Speaker: Holly Oldroyd
Institution: UC Davis
Steep terrain imposes several challenges to interpreting and modeling turbulent surface-atmospheric exchanges because it violates key assumptions (e.g., horizontal terrain) necessary for traditional parameterizations and classical theories 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. These parameterizations translate how surface conditions, such as roughness, temperature and soil moisture, influence the atmosphere, and vice versa, via turbulent fluxes of momentum, heat and water vapor. Therefore, we cannot expect them, a priori, to represent the fundamental physical processes that arise from the atmosphere’s interactions with the heterogeneous and sloping terrain inherent to mountainous regions. As micro-meteorological 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. In this presentation, I will show near-surface turbulence observations of winds over a steep (35.5 degree), Alpine slope to investigate how slopes impact the near-surface turbulence structure and hence, exchange processes under a various local and non-local forcing conditions. A key outcome of these investigations is the interesting, and to some degree counterintuitive, role that buoyancy plays in generating/suppressing turbulence kinetic energy (TKE). As an important consequence of this behavior, conventional stability characterizations require careful coordinate system alignment and interpretation. In addition, traditional flux-gradient relations, used in almost every weather prediction and hydrological model are shown to inadequately parameterize scalar transport over steep terrain. Finally, the observations provide an avenue to propose new parameterizations and theories for various slope-flow regimes and explore key objectives for future field observation strategies.