Nocturnal buoyant TKE production in steep-slope katabatic flows: Observations, theory and multi-scale transport dynamics

As micrometeorological research shifts to increasingly non-idealized environments, the lens through which we view classical atmospheric boundary layer theory must also shift to accommodate unfamiliar behavior. We present turbulence observations in katabatic flow over a steep (35.5 degree), alpine slope and draw comparisons with classical theories for nocturnal boundary layers (NBL) over flat terrain and shallow-angle slopes to delineate key physical differences and similarities. In each case, the NBL is characterized by a strong, terrain-aligned thermal stratification. Over flat terrain, this temperature inversion tends to stabilize perturbations and suppresses vertical motions. Hence, the buoyancy term in the TKE budget equation acts as a sink. In contrast, the observed steep-slope katabatic jet is characterized by buoyant TKE production despite the NBL thermal stratification that buoyantly drives these flows. Since the gravity vector is not terrain-normal, this buoyant TKE production occurs when the streamwise (upslope) buoyancy flux vector component is larger than that of the slope-normal buoyancy flux, and the net vertical buoyancy flux becomes positive, or a source of TKE. Due to a relatively small number of observations over steep terrain, the turbulence structure of such flows and the implications of buoyant TKE production in the NBL have gone largely unexplored. As an important consequence of this characteristic, we show that conventional stability characterizations require careful coordinate system alignment for katabatic flows and further, that the interpretation of stability over a slope may need to be reassessed, more broadly. Since these results have strong implications for stability-based modeling, we present multi-scale statistics and budget analyses to describe physical interactions between turbulent fluxes at various scales, and to interpret similarities and differences between the observations and classical theories regarding streamwise buoyancy fluxes. Finally, we use these analyses to propose new conceptual theories for the steep-slope katabatic flow regime and key objectives for future field observation strategies.