Add a design documents for the unified base mesh

[xylar]

This mesh is intended to work for the E3SM land, river, ocean and sea-ice components.

This PR also adds design docs for 4 shared steps related to the unified base mesh, tentatively named :

• prepare_coastline
• prepare_river_network
• build_sizing_field
• create_base_mesh

Checklist

• Documentation has been built locally and changes look as expected

[xylar]

This is a draft, but feel free to comment and give feedback.

[xylar]

I'm going to fold #522 into this branch. It is too much effort to keep the designs separate.

[changliao1025]

Since buffer zone will be introduced "land-ward" during the coastline preprocessing, we might need to record both the raw and buffered spacing rasters for coastline.

[xylar]

Since buffer zone will be introduced "land-ward" during the coastline preprocessing, we might need to record both the raw and buffered spacing rasters for coastline.

I don't quite follow what you mean. But we can easily recover a mask from the signed distance function that includes all cells that are a given distance or further from the coastline in either direction. For example, signed_distance > 0 would recover the ocean mask. signed_distance < -buffer_distance would recover the land that is further inland than buffer_distance. So I believe everything we need should be available.

[changliao1025]

Since buffer zone will be introduced "land-ward" during the coastline preprocessing, we might need to record both the raw and buffered spacing rasters for coastline.

I don't quite follow what you mean. But we can easily recover a mask from the signed distance function that includes all cells that are a given distance or further from the coastline in either direction. For example, signed_distance > 0 would recover the ocean mask. signed_distance < -buffer_distance would recover the land that is further inland than buffer_distance. So I believe everything we need should be available.

I assume you meant geodesic distance?
In a raster based buffer calculation, the distance can mean pixel and the actual distance varies with latitude.

[xylar]

I assume you meant geodesic distance?
In a raster based buffer calculation, the distance can mean pixel and the actual distance varies with latitude.

It actually uses Cartesian distance but you have to be very far from the coastline for that to make a difference.