On-demand coastal modelling with two-way nesting

The development and tuning of oceanographic models for coastal regions can be a time-consuming process. However, there are many situations in which an easily producible, user-friendly and still reliable tool is desirable for unresolved coastal regions, or planning modifications of coastal infrastructure, wind farms, assist in search and rescue operations, etc. This paper develops a prototype of an on-demand, relocatable modelling tool enabling to create the coastal setup easily by simply attaching higher resolution domain(s) to the base setup of open seas. The prototype runs on internal system at Danish Meteorological Institute (DMI), that is designed for further public use in a later stage in EU Digital Twin Ocean. To accomplish this, we apply the oceanographic circulation model HIROMB BOOS Model (HBM) which implements a two-way nesting technique to ensure a seamless transition from open seas to coastal waters. This tool is based on publicly available resources: bathymetry, oceanographic software, weather forcing, boundary forcing, etc., so that, it can be accessible by public community. The base bathymetry of the model at resolution of 2 arc seconds is derived from EMODnet bathymetry with coastal correction by using OpenStreetMap coastline. Lower resolution bathymetries in discrete resolutions are derived from the base bathymetry and processed beforehand, ensuring that users can apply them directly. To ensure close connections (strait, fjord, large river, channel, etc.) between water bodies, a vector layer of waterways is used for each low-resolution bathymetry. In the same way, narrow land forms (dam, narrow-long island or peninsula, road, etc.) are ensured by vector layer of dams. The on-demand tool is specifically designed for the North Sea-Baltic Sea region and is based on a tuned setup for open seas with properly-resolved Danish straits. As the on-demand tool will not use data assimilation, salinity inflow in the Baltic Sea is enhanced by slight smoothing of the bathymetry from Danish straits to Baltic proper. When comparing the results of base setup with existing reanalysis products in the North Sea - Baltic Sea, sea level of the HBM base setup shows favourable results. On-demand high resolution downscaling simulations have been tested for some major storm surge cases, showing benefits of using high resolution in storm surge modelling. The stability and quality of the resulting two-way nested coastal setups can be enhanced by implementing a cascade of two-way nestings. In summary, the paper demonstrates that creation of on-demand coastal oceanographic applications by attaching highly resolved coastal domains to existing base setups for open seas is achievable task.