The added value of the hight-resolution HARMONIE runs for deriving the HBCs : contribution to P 1 of the WTI2017-HB Wind Modelling Project

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 simulation 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. This report aims to determine the added value of the high-resolution HARMNIE model over the coarse resolution ERA-Interim dataset obtained from the European Centre for Medium-Range Weather Forecasting (ECMWF). A detailed comparison of the 10-m wind and surface-stress 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 becomes less accurate as a result of the coarse resolution. Far from the coast, 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 HARONIE, storm surge calculations with the WAQUA model are closer to the observed water levels than when input from ERA-Interim is uses.