Evaluating the role of Urban Canopy Parameters in ICON TERRA_URB

Urban canopy schemes are essential for urban climate modeling, yet their performance depends on the level of detail in Urban Canopy Parameters (UCPs). In this study, we evaluate the ICON TERRA_URB urban scheme against dense urban observations (near-surface sites, higher-level sites, and eddy-covariance towers) and satellite surface-temperature products, focusing on air and surface temperature, sensible and latent heat fluxes, and wind speed over Zurich and Basel during summer 2023 using four experimental configurations at 500 m resolution. These include simulations without an urban canopy scheme (No TU), TERRA_URB with spatially uniform UCPs (Constant TU), local-climate-zone-based spatially varying parameters (LCZ TU), and city-specific urban canopy parameters (Real TU). Results show that activating TERRA_URB, regardless of parameter detail, provides the largest improvement by reducing the nocturnal cold bias, increasing urban surface temperatures, reproducing slightly higher nocturnal wind speeds, and improving surface energy flux partitioning. Spatially varying UCPs further refine the simulations. In particular, Real TU best captures the nocturnal urban signal, improving agreement with observed nighttime temperatures and sensible heat fluxes, although some biases remain in city centers. Their added benefit is modest compared to the main gain from activating the urban scheme itself. A trade-off emerges aloft, as TERRA_URB tends to overestimate warming, with No TU often performing better at most higher-level sites, although this may also reflect coupled boundary-layer processes beyond the urban scheme itself. These results suggest that while bulk urban canopy schemes effectively capture surface exchanges, improved boundary-layer representation likely requires multi-layer canopy approaches.