[MITgcm-support] eta in the atmospheric model

[Roland Young]

Hi,

I’m a bit confused about what eta is in the atmospheric model (partly because the units in the output file are for the ocean model). In particular the relationship between the surface pressure (which appears to change) and the model cell centres and faces (which appear to be fixed).

The surface boundary condition given by Eq. 1.55 is p = ps, and (I thought, at least until now) that ps is constant and specified in the “data” file as Ro_SeaLevel.

But Sect. 3.6.3.1 describes eta as surface pressure anomaly, which implies that the surface pressure changes in time.

So (1) in the atmospheric model, is ps = Ro_SeaLevel + eta?

(2) Is the lowest grid cell face at Ro_SeaLevel (fixed), or ps (varying)?

(3) The vertical grid cell faces are at rF, and rF[1] = Ro_SeaLevel. When eta<0, does this mean that the lowest cell face is below the model surface? If so, how are you supposed to interpret the vertical velocity there? Similarly, if eta is large enough such that it is more negative than rC[1] - rF[1], how should I interpret the values of the prognostic variables at the first cell centre?

Thanks,

Roland