模拟的对流边界层光学湍流的特征尺度分析

边界层作为地面与自由大气的过渡带对人类活动影响很大。边界层湍流结构尤其是夹卷层的湍流特征,是边界层研究的一个重要方面,这对于加深对边界层的认识以及研究边界层参数化具有十分重要的意义。由于边界层上部位置较高,难以进行精细结构观测。在室内对流水槽(150cm×150cm×60cm)中模拟了大气对流边界层的发生发展。将准直光通过模拟对流边界层得到光斑图像数据。利用改进协方差法对光斑图像进行功率谱分析,找出功率谱密度最大值所对应的频率即峰值频率,峰值频率对应的波长为涡的特征尺度。研究结果表明,在混合层,峰值波长较小,说明此处混合比较均匀,小尺度结构占主导地位。而夹卷层的峰值波长较大。夹卷层的平均峰值波长与对流Richardson数存在一定的关系,这种关系受下垫面类型以及对流状况的双重影响。

Characteristic Scale Analysis of Atmospheric Optical Turbulence in Simulating Convective Boundary Layer with Water Tank

The boundary layer is very important to human activity as the transition zone between the surface and free atmosphere. Turbulence structure of the boundary layer especially the entrainment layer is an important aspect for the boundary layer research, which is very important to improve understanding of the boundary layer and the research of the boundary layer parameterization. Because of the high level of the top of the boundary layer, it is difficult to detect extensively. Generation and evolution of the atmospheric convective layer are simulated in laboratory convective water tank with the dimension of 150 cm×150 cm×60 cm . When the collimated light beam propagates through the simulated atmospheric convective boundary layer, facula image data can be obtained. Using improved covariance method for power spectrum analysis on the facula image, the peak frequency can be found by finding the maximum power spectral density corresponding to the frequency, which is needed to obtain the peak wavelength as the characteristic scale of eddy. It is found that, in the mixed layer, peak wavelength is small, which means that the mixed layer is homogeneously mixed and small scale dominated there. But in entrainment layer the peak wavelength was larger. The average peak wavelength in the entrainment zone has some relation with the convective Richardson number, and the relation are influenced by surface types and convection condition.