External controls on the transition between stable boundary-layer turbulence regimes

The mean wind speed at which the stable boundary layer (SBL) experiences a turbulence regime transition (V-r) depends on other flow characteristics, such as its thermal stability. Here, V-r variability is examined both at a single site and across sites using three multiple-level datasets: Santa Maria, Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), and Fluxes over Snow Surfaces (FLOSS II). A method to determine V-r is introduced and validated. It is taken as the mean wind speed at which the vertical gradient of the turbulence velocity scale switches sign. Emphasis is given to the control exerted on V-r by quantities that are external to the SBL, such as radiation, roughness, and soil properties. In each of the experiments, V-r increases with net radiative loss at the surface (R-n) at a rate that is site-dependent. It also increases for smaller roughness lengths, as indicated by its wind direction dependence. No conclusive relationship has been found between V-r and downward longwave radiation observed at the surface. The across-site comparison indicates that soil heat capacity influences the rate at which V-r increases with R-n.