Observed Characteristics of the Boundary-Layer Evolution during Evening Transitions in North-Central Oklahoma

In the late afternoon and early evening, the planetary boundary layer (PBL) undergoes a transition in response to the diminishing solar radiation. The sensible heat flux decreases, eventually becoming negative shortly before sunset. As the sensible heat flux becomes negative, typically air near the surface begins to cool and the PBL stabilizes. This results in rapid changes in properties (i.e., temperature, moisture, turbulence, momentum) of the PBL, from the surface through the top of the daytime mixing layer. While the changes in these properties near the surface have been studied and are relatively well-documented, the evolution of the entire PBL and its properties, including the interplay between wind, temperature, and turbulence profiles, as a whole is more unknown.

The Land-Atmosphere Feedback Experiment (LAFE) took place in north-central Oklahoma at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) facility in August 2017. During LAFE, several instruments were deployed and operated at the SGP site to primarily investigate variability in the surface layer and PBL over short distances that are at subgrid scale in most numerical weather prediction models. Measurements from these systems temporarily installed at the central facility, as well as other instrumentation permanently installed at extended facilities located 40-50 km away, are used here to quantify changes in PBL during the afternoon and evening transition period. Doppler lidars are used to measure profiles of the mean wind and turbulent quantities, while Atmospheric Emitted Radiance Interferometer measurements are used to retrieve temperature and humidity profiles. Combined, these observations give a complete depiction of the atmospheric state in the lowest 1+ km of the atmosphere. These measurements are analyzed at the central facility and surrounding sites to quantify changes in the PBL, and how the PBL evolves differently spatially.