Evaluation and Advancement of Similarity Scalings for a Steep Alpine Slope

Abstract:

Monin-Obukhov similarity theory was initially developed for atmospheric boundary layer flows over flat and statistically homogeneous terrain, and verified with flux and profile measurements over such terrain. Even though ‘flat and statistically homogeneous terrain’ implies a very strict classification for real terrain to satisfy, Monin-Obukhov similarity theory has been shown to hold for a variety of flow situations outside of the narrow terrain categorizations of flows initially proposed. Furthermore, through the use of extended stability functions, it has been shown to hold even for specialized events in the stable boundary layer such as nonbreaking internal gravity waves and low-level jet formation over relatively flat terrain. The fact that it typically works, is one reason why Monin-Obukhov similarity scaling is so widely used in meteorological and hydrological modeling for real terrain, even when theoretically it should break down. Another reason why it is so widely used (potentially when it is inappropriate) is due to a lack of alternative, or more accurate, methodologies to relate the land surface forcing (boundary conditions) to the atmosphere in models. One such example for which Monin-Obukhov similarity theory has been shown to breakdown is steep slope flows. These steep slope flows are characterized by a very shallow jet layer, over which the fluxes vary by more than 10%, and the traditional ‘constant-flux’ surface layer is not observed. However, thermally driven slope flows exhibit a jet-like velocity profile. This suggests a ‘universal’ functionality, which could be revealed with the proper similarity scaling. The current work seeks to evaluate the validity of Monin-Obukhov similarity theory, with and without the extended stability functions and more recent similarity theories, proposed specifically for steep slope flows, using field data taken over a steep (35.5°) slope in Val Ferret, Switzerland. In addition, alternative, empirically driven, flux-gradient relationships are proposed and evaluated using the field data. The scalings associated with these relationships imply that they may be transferrable to other slope sites, and therefore may be used in place of traditional similarity theories.