Hybrid Numerical Study Of Actual Downburst Near-surface Wind Field

Downbursts occurring in thunderstorms have been resulted in a large number of damage of many transmission towers and other engineering structures in the world. In order to improve the ability of structures in resisting downburst wind, many domestic and foreign scholars working in structural wind engineering have made extensive research in the downburst near-surface wind field and wind loads by various means in the last three decades. The physical simulation in laboratory can only get simple flow field similar to downburst and costs highly. Field measurement would be too expensive and inefficient, although it is the most direct and reliable mean. In recent years, accompanied with rapid development of the computer science and technology, CFD numerical method has gradually become the main method of researching downburst wind field and wind profiles in structural wind engineering field. However, a large number of key initial flow parameters of CFD model, such as the initial diameter of the jet, the initial velocity and height could not be obtained by actual measurement. These parameters are all assumed according to the statistical characteristics of downbursts, so the research progress of actual downburst wind loads is very slow. In order to solve the practical difficulties in the study of downburst engineering, based on single sounding, a cloud model is established to simulate the strong convective flow field of downbursts to provide the necessary initial outflow parameters for CFD model to simulate the actual downburst near-surface wind field and wind profiles in accordance with the accuracy requirements of structural design.Firstly, the cloud model developed by Institute of Atmospheric Physics is used to simulate the occurrence and development process of strong convective weather, in which small-scale downbursts occurred. The horizontally homogeneous initial field of the cloud model is constructed by the measured single-sounding at eight in every morning and evening. Three actual downburst case studies could be showed that simulation results of the cloud model depend on the single sounding whose spatial and temporal distribution is too sparse. If the temporal and spatial distance between single sounding and actual downburst is large, downburst wind field simulation would be likely to fail as the initial sounding condition is not ideal. In addition, simulation results were also affected by the parameter configuration for model. For different cases, the optimal time step and initial disturbance should be determined on basis of a large number of simulation tests.In order to resolve the problem which the measured single sounding is too sparse for cloud model to simulate actual downbursts, the triple-nested computing gridded data of the mesoscale ARPS model with horizontal grid length of1km and vertical grid length of0.5km is introduced. By selecting the ARPS gridded data in the corresponding space-time position and when&where downburst is going to occur to be the initial single sounding of cloud model, Wuhan "6.22" downburst and Guangzhou "7.21" downburst have been simulated successfully. Analysis has been showed that the triple-nested computing physical quantities of the ARPS model are relatively objective. The computing sounding curves have the basic characteristics of unstable weather on that day. The differences between computing sounding and measured sounding also reflect that the atmospheric stratification of unstable weather keeps in changing with time. The nested simulation method by combining ARPS model and cloud model overcomes the lack of applying cloud model alone to simulate strong convective process of downburst. By this method, downburst turning out in severe convective weather at any time and any place could be simulated.A higher-resolution cloud model is modified to simulate actual downburst weather process in this paper, which is in order to meet the initial data density requirements of the high-resolution CFD model. Wuhan "7.27" downburst and Wuhan "6.22" downburst as well as Guangzhou "7.21" downburst have been simulated by cloud model with grid resolution of250*250m*250m on the basis of the measured sounding or ARPS computing sounding. Results are showed that bottom of the simulated convective cloud rises, the convection process intensifies, the maximum lifting speed and sinking speed significantly increase, the maximum lifting speed and the maximum sinking speed as well as the maximum surface wind speed happen several minutes in advance. Fine flow field with more than one sinking monomer generated in succession in the convective process could be also simulated. The earlier sinking monomers could not cause strong surface winds due to less precipitation particles as well as the higher height of sinking starting. But the last strong downdraft which develops into downburst generally contains multiple sinking centres with more significant vortex ring structure in the near-surface wind field. The multi-monomer characteristic of downbursts is more in line with actual storms.CFD model is used to carry out fine simulation of the near-ground divergent wind field of actual downbursts in this paper, based on sinking inlet conditions calculated by the higher-resolution cloud model. Results show that the near-surface divergent wind field of actual downbursts is extremely complex. Wind speed time history and wind profiles in different locations in wind field continue to change over time. Especially in the process of downdraft hitting ground, the maximum speed of wind profiles increases and the height of maximum speed reduces rapidly. However, when wind speed reaches to the maximum value rapidly, the wind profile shape and height of the maximum speed almost don’t change. It is found that the outflow angle range of strong and weak downdraft in sinking entrance and the distance from sinking center are important factors that affect the maximum speed value, the height of maximum speed and attenuation tendency of the vertical wind profiles in different locations in the wind field. Moreover, the hybrid simulated wind profiles are quite different from the several existing empirical and analytical wind profiles which contain less characteristic parameters associated with actual downbursts and cannot take some important features of the actual downburst wind profiles into account. In future study of analytical wind profiles, it would be considered to increase the number of main characteristic parameters to make the constructed analytical wind profiles more in line with actual downbursts.