Hybrid vertical coordinates

[shoyer]

Hybrid sigma-pressure coordinates are used by most modern dynamical cores. Switching from sigma to hybrid coordiates would make it possible to run the Dinosaur dycore on native ECMWF/UFS grids, and presumably would improve model performance in mountainous areas.

Implementation wise, this should be relatively straightforward. The math on hybrid coordinates is pretty straightforward, and described in standard textbooks (e.g., Dale Durran's book).

The NeuralGCM team does not currently have any plans to implement this feature, but we would welcome external contributions.

[milankl]

There's a visualisation of this here https://github.com/SpeedyWeather/SpeedyWeather.jl/issues/329#issue-1711108442

(green) sigma-pressure coordinates (black) sigma coordinates