The role of sub-grid orography for global high-resolution flood forecasting

River discharge directly affects the water-food-energy-environment nexus and can have devastating impacts during floods. Floods often occur after extreme precipitation events, which are challenging to forecast accurately, both in time and space. Unresolved small-scale processes and features, including convection and orography, have direct impacts on our ability to accurately simulate precipitation, its partitioning into surface and sub-surface runoff, and consequently hydrological forecast skill. This motivates a spatial resolution increase in Numerical Weather Prediction (NWP) models, including their land and river components, and the revision of parametrizations suitable for those small-scale processes, such as runoff generation over regions with complex orography.

The Destination Earth programme of the European Commission addresses these issues through the global Extremes Digital Twin (C-EDT): globally coupled simulations at spatial resolutions of ~4.4km. These meteorological simulations are used to force ECMWF’s Land Surface Modelling System (ecLand), the land component of the Integrated Forecasting System (IFS), which in turn generates runoff. By effectively 1-way coupling the hydrodynamic Catchment-based Macro-scale Floodplain model (CaMa-Flood) to the IFS, grid-wise generated runoff is routed as streamflow in rivers and to simulate hydrological events.

Here, we investigate the added value of high resolution for global hydrological simulations by comparing the hydrological C-EDT-CaMa-Flood simulations at ~4.4km with a similar configuration at the operational resolution ~9km. Using the same input data, higher model resolution yields a local mean orography which is more consistent with the true conditions. Such an improved mean orography leads to better representation of the terrain, which is particularly important in mountainous regions. This fosters local precipitation maxima of a higher magnitude, due to convective processes. Conversely, a higher resolution also leads to less variability in sub-grid orography, which is a determining variable when modelling the saturated fraction per grid area over which saturation excess surface runoff is generated. Consequently, a change of the variability in sub-grid orography will directly affect the partitioning of precipitation into runoff and infiltrating water, and therefore the downstream streamflow accumulation and locally also the plant water availability. The results presented here explore the impact of model resolution changes on both the precipitation and runoff generation.

Earlier results focus on several streamflow events in the European Alps and first analyses show that, despite slightly higher precipitation totals over complex orographic regions, less surface runoff is generated, and lower peak streamflow values are predicted. As differences between initial soil moisture conditions between the two resolutions were found to be marginal for those events, the reduction in the sub-grid orography is the remaining factor leading to lower surface runoff generation at the higher resolution. Those results suggest that scale-dependent parameterisations for the runoff-generating processes are needed to minimise uncertainty in the streamflow predictions at varying scales.