Wave data on the CDS and interpolation

The ERA5 wave data on the Climate Data Store (CDS) have been made available already * pre-interpolated* onto a regular lat-long grid of 0.5 x 0.5 degrees.

However, the actual grid used by the wave model component of ERA5, is an irregular lat-long grid in which the latitude are spaced regularly every 0.36 degrees, but points on each latitude are spaced such that the actual distance between them is roughly 40km (0.36 at the equator)

For an area around Crete for example, it looks like:

Note that in this plot, the red star symbols are the locations of the grid points, the numbers above those symbols are the actual grid point value.

Please note that Crete is indeed *not *'flooded' with waves. There are a few points that are encroaching a bit on the Island, but again remember that with ERA5 native grid, one is trying the represent the coastline with grid boxes of 40 by 40 km and a grid box is found to be sea when the majority of its area is over sea. 

Ideally, the CDS data should have been made available on a regular lat-long grid that minimises the impact of interpolation from the native grid (0.36x0.36)

The interpolation scheme used by ECMWF for wave data uses a scheme based on weighted distances between the target point and the 4 neighbouring native grid points, *except *when land is present, where only values from sea points are used. However, the scheme does not know at all of the actual land sea mask of the target grid. For this reason, when the data were pre-interpolated on a 0.5x0.5 grid to be stored in the CDS, points along the southern coast of Crete were made to encroach at bit over land. They are to be interpreted as values representative for the closest coastline.

Unfortunately for Crete, the data appears as if the whole Island has gone,

but as I have shown with the plot of the native grid points, Crete is well resolved. Moreover, within the numerical scheme used by ECMWF wave model, there is a scheme that uses a high resolution bathymetric data set to account for the impact of islands on the propagation properties of waves. For this reason, waves from the North would not propagate across Crete as it it was not there.