Counter-Gradient, Co-Gradient and ‘Surface-Layer' Momentum Fluxes in Nocturnal Slope Flows over Steep Alpine Terrain

Generally, the more an underlying terrain deviates from being flat, uniform and homogeneous, the less that classical theories and models of the atmospheric boundary layer hold. At times this leads to high uncertainties in turbulent flux predictions, and at other times the models completely break down. Hence, to better understand, model and subsequently predict surface turbulent exchanges over non-idealized terrain, an important objective of many recent field campaigns has been to investigate the near-surface turbulence structure. We present observations of momentum fluxes in nocturnal slope flows over steep (35.5 degree), alpine terrain in Val Ferret, Switzerland. Under clear-sky conditions, we observe two distinct flow regimes with mean winds directed down the slope: (1) buoyancy-driven, ‘katabatic flow', for which an elevated velocity maximum (katabatic jet peak) is observed and (2) ‘downslope winds', for which larger-scale forcing prevents formation of a katabatic jet. In downslope wind cases, the velocity profile is quite similar to a logarithmic profile often observed over flat terrain, and the corresponding momentum fluxes roughly resemble a constant-flux surface-layer. In stark contrast, the velocity profiles in the katabatic regime exhibit a jet-like shape. The katabatic jet strongly modulates the corresponding momentum fluxes, which show steep gradients over the shallow katabatic layer and typically change sign near the jet peak as the velocity gradients change sign. However, frequently a counter-gradient momentum flux is observed near the jet peak (and at times at higher levels), suggesting significant non-local turbulent transport within the katabatic jet layer. We compare and contrast this behavior with katabatic flow theories and observational studies over shallow-angle slopes, and use budget and co-spectral analyses to better understand the non-local transport dynamics. In addition, we show that as a consequence of the counter-gradient momentum fluxes, even local stability can be difficult to characterize because a counter-gradient momentum flux represent a sink in the shear term of turbulence kinetic energy budget equation. These results have broad implications for stability-based modeling of katabatic flows.