The added value of the high-resolution HARMONIE runs for deriving the HBCs

For the derivation of the Hydraulic Boundary Conditions (HBCs), information on extreme winds over open-water areas is required. To this end, a new method is developed that will answer the need for a description of both the strength and the space- and time-characteristics of extreme storms. The method is based on high-resolution atmospheric
model simulations rather than on a spatial interpolation of sparse point measurements of wind speed. The HARMONIE model, which has a grid spacing of 2.5 km, has been selected to perform the simulations with.
This report aims to determine the added value of the high-resolution HARMONIE model over the coarse resolution ERA-Interim dataset obtained from the European Centre of Medium-Range Weather Forecasting (ECMWF). A detailed comparison of the 10-m wind and surface-stress fields of both models is presented. Coastal areas and large inland water bodies like Lake IJssel benefit most from using the high-resolution model. In those areas, wind and surface-stress values of ERA-Interim become less accurate as a result of the coarse resolution. Far from the coast, HARMONIE winds are systematically higher than ERA-Interim winds. Especially during storm situations,
HARMONIE winds verify better with observations than ERA-Interim winds. Differences in surface stress between HARMONIE and ERA-Interim are smaller than differences in wind speed. When using input from HARMONIE, storm surge calculations with the WAQUA model are closer to the observed water levels than when input from ERA-Interim is used.