Formation and development of nocturnal boundary layer clouds over the southern Great Plains

The formation and evolution of nocturnal boundary layer clouds over land ar e studied using a simple well-mixed boundary layer theory. By analyzing the deepening rate of the mixed layer depth based on the turbulent kinetic ene rgy budget of the whole boundary layer, the authors studied how the formati on of idealized nocturnal boundary layer clouds is related to the physical processes associated with the land surface and the boundary layer. Prelimin ary analysis indicates that for a range of surface moisture and heat fluxes , wind shear can be an important factor in triggering the formation of noct urnal stratus. The relative importance of different physical processes resp onsible for cloud formation can be evaluated by the ratio between the lifti ng condensation level and a critical level, which is proportional to the Mo nin-Obukhov length scale. In this study, data collected from the Atmospheri c Radiation Measurement site in the southern Great Plains are used to exami ne the results of the theoretical analysis. The analyses of the two nocturn al stratus cloud cases observed on 25 October 1996 and 6 November 1997 indi cate that the turbulent mixing induced by the wind shear plays a pivotal ro le in the cloud formation during these two cases.