对流边界层顶部光学湍流的室内模拟研究

对流边界层顶部夹卷层是边界层和自由大气之间的过渡带,夹卷层的湍流结构是其主要特征之一。由于夹卷层的位置较高,进行精细结构观测很难,对此认识较少。利用尺寸为150 cm×150 cm×60 cm室内对流水槽模拟了大气对流边界层的发生和发展。将准直光通过模拟对流边界层,根据光在湍流介质中的传输理论得到混合层顶部夹卷层的温度结构常数的特点,在夹卷层内,归一化的温度结构常数满足相同的规律;在高度0.8zi处,温度结构常数最小,随着高度增加,温度结构常数增大,zi处达最大,然后又随着高度减逐渐小,在高度1.2zi处湍流消失。在边界层顶部zi处,归一化的温度结构常数处于一定的变化范围,归一化温度结构常数处于0.08~0.3之间,这可能与夹卷层的大尺度结构有关。

Laboratory Study of Optical Turbulence at the Top of Convective Boundary Layer

Entrainment zone (EZ) at the top of convective boundary layer is a transition layer between the atmospheric convective boundary layer (ACBL) and free atmosphere, the turbulence structure of entrainment zone is one of the main properties. Because of the high level of entrainment zone, it is difficult to detect extensively, and little comprehension is obtained. Generation and evolution of the atmospheric convective boundary layer are simulated in laboratory convective water tank with the dimension of 150 cm×150 cm×60 cm. When the collision light beam propagates through the simulated atmospheric convective boundary layer, distribution of intensity of beam is fluctuating. Based on the theory of light wave propagation in turbulent media, the characteristic of temperature structure constant in entrainment zone can be given. In entrainment zone, the normalized temperature structure constant accords with rule. At the level 0.8zi, the temperature structure constant is minimum, then increases gradually with height, and gets maximum at the level zi. Over the level zi, the temperature structure decreases gradually with height, and gets to zero at the level 1.2zi. At the level zi of the boundary top, the normalized temperature structure constant varies within a certain range of 0.08～0.3, this could be contributed to the large-scale structure in entrainment zone.