沙尘暴过程中的混合层特征

本文基于大气气溶胶垂直分布的一些观测事实和针对两例不同天气类型的沙尘暴过程进行的观测分析,得出在行星边界层中,气溶胶浓度远大于其上方的自由大气;且其层结越接近中性,其浓度越接近均匀分布。在冷锋型沙尘暴的维持阶段,地面风速较强且其大小与沙尘暴强度是比较一致的。在高压底部偏东风型沙尘暴过程中,使沙尘暴触发和维持的地面风速弱于冷锋型,且沙尘暴强度与地面风速大小没有明显关系。混合层是影响沙尘暴发生及强度的另一个重要因素。混合层总与沙尘暴伴随,且其维持滞后于沙尘暴。其原因是沙尘气溶胶对地面吸收太阳辐射的削弱使混合层的日变化减小,从而产生一种使混合层维持的机制。这一辐射影响在沙尘暴过程中具有普遍性。在冷锋型沙尘暴过程中,沙尘辐射对冷锋强度的强迫也导致混合层日变化出现增强的趋势。它们的共同影响使冷锋型沙尘暴过程中混合层能够始终维持但其日变化强于高压底部型。混合层与沙尘暴的关系应该结合沙尘暴的不同发展阶段考虑。

The Characteristics of Mixed Layer in the Dust Storm Processes

Based on some facts about the vertical distribution of the atmospheric aerosol and the observational study on two different types of dust storm (DS) processes(Cold Front DS and High bottom DS), some valuable conclusion were achieved. The concentration of the aerosol in the PBL is larger than that in the free atmosphere. The atmospheric stratification can affect the vertical distribution of the atmospheric aerosol : while the stratification is close to neutral the concentration of the aerosol is close to homogeneous distribution vertically. The surface wind speed in the Cold Front DS process is larger than that in the High Bottom DS process. Moreover, it is close correlative to the strength of DS in the Cold Front DS process while they have not clear connection in the High Bottom DS process. In all types of DS process, the Mixed Layer (ML) is closely going with the DS and it dead later than the DS. The reason for that is the radiant effect of the dust aerosol, which minimized the daily change of the ML and consequently induced the maintain of the ML. It is prevalent in all of the DS process. Another dust radiant forcing on the strength of cold front, which only exists in the Cold Front DS process, could strengthen the daily change of the DS and ML. The two different dust radiant forcing induce that although the strength of ML and DS maintain in the Cold Front DS process, its daily change is greater than that in the High Bottom DS process. In the High Bottom DS process the ML is lower but it is strong and could maintain steadily through the DS process. The connection between the ML and DS also depend on the development stage of the DS.