Observations and one-dimensional modeling of the atmospheric boundary layer during the afternoon-evening transition time period

A more complete understanding of the afternoon-evening transition (AET) of the atmospheric boundary layer and the phenomena that occur during this time period is developed through the utilization of a diverse set of tools, including a 1-D boundary layer model and observational assets. The use of a multi-sensor approach to analyze the clear air boundary layer transition provides a more comprehensive description of the AET than has been previously available.;An analysis of 82 cases during summer 2007 and autumn 2008 provides an in-depth examination of the boundary layer transition with and without the presence of boundaries and deep convection. The multi-sensor approach illustrates that the most significant changes during the AET occur near the surface, including a significant decrease in 10 m wind speeds. This decrease in wind speeds is hypothesized to produce a decrease in surface layer convergence, and an increase in convergence above the surface layer in association with accelerating winds during the AET.;The observations of outflow boundaries and gust fronts during the AET showed that the evolution of the boundaries themselves made it difficult to determine the effect of the AET on the boundaries. Observations of CI cases also appeared to provide little evidence regarding the AET influence, likely due to the strong signal associated with convective-scale motions.;Observations from cases where cold fronts and drylines were present give reason to believe that the AET plays a role in CI. A non-convective dryline case from 22 May 2002 during IHOP showed that convergence increased during the AET, providing support to the hypothesis that convergence within pre-existing boundaries increases during the AET. Evidence of an increase in convergence prior to CI was also observed for two cold front cases. The convective signal possibly overwhelms the signal for other active CI cases. The similar timing of all of the different events suggests that the AET plays a role in the evolution of boundaries and convective initiation during the early evening hours. The results of this study provide support for the hypotheses being investigated.