[MITgcm-support] 2deg global experiment: advection schemes

[Martin Losch]

Hi there,

I would like to share my experience with this (and maybe get some 
general advice):

I am running a 2deg global ocean model (much like the 2deg ECCO 
configuration, that is, from -80 to 80 deg, 23 layers, GM, KPP) and I 
make the interesting observation, that the solution is very sensitive 
to the advection scheme (and the parameter staggerTimeStep) that I use.
I have reason to believe that the default 2nd-order scheme is 
inadequate for my purposes (I restore T and S along the southern 
boundary in order to model the effects of the Antartic shelf ice 
caverns and thus get sharp gradients there, that lead to weird T and S 
properties near the southern boundary), so I tried 3rd-order upwind, 
DST3 with an without flux limiter, and 2nd-order with flux limiter.
First of all, when staggerTimeStep=.false. (default), then these 
advection schemes (together with a set of GM, diffusivitity, viscosity 
parameters that produced a reasonable circulation with the default 
2nd-order scheme: 133Sv ACC, 20Sv MOC, etc) give me a dramatically 
intensified circulation: ACC order 400-500Sv, etc. watermasses change 
drastically (not the patterns, just the horizontal and vertical 
gradients, which explains the stronger circulation).
Second, staggerTimeStep=.true. brings down the values to a realistic 
126-130Sv (and realistic MOC, etc), except for DST3 without flux 
limiter, where the ACC is still O(250Sv).

2 years ago, Jean-Michel recommended to use the staggerTimeStep flag 
with higher order schemes, so I am not so surprised that this option 
salvages most of the runs, but still I am surprised that these 
advection schemes have such a fundamental impact on the solution.
I had expected that the additional diffusion introduced by these 
schemes (in particular the flux limited schemes) would actually reduce 
the circulation (as they mostly do with staggerTimeStep=.true.), but 
not increase it.

Was I wrong to keep the GM-diffusivity and viscosity parameters while 
switching the advection shemes?

Martin

Martin Losch // Alfred-Wegener-Institut für Polar- und Meeresforschung
Postfach 120161, 27515 Bremerhaven, Germany
mailto:mlosch at awi-bremerhaven.de // Tel./Fax: ++49(471)4831-1872/1797
http://www.awi-bremerhaven.de/People/show?mlosch