[MITgcm-support] tracers

[Dimitris Menemenlis]

Alan, I tried to send this yesterday from hotel but internet crashed.   
My answer is superseded by Martin's much more useful one.  Here it is  
nevertheless:

I think that the best you can do is to add a passive tracer with  
concentration proportional to relative volume of runoff entering a  
particular surface grid box at a particular time step.  As soon as the  
runoff enters a model grid box, it mixes completely with the water in  
that grid box (since a grid box is the smallest volume of water that  
the model knows of) so adding a passive tracer with the correct  
concentration in that grid box would be equivalent to what you  
describe in your message.

Dimitris

On Sep 23, 2009, at 9:28 AM, Martin Losch wrote:

> Hi Alan,
>
> I am not sure if I understand exactly, what you want to do exactly,
> but (if you are using pkg/exf) there is a field called runoff (units
> are m/s). All you need to do is add this to a ptracer-field, the best
> place would be S/R PTRACERS_FORCING_SURF:
> surfaceForcingPTr(i,j,bi,bj,iYourTracer) =
> runoff(i,j,bi,bj)*_ra(i,j,bi,bj) * drF(ks)*_hFacC(i,j,ks,bi,bj)
> (ks=1 in z-coordinates); because this is an absolute volume add it at
> the end of the routine (to avoid the dilution of volume). Then you'd
> have a volume of freshwater from runoff in m^3 in the ptracer
> iYourTracer (the cell thickness is taken out again when
> surfaceForcingPTr is applied in ptracers_forcing). The disadvantage is
> that you can accumulate more fresh water from runoff in a grid cell
> than the cell can hold which is kind of unsatifactory. Alternatively
> you could add a tracer of some concentration (e.g. 1units/m^3) to the
> run-off, but that appears to me more complicated: In this case I would
> add the tracer at the beginning of the routine as
> surfaceForcingPTr(i,j,bi,bj,iTrc) = runoff(i,j,bi,bj) * 1 (units/m^3)
> Because this is a now a concentration, it should be diluted by
> additional EmP (but not mR), so that you'd have to remove the runoff
> from the EmPmR and PmEpR in the following lines of code.
>
> Martin
>
> On Sep 22, 2009, at 9:51 PM, Alan Condron wrote:
>
>>
>>
>> Does anyone know if it possible to add a tracer directly to the
>> model runoff
>> field? For example, instead of adding a ptracer field in a grid cell
>> that
>> corresponds to a river mouth, I would like the freshwater that runs
>> off into
>> the model in this region to already have a tracer in it. In this
>> way, I will
>> know that I'm tagging only freshwater from this source. Is this
>> possible to do?
>>
>> Many thanks,
>> Alan
>>
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