Investigate most stable sponge parameters

[LenkaNovak]

Currently, the Rayleigh sponge makes the target longrun unstable after 140d. Removing the sponge allows running the same config for 300d+).

It is clear that we need to use some dissipation in the upper atmosphere to make the simulation more realistic, so we need to find a better more stable sponge configuration for this simulation set up.