Greetings,
I am running a simulation of the North Aegean region for the period 2002-2007. The general picture, concerning circulation and tracers, seems ok except an small area close to the southern boundary. A cyclone is created in that area and also the temperature field is cooler from that was expected. Μy question is this: it's an artifact because of the reflection(???) to and from the southern boundary or not? If not what is causing it? The boundary conditions file was created from MFS system reanalysis (http://gnoo.bo.ingv.it/mfs/dailysys_des.htm). I am using BULK_FLUXES and my atmospheric forcing field have a 6 hours time step. The OBC options were closed for east, north and west edge. For the south edge was clamped for u,v,salt and temperature, chapman for zeta, flather for ubar,vbar and gradient for tke. I am attaching my *.h file and a figure from the run. Any help or suggestion will be highly appreciated.
Giannis
Is it artifact or not?
Is it artifact or not?
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north_aegean.h- (1.79 KiB) Downloaded 275 times
Re: Is it artifact or not?
Ill-behaved boundary conditions are quite capable of creating odd eddies like yours. Since you have reasonable values from outside for your southern boundary, I suggest you try the RadNud option for LBC instead of clamped. It has served us well.
Re: Is it artifact or not?
Greetings,
Kate thank you very much for your answer. I would like to ask a couple more thing about what you have suggested. If i am understanding right you suggest to use RadNud boundary conditions for my 3D fields (T,S,U,V). Is there any specific/accurate(??) way to calculate/set some how optimum nudging time scales for my variables and OBCFAC? Maybe it's silly to ask but i am little confused with this issue: if i am nudging only the 3D fields do i have to set values for the time scales for the other variables in ocean*.in file (ZNUDG, M2NUDG)? I am using a climatology file which i have create it from MEDATLAS database and contains values only for tracers (T,S). Is it necessary to also nudge the climatology? Until now i haven't see bad drifts. Thanks again for your time.
Giannis
Kate thank you very much for your answer. I would like to ask a couple more thing about what you have suggested. If i am understanding right you suggest to use RadNud boundary conditions for my 3D fields (T,S,U,V). Is there any specific/accurate(??) way to calculate/set some how optimum nudging time scales for my variables and OBCFAC? Maybe it's silly to ask but i am little confused with this issue: if i am nudging only the 3D fields do i have to set values for the time scales for the other variables in ocean*.in file (ZNUDG, M2NUDG)? I am using a climatology file which i have create it from MEDATLAS database and contains values only for tracers (T,S). Is it necessary to also nudge the climatology? Until now i haven't see bad drifts. Thanks again for your time.
Giannis
Re: Is it artifact or not?
Like grid generation, open boundaries are something of an art. There's a lot of "try it and see". Still, I haven't heard that clamped boundaries are a good idea and we have gotten a lot of use out of the RadNud option. I don't know what you are using for u,v for your clamping now, but letting it have some freedom there will probably be good, especially if you don't have a u,v that's consistent with your T,S.
As for timescales, we've been using 360 days for the outgoing nudging times, 2-3 days for incoming nudging times. We're nudging the boundary to 5-daily fields of u,v,T,S from a larger (data assimilating) model. Things behave better with that than with something like a monthly Levitus climatology - but that's just been my experience.
When we do climatology nudging, it's only in a narrow band by the boundary using monthly fields - again from a larger model.
As for timescales, we've been using 360 days for the outgoing nudging times, 2-3 days for incoming nudging times. We're nudging the boundary to 5-daily fields of u,v,T,S from a larger (data assimilating) model. Things behave better with that than with something like a monthly Levitus climatology - but that's just been my experience.
When we do climatology nudging, it's only in a narrow band by the boundary using monthly fields - again from a larger model.