Atmospheric boundary-layer evening transitions: a comparison between two different experimental sites

Abstract

The planetary boundary-layer (PBL) afternoon and evening transition is investigated with measurements from two-month datasets, gathered at two experimental sites significantly different regarding heterogeneity, the degree of terrain wetness, and proximity to mountains. The period of 4 h prior to and after astronomical sunset is extensively analyzed. We show the mean evolution, average, maximum and minimum values of PBL variables, including wind speed, turbulent kinetic energy and potential temperature vertical gradient. Characteristic events, such as the wind minimum around sunset and a common pattern in the evolution of other variables, are identified. Results suggest that, for the establishment of the nocturnal stable boundary layer, moisture plays a more decisive role than turbulence. We also look into the occurrence of katabatic flows, finding more intense but less frequent events at the driest site. In contrast, at that location the crossover of the sensible heat flux takes place later. Time-scale evolution is investigated through case studies, and air humidity and soil moisture are found to have crucial importance explaining most of the site-to-site differences. Therefore, a humidity sensitivity experiment with the Weather Research and Forecasting model is performed, evaluating the role of moisture during the transition by increasing the soil humidity at the driest site and reducing it at the other location. The simulations reveal that humidity effects are more important until 1 h before sunset, both near the surface and at upper levels in the PBL. Furthermore, the moisture change is more relevant at the less humid and more homogeneous site, with intense and long-lasting effects after sunset.