Observations of the afternoon transition

One of the most important and least well understood aspects of boundary layer physics is the transition from the relatively deep and vigorously mixed daytime convective boundary layer to the shallow and poorly mixed nighttime stable layer. This transition also often leaves behind a nearly neutrally stratified residual layer, formerly the upper 80% or so of the daytime boundary layer. This layer, released from surface influence but still having the composition of the daytime boundary layer, may transport that composition over considerable distances during the night, and is also subject to interesting physical processes such as the formation of jets in the wind profile.

In this study, we use radar wind profiler data to characterize the afternoon transition and subsequent residual layer behavior by examining two main categories. The categories are based on the radar reflectivity patterns, which show the variations of humidity, temperature, and turbulence intensity, a more direct measure of turbulence intensity, and our understanding of the expected behavior of the boundary layer. This work is intended to contribute to the goal of predicting afternoon transition behavior from measurements earlier in the day. Radiosonde profiles taken at noon LST in each of the categories show significant differences indicating the feasibility of prediction.