The Characteristics of Boundary Layer Structure under Strong Wind Weather

Observation of surface-layer strong winds shows there are two kinds of velocity fluctuations superimposed on the average flow:random turbulent fluctuations, and coherent gusty wind disturbances. Studies on the boundary-layer structure under strong wind weather infer that the shear effect of strong wind in the upper layer produces gusty wind disturbances and transports energy downward. This is important for flux transportation in the surface layer. In this study, data were obtained from the Beijing 325 m meteorological tower, the wind tower at a height of 1257 m on Miaofengshan Mountain, and the tower at a height of 1688 m on Lingshan Mountain. Based on these data, the gusty wind disturbances in the upper layer and the downward transportation under strong wind weather were analyzed. Results showed that there were gusty wind disturbances in both the surface and upper layers (coherent structures whose periods were between one and ten minutes), superimposed on the average flow, as well as high-frequency turbulent fluctuations during strong wind periods. The coherent structures showed that the vertical velocity was downward when the horizontal velocity was in its peak phase, and vice versa. Furthermore, they were produced in the upper part of the boundary layer, and affected by the shear of the average flow and friction of the ground during their downward movement and transport process, before being broken up to form turbulence. The momentum fluxes of windy gusts and turbulence in the upper layer transported downward. During strong wind periods, the downward fluxes of momentum by windy gusts and turbulence were the same, both being important for the transportation of sand, dust, pollution, and other aerosols. This study provides an observational and theoretical basis for the development of flux transportation parameterization in models.