Scale-aware evaluation of complex mountain boundary layer flow from observations and simulations

Boundary layers over complex, mountainous terrain are characterized by multi-scale, complex flow structures, where scale separation poses a fundamental challenge. In this study, we apply the novel multiresolution coherent spatio-temporal scale separation (mrCOSTS) method to LIDAR observations and numerical data of the velocity components of complex mountain boundary-layer flow. We are able to identify three distinct time scales (i.e., turbulent scales, mountain boundary layer, and diurnal scales) and explore the underlying physical processes. Furthermore, we identified the dominant flow patterns for each time scale, e.g., down- and up-valley flows, cross-valley vortices, small-scale turbulence, and large evening transition eddies . Applying mrCOSTS to simulated velocity components enables us to identify how coherent structures and the flow patterns are represented at various mesh sizes in the model. Using mrCOSTS we trivially retrieved complex dynamics that were previously difficult to resolve, enabling a direct, scale-aware evaluation between the LIDAR observations and model results.