| A better understanding of the evolution of the convective boundary layer (CBL) improves our ability to forecast nighttime conditions and enables us to make better decisions about pollutant release, which in turn, determines the conditions in the lower part of the atmosphere where we all live.; In this study, I examine the structure of the CBL during the afternoon transition using data collected during the summer months in Illinois. Within these data a pattern of behavior was identified, where the strength of turbulence and the depth of the CBL declined over several hours during the afternoon. This pattern was found to be quite insensitive to the different atmospheric conditions occurring during the experiment.; A 1D numerical model, developed and tested to simulate the afternoon transition, was shown to reproduce the observed pattern. The model used first-order closure and included both longwave and shortwave radiation. The effect of radiation is expected to become significant, especially at the CBL top, as the surface forcing is reduced.; The model analyzed in detail the CBL evolution corresponding to one of the observed days and reproduced the observed pattern reasonably well. Large-scale subsidence was shown to have a major effect on the CBL behavior and may need to be included in future work.; This study illustrates the complexity of the CBL behavior during the afternoon, but also identifies a consistent pattern of behavior. The transition was not abrupt and it was found that the transition can begin at widely varying times during the afternoon. A simple model was developed which was able to capture much of the relevant physics of the transition. |
