Validation of DCSM-FM (2020-2023) : HARMONIE and D-HYDRO migration

This report assesses the impact on water level accuracy resulting from two key model transitions: the migration from the HARMONIE40 to the HARMONIE43 meteorological model, and the intended shift from the fifth-generation WAQUA model (DCSMv6-ZUNOv4) to the sixth-generation D-HYDRO DCSM-FM 100m model. The analysis considers the effects of the HARMONIE migration on both the two-dimensional operational DCSM-FM 100m setup and the three-dimensional 3D DCSM-FM model, with specific focus on atmospheric exchange processes. To evaluate these transitions, multi-year hydrodynamic hindcast computations were conducted and validated against tide gauge data from Dutch coastal waters. This analysis encompasses a wide range of weather and hydrodynamic conditions, including storm Pia in December 2023 (for ECMWF and HARMONIE40). The results from HARMONIE were compared to those using ECMWF meteorological forcing. Findings indicate that ECMWF provides the best overall results for DCSM-FM 100m, achieving the lowest average surge error, with an RMSE of 5.6 cm, followed closely by HA40 (5.9 cm) and HA43 (6.0 cm). Although HA43 exhibits slightly higher average errors than HA40, it performs better at specific inland stations in the Western and Eastern Scheldt, Wadden Sea, and Ems-Dollard estuary. For skew surge under calm conditions, ECMWF also records the lowest errors, with an RMSE of 5.2 cm compared to 5.6 cm for both HA40 and HA43. During extreme conditions, where skew surge underestimation contributes significantly to high water errors, HA40 displays the lowest negative bias, followed by HA43, and finally ECMWF. Overall, DCSM-FM 100m consistently outperforms its predecessor, DCSMv6-ZUNOv4, in tide, surge, and total water level predictions, as well as skew surge estimates under both calm and storm conditions. Based on these findings, it is recommended that DCSM-FM 100m replace DCSMv6-ZUNOv4 as the primary operational tide-surge model at HMC and WMCN. The report concludes with several targeted recommendations for improvements to further enhance forecast quality.