Abstract

Over complex, mountainous terrain, the lowest layer of the atmosphere has to be considered as the ‘Mountain Boundary Layer ‘(MoBL). Different to its counterpart over flat terrain, the MoBL is characterized by intrinsic horizontal heterogeneity, three-dimensionality and failure of many of the well-known boundary layer parameterizations. For applications such as hydro- power (renewable energy) simulations, avalanche warnings, landslides (and other consequences of extreme mountain weather), there is therefore a fundamental problem: the available input information stems from atmospheric (mostly NWP) modeling output, which in turn is based on horizontally homogeneous and flat conditions. In more general terms: as long as output from NWP is used for traditional weather prediction purposes (temperature, precipitation and wind), it may be not so important whether the boundary layer parameterization is physically sound or only well-tuned. If it is used as input for application models, the MoBL forecast must be ‘right for the right reason’. If we extend the world’s land surface area from the truly mountainous fraction to hilly terrain, almost 70% of the planet’s land surface are affected, thus rendering a proper, physically based characterization of the boundary layer under such conditions crucial. In this contribution, the MoBL is introduced, as well as the largest challenges we face in its description, physical characterization and modeling. TEAMx (Multi-scale Transport and Exchange processes in the Atmosphere over Mountains – programme and experiment) is furthermore introduced as a large international research program, that seeks to advance or understanding of MoBL and meso-scale atmospheric processes over mountains.