Observational and Modeling Study of the Wind Controls on the Stable Boundary Layer

Many studies show the existence of two regimes or states in the stable boundary layer. The SBL is usually classified as very or weakly stable but this classification varies between studies. Recent studies have identified that the transition between the two SBL regimes is controlled by mean wind speed.

In the present study, both observational and simplified model results are compared to show the existence of simple relationships between different statistical moments of turbulent quantities and the mean wind speed. The observations are taken at two micrometeorological towers with multiple levels of observations, while the model is based on prognostic equations for mean wind speed, mean temperature, turbulent kinetic energy, heat flux and temperature variance.

In both observational sites, it is shown that the drag coefficient can be approached by two constant values, depending on the mean wind speed: a smaller value for the weak wind regime and a larger one in the strong wind case. The model results support this finding. Both model and observations agree that the mean wind speed profile tend to be vertically linear in the very stable limit and logarithmic on the strong wind limit. A somewhat abrupt transition between the two profiles happens in the transition between the two regimes in the model. As a consequence, the mean wind probability distribution is bimodal near the surface in the model, and this result is confirmed in some observations. Two regimes are evident for the wind shear dependence on wind speed. For the weak wind regime, the wind shear is independent of the mean wind speed, but it increases linearly with wind speed. As a consequence, the shear production term scales with the cube of the mean wind speed both in the model and observations in the strong wind regime, but not in the weak wind case. The model and observed dependence of the other terms of the TKE with respect to wind speed are also compared.