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

The formation and evolution of nocturnal boundary layer clouds over land are studied using a simple well-mixed boundary layer theory. By analyzing the deepening rate of the mixed layer depth based on the turbulent kinetic energy budget of the whole boundary layer, the authors studied how the formation of idealized nocturnal boundary layer clouds is related to the physical processes associated with the land surface and the boundary layer. Preliminary analysis indicates that for a range of surface moisture and heat fluxes, wind shear can be an important factor in triggering the formation of nocturnal stratus. The relative importance of different physical processes responsible for cloud formation can be evaluated by the ratio between the lifting condensation level and a critical level, which is proportional to the Monin-Obukhov length scale. In this study, data collected from the Atmospheric Radiation Measurement site in the southern Great Plains are used to examine the results of the theoretical analysis. The analyses of the two nocturnal stratus cloud cases observed on 25 October 1996 and 6 November 1997 indicate that the turbulent mixing induced by the wind shear plays a pivotal role in the cloud formation during these two cases.