[MITgcm-support] Internal wave - boundary

[Flavien Gouillon]

Dear all,

I’m trying to do quite a simple configuration: a barotropic tidal flow 
interacting with a Gaussian ridge (i.e. 2D problem, no y-dependency, no 
Coriolis). My basin is 1200km long and I use dx=1.5km as my model grid 
spacing with 60 vertical z-levels (i.e. grid dimensions is 800x1x60). My 
stratification is uniform and only the temperature is an active tracer. 
Both West and East boundary are open and I use Orlanski OB conditions to 
radiate away my barotropic component. I stop my simulations after ~10 
days so my internal wave does not reach any of the boundaries (i.e. I do 
not need sponge condition, I think). Rather than prescribing the tidal 
forcing as a body force I prescribe a barotropic tidal flow at my 
Western boundary by modifying the orlanski_west subroutine (attached) 
based on some Jody's code. The barotropic tidal flow is well prescribed 
(I checked velocity at my Western boundary) and IW generation and 
propagation is also well simulated.

The problem is that I generate some strong baroclinic velocities 
anomalies of order 1e-5 m/s at this same boundary that will at some 
point interact with my internal wave field. I do not understand why: is 
there an inconsistency between the free-surface and the barotropic flow 
I prescribe or a problem with how I handle my temperature at the OB 
(Temp field looks OK though)? I have attached figures that show this 
baroclinic component propagating from the West into my basin.
It is also worthwhile to mention that when I reduce my dt, the magnitude 
of these anomalies is reduced (anomalies are of order 1e-4 m/s for a dt 
of 10s and order 1e-5 m/s for a dt of 1s, what I'm showing). So is this 
the proof of some numerical instability at the OB…? I also tried to put 
land for the Eastern boundary (still using Orlanski) to make sure it 
wasn't a periodic basin type of problem and I still have it (even 
stronger anomalies).

I also attached my data file for anyone who is curious about the 
different options and parameters I use.

I’ll appreciate any help/advices; if you need more info let me know.
Thanks in advance.

Flav

-- 
Flavien Gouillon
GRA, Physical Oceanography
COAPS/FSU
www.coaps.fsu.edu/~gouillon

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