Passive Tracer Evolution over Complex Terrain: A LES Case Study of the Beromünster Region

Storage and transport processes over complex terrain remain a major source of uncertainty in atmospheric models, particularly under stable boundary layer (SBL) conditions. This work builds upon previous idealized studies and presents a high-resolution large-eddy simulation (LES) case study of passive tracer evolution in the Beromünster region of the Swiss Midlands using realistic terrain, land use, and surface tracer fluxes.

The simulations are designed to capture both stable and convective boundary layer regimes over a 48-hour period, allowing investigation of the full diurnal cycle of tracer evolution. Emphasis is placed on nighttime tracer accumulation within the SBL and its subsequent depletion during the morning transition. In addition, the sensitivity of tracer transport to horizontal grid spacing is assessed to better understand resolution requirements in complex terrain.

Spatially heterogeneous and temporally varying surface tracer fluxes representative of CO₂ exchange, together with realistic topography and land use, enable a more physically consistent representation of lower boundary conditions compared to previous idealized studies. The simulations are evaluated against CO₂ observations from the Beromünster tall tower, which provides measurements at multiple heights (2–212 m), offering detailed insight into the vertical structure and temporal evolution of tracer concentrations.

The results indicate that cold-air pooling and terrain-driven drainage flows lead to pronounced spatial variability in near-surface tracer concentrations and strong stratification during nighttime conditions. During the morning transition, tracer concentrations rapidly decrease as boundary layer growth and slope and valley flows promote vertical export. Initial comparisons suggest that the simulations reproduce the general temporal evolution, while showing a tendency toward stronger stratification during SBL conditions. It suggests that the diurnal evolution of tracer concentrations is primarily controlled by cold-air pooling and local circulation patterns.

Overall, this study extends process understanding from idealized configurations to a more realistic setting and provides new insight into passive tracer dynamics over the Swiss Midlands, with implications for the evaluation and development of atmospheric models such as ICON and ICON-ART.