On clear, calm nights, the cooling of Earth’s surface creates a unique layer of air between 10 and 100 meters above the ground in which the temperature increases with height. Because the presence of warmer air overlying denser, cooler air is thermally very stable, these inversions—also called stable boundary layers (SBLs)—suppress turbulence and vertical mixing. This can result in adverse effects, including the buildup of high concentrations of pollutants near the ground that may affect air quality and human health.
Although previous studies have demonstrated that SBLs can be either weakly or strongly stratified, the mechanisms responsible for these variations are not well understood. To determine how turbulence varies under different conditions, Lan et al. conducted a field campaign at the Idaho National Laboratory. There, the team measured the wind field in three dimensions—as well as temperature, water vapor density, and other atmospheric conditions—using four eddy covariance systems mounted at heights of 2, 8, 16, and 60 meters on a tower over flat terrain.
The results indicate that the speed of winds measured a couple of meters above the ground determines the structure of a swirling type of turbulence known as eddies. When these near-surface winds are weak, the eddies are confined to a thin layer of air, which suppresses vertical mixing and reduces the downward transport of heat. These changes ultimately lead to a more strongly layered inversion. By contrast, when stronger winds are present near Earth’s surface, larger-scale eddies can form. These enhance vertical mixing, which weakens the stratification and allows even larger eddies to develop.
These findings suggest that SBLs can be classified according to average near-surface wind speeds. By showing that the structure of turbulence varies significantly between weakly and strongly stratified inversions, the authors provide important insights into the differences between these end member states and how turbulence within SBLs should be characterized in both climate and weather models. (Journal Geophysical Research: Atmospheres, https://doi.org/10.1029/2018JD028628, 2018)
—Terri Cook, Freelance Writer