Atmospheric Feedbacks to Improved Subsurface Hydrology in a km-Scale Earth System Model (ICON–JSBACH)

As Earth System Models (ESMs) move toward kilometer-scale grid spacing, resolving small-scale atmospheric processes substantially improves the representation of convection and precipitation. However, the land component remains a major source of uncertainty in the atmospheric water cycle. Inadequate soil moisture and groundwater representations affect evaporation, land–atmosphere coupling, and ultimately the atmospheric supply of water as precipitation. These hydrological biases therefore influence not only local surface conditions but also remote moisture transport and recycling. In this work, we improve the representation of subsurface hydrology in the JSBACH land surface model, coupled to the ICON atmospheric model. We introduce additional soil layers, implement lateral groundwater flow between grid cells, and connect shallow groundwater to the river network. We evaluate the new developments using standalone kilometer-scale JSBACH simulations against flux tower measurements of latent and sensible heat fluxes and soil moisture observations in the Pyrenees (Spain and France). We then assess their impact on atmospheric variables, specifically 2 m temperature and precipitation, within ICON simulations at 3-km grid spacing over Europe.