On the structure of the atmospheric boundary layer over highly complex terrain

The atmospheric boundary layer (ABL) over mountainous regions plays a crucial role in exchange processes between the surface and the free atmosphere, influencing weather, climate, and air quality. Unlike the relatively uniform ABL over flat terrain, the structure of the mountain boundary layer (MoBL) is highly complex due to the wide spectrum of scales of motion induced by the multi-scale orography. These scales range from small-scale turbulence and coherent structures to slope and valley winds, encompassing both thermally and dynamically forced flows. This intricate interplay of processes creates a highly heterogeneous and variable boundary layer that challenges traditional modeling approaches and necessitates detailed investigation. This study aims to enhance understanding of the convective boundary layer (CBL) over highly complex terrain by addressing the following questions: What are the characteristics of the coherent structures (e.g., thermals) in the CBL and how stationary are they? What is their diurnal cycle, and how do their statistics, such as preferred locations, vary from day to day?

To answer these questions, we utilize the ICON model to perform large-domain, real-world large-eddy simulations (LES) at a resolution of 65 m, incorporating 1.5 million grid points. The simulations employ a nesting strategy with four domains at resolutions of 520 m, 260 m, 130 m, and 65 m, progressively refining the model to capture fine-scale dynamics. Conducted over the Swiss Alps for seven days in August 2022, the simulations reveal a highly heterogeneous boundary layer with preferred locations for thermal formation. These locations exhibit a rather consistent diurnal cycle and remarkably small day-to-day variability, despite changing large-scale forcings. Comparisons with Alptherm, a Lagrangian model designed for forecasting gliding conditions, provide additional context. Insights from this study advance our understanding of the mountain ABL and support improvements in mesoscale and forecasting models for complex terrain.