Field Measurement Of Wind Characteristics In Typhoon Boundary Layer With Different Terrain

Typhoon is a kind of natural disaster with strong destructive power and wide influence range.In China,many large civil engineering structures are located in coastal areas with frequent typhoons.In consequence,the safety,comfort and durability of these wind-sensitive structures under typhoon action is a key problem in their design,construction,operation and maintenance.The wind characteristics of typhoon are the premise and foundation of the research on building resistance to typhoon.After researching,it is of great theoretical significance and practical engineering value to develop and improve the related theories of structural wind engineering as well as to guide the wind-resistant design of wind-sensitive structures in typhoon-affected areas.However,due to the complexity of typhoon wind characteristics and many challenges in the process of typhoon measurement work,there are still a lot of problems which need to be further studied in this field.On the one hand,most of the current researches on typhoon wind field are limited to the near surface,which leads to the lack of in-depth understanding of typhoon structure and wind field characteristics in a deeper range.On the other hand,there are lacks of research on the wind characteristics of typhoon under different terrain.In view of the above situation,the global characteristics of typhoon and the wind characteristics of typhoon boundary layer under different terrain are systematically studied in this paper.The results show that the vertical structure of the core area of typhoon can extend to the top of the troposphere（～17km）,and the airflow above it is controlled by large-scale background atmospheric movement;the center of the outflow layer is located near 15km.In this range,horizontal wind speed and direction fluctuate drastically.The thaw layer is located at a height of～5km,which divides the typhoon structure into an ice layer on the upper part and a rain layer on the lower part.Based on measured results of the vertical wind profile,it is shown that the vertical wind profile of typhoon exhibits obvious low-level jet characteristics at a height of～1-2km.In other words,horizontal wind speed below the gradient wind increases with height,while horizontal wind speed above the gradient wind decreases with the height.The measured results also indicate that the height of the center of the low-level jet stream（the gradient wind height）is greatly affected by the inflow terrain.The height of the low-altitude jet stream under exposure terrain of inflow is significantly lower than that under the mountainous terrain.In addition,the wind field characteristics of the typhoon boundary layer are also closely related to the characteristics of the inflow terrain.In order to further discuss the influence of different terrain features on the wind field of typhoon,the mean and fluctuating characteristics under different terrain during typhoon were systematically studied based on offshore platform,meteorological gradient observation tower（356m）and several near-surface meteorological observation stations.The mean wind profile and turbulence parameters,i.e.,gust factor,turbulence intensity,turbulence integral length scale,etc.,are discussed under the sea surface landform,urban terrain,and complex mountainous terrain,and the measured results are compared with the wind load code.Main findings and conclusions are summarized below:（1）The wind characteristics of the sea surface inflow terrain show that the mean wind profile follows the Log-law or Power-law distribution within the observed height of 200m,and the surface roughness z₀and exponentαare different between different wind speed levels.The results of turbulence characteristics show that gust factor and turbulence intensity first decrease and then increase with the increase of wind speed.When the wind speed exceeds a certain critical value,turbulence parameters no longer increase significantly with the increase of wind speed.（2）Under the urban landforms,the thickness of atmospheric surface layer（i.e.,mean wind speed meets the requirement of Power-law distribution）becomes thicker with the increase of wind speed.The measured exponentαis significantly larger than the same value of the surface inflow landforms,which is consistent with the prescribed value in China’s code.In addition,results of wind parameters（i.e.,turbulence intensity,turbulence integral length scale,gust factor）measured at different heights also demonstrated inconsistent features.The measured peak factors were found to be noticeably smaller than those predicted by an existing model especially under strong wind conditions.（3）The wind field becomes very complex under the complex mountain landform,which will show the feature of shielding effect,speed-up effect and canyon effect.Under special circumstances,the phenomenon of flow separation and generation of vortex were observed.The mean and fluctuating characteristics of the corresponding near-surface wind field will also show the difference of abnormal increase or decrease.The turbulence parameters represented by gust factor are significantly affected by the nearby hill terrain,and the turbulence increases significantly.Especially,nearer to the upwind topographic obstacles,the more evident the effect became.The canyon effect dominates the nearby wind climate and is inversely proportional to the distance away from the saddle point.The measured results in this paper provide a valuable basis for in-depth understanding of global structure characteristics of typhoon and reasonable determination of typhoon wind field information under different landforms.The relevant research results also provide reference for the typhoon resistance design and disaster prevention and mitigation practice of civil engineering structures in coastal areas.