical characterization and establishing a high precision model of thunderstorm wind field,which is preliminary for understanding the wind loading of structures under thunderstorm outflows.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 and applying a refined mixed statistics method,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.