LAND-ATMOSPHERE COUPLING MECHANISMS OVER STEEP TERRAIN: TURBULENT TRANSPORT AND MODELING CHALLENGES (Invited Presentation)

Steep terrain poses several challenges to modeling turbulent surface-atmospheric exchanges, ranging from difficulties associated with the numerical grid (e.g., insufficient resolution) to violating key assumptions necessary for traditional parameterizations to hold. State-of-the-art numerical weather, climate, hydrologic and remote sensing models rely on empirically-based, surface-atmosphere exchange parameterizations that were developed for horizontal and homogeneous terrain within the Monin-Obukhov similarity theory scaling framework. These classical parameterizations translate how surface conditions, such as roughness, temperature and soil moisture, influence the atmosphere, and vice versa, via turbulent fluxes; hence, they cannot represent the fundamental physical processes that arise from the atmosphere’s interactions with the heterogeneous and sloping terrain. We present observations of the turbulence structure over steep (35.5 degree), alpine terrain in Val Ferret, Switzerland to highlight specific modeling challenges arising from deviating from the horizontal-terrain assumption. Under clear-sky, nocturnal 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. These distinct flow regimes exhibit very different vertical turbulence structures. Hence, improved numerical forecasting models should seek the capabilities to accurately reproduce these regimes. Additional modeling challenges posed by steep-slope flows are that turbulent fluxes rarely exhibit a constant-flux surface layer, violating another key assumption for most turbulence parameterizations (e.g., land-surface wall models and turbulence closure models). We additionally show that the traditional concept of atmospheric stability can become unclear for flows over steep terrain because turbulence kinetic energy can be buoyantly produced despite statically stable thermal stratification. This further complicates stability-based numerical modeling. Finally, we propose key areas for future research toward improving modeling capabilities over steep terrain.