Surface Fluxes and Flow Structure for Stably Stratified Near-Calm Conditions

The dependence of fluxes on the wind speed near the surface of the stable nocturnal boundary layer has been organized in terms of a threshold wind speed. When the wind speed is smaller than the threshold speed (near-calm conditions), the eddies at the observational level tend to be decoupled from the surface. When the wind speeds are larger than the threshold value, the main eddies are thought to directly interact with the underlying surface. As an example, the threshold wind speed at the 2-m observational level is typically 1 m \(\hbox {s}^{-1}\) ${\text{s}}^{-1}$ . Investigation of the heat flux as a function of the wind speed is relatively straightforward. The dependence of the friction velocity \(u_*\) $u_{\ast }^{}$ on the wind speed requires somewhat different strategies. The friction velocity is derived from the momentum flux averaged in different ways. For near-calm conditions, the stress vector and wind vector are often not aligned. The wind direction frequently varies rapidly with height near the surface for near-calm conditions.