[MITgcm-support] using RBCS to generate flow

[katsman]

Dear MITgcm developers,

Over the past years, I used the RBCS package to generate a flow in an 
(otherwise unforced) basin representing the Labrador Sea, by restoring 
the flow to a prescibed 3D-temperature and velocity field in a corner of 
the basin (T and velocity in geostrophic balance). This worked excellent 
in a (admittedly old) checkpoint58-version in which we manually added a 
restoring term on U and V analogous to the programmed T, S part.

When I got a new workstation I thought it time to upgrade to the most 
recent MITgcm version, but I cannot get the package to work in the same 
way (and I tried many things over the past months, so I am getting a bit 
desperate...).

The new checkpoint64 is supposed to be able to restore T,U and V, but I 
find that - when I prescribe my T,V fields again as before - in the 
sponge region defined by the mask, T is restored fine, while V is 
restored fine only during the first few timesteps of the simulation. 
Then a purely baroclinic U develops as well, while V becomes barotropic 
(despite RBCS acting on it). Intriguing, but not what I wanted.

A test that really makes me think there is a bug in the rbcs package 
rather than me doing something stupid is that when I only restore T with 
a gradient in x, I expect to see a purely baroclinic V develop in 
geostrophic balance with that. In stead, nothing happens (I can run it 
for a month, no flow develops at all).

One of the earlier versions (c62) has yet another version of the RBCS 
package. When I do a test with T-restore only (U,V is not standard in 
that one), a baroclinic flow does develop obeying geostrophy.

Any ideas what is wrong here? I checked - the  model does frequent the 
appropriate lines of code  in rbcs_add_tendency in the c64 version  to 
change the gV parameter - I suspect somewhere in the code U and V are 
mixed up. Notably, the baroclinic flow that develops in the sponge 
region is roughly the same strength as the V I prescribe

Any ideas?? (if the description is unclear I can send pictures)

Thanks in advance & happy holidays
Caroline