Terrain effects on the turbulence relationship with wind speed in the nocturnal boundary layer

It has been recently established that the two major distinct regimes of the stable boundary layer (weakly stable and very stable) are characterized by well-defined different dependences of turbulence kinetic energy (TKE) on wind speed (Sun et al.,2012), a relationship that has been called the HOckey-Stick Transition (HOST) (Sun et al. 2015). Over flat land, TKE increases slightly with wind speed for weak winds, and increases significantly and linearly with wind speed for winds stronger than a threshold wind speed at a given height. Since then, the described relationship has been observed in datasets collected over a variety of surfaces, which include terrains of different degree of complexity, vegetated canopies or ocean. In the present study, we analyze the HOST using data from FLOSSII experiment over a moderately complex terrain with distinct wind-direction-dependent topographical features. The overall dependence of TKE on wind speed deviates from the flat terrain case, mainly for the weak wind regime, for which TKE shows a relatively larger increase with wind speed. The main purpose of the present study is to explain such differences. The HOST at FLOSSII is highly dependent on wind direction, indicating that upwind terrain characteristics affect the turbulence dependence on wind speed. When wind is from a relatively homogeneous terrain, southeast of the site, the resulting HOST resembles the CASES-99 from flat terrain. In contrast, when wind blows from a mountainous area west of the site, the two regimes are almost not discernible. Our data analyses suggest that the distinctive difference between two different wind directions is that strong wind from the flat terrain can lead to strong shear-generated turbulent mixing, which can nearly eliminate vertical temperature gradients; while strong wind from the mountainous terrain is associated with strong downslope flow with adiabatic warming, which sustains the atmospheric stable stratification regardless of wind speed. As a result, the distinction between the stable and the nearly neutral regimes separated by the threshold wind speed that is observed over flat terrain is not discernible for wind from the mountain direction.