Evaluation of HARMONIE simulations for 16 historical storms : contribution to WP1 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 for extreme storms. The method relies on using high-resolution atmospheric model simulations rather than on using 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. Following the model setup described by Van den Brink et al. (2013), this report presents a verification of HARMONIE simulation of 16 historical storms that were recommended by Groen and Caires (2011). Based on a verification with observations, we conclude that the wind fields produced by the model are realistic. Over sea, modelled wind speeds show a positive bias of about 0.5 m/s; for most stations the rms error is between 1.5 and 2.0 m/s. The bias in wind direction is a few degrees, the rms error is order 10° for winds of 8Bft and higher. Temporal correlation modelled and observed wind speed is 0.95 over sea. Spatial characteristics are generally well-captured. Spatial correlation between observations valid at the same time amounts 0.87 on average. HARMONIE represents spatial gradients between a selection of stations in 10-m wind speed and surface pressure rather well. Wind patterns over Lake IJssel are accurately reproduced by the model, including the impact of stability on the near-surface wind speed. It is demonstrated that for high wind speeds stability effects leave the vertical wind speed ratio relatively unaffected, while the water/land ration changes significantly. Given the results of the validation, the HARMONIE model set-up used for the simulation of the 16 validated storm periods will also be used for the long-term HARMONIE simulations.