[MITgcm-support] Negative concentrations in pTracer

[Menemenlis, Dimitris (329D)]

Roland, is that really a "d8" or "d-8" for the negative numbers?
If "d8", answer below is not relevant.

If "d-8", I think it's probably just normal advection noise.  See, for example,
http://mitgcm.org/public/r2_manual/latest/online_documents/node72.html
http://mitgcm.org/public/r2_manual/latest/online_documents/node81.html

Typically advection schemes with flux limiters, as is 77, should remove or
minimize this type of overshoot, but maybe not perfectly so.  You could try
using 33 or 7, see if it helps get rid of undershoot.
 
> On Nov 27, 2014, at 7:33 AM, Roland Young <young at atm.ox.ac.uk> wrote:
> 
> Hi,
> 
> I'm using a number of atmospheric tracers carried around by the pTracers package. These typically begin as a thin layer over a handful of model levels, with the other levels set to zero. I have found that when tracers are advected vertically (tracer diffusion coefficients are zero) the concentration in the cells where initially there is no tracer can go negative. The effect is small but noticeable - typically 7-8 orders of magnitude smaller than the initial concentration. For example, schematically:
> 
> Initial condition
> 
> space
> 0.0d0
> 0.0d0
> 0.0d0
> 1.0d0
> 1.0d0
> 1.0d0
> 1.0d0
> 0.0d0
> 0.0d0
> 0.0d0
> ground
> 
> After 1 timestep
> 
> space
> 0.0d0
> 0.0d0
> -2.0d8
> 1.0d0
> 1.0d0
> 1.0d0
> 1.0d0
> 3.0d-8
> 0.0d0
> 0.0d0
> ground
> 
> After 2 timesteps
> 
> space
> 0.0d0
> 3.0d-15
> -1.0d8
> 1.0d0
> 1.0d0
> 1.0d0
> 1.0d0
> 2.0d-8
> -4.0d-16
> 0.0d0
> ground
> 
> I was wondering why it's going negative and whether there is an option that will keep these values positive? I'm currently using pTRACERS_advScheme=77 as that's what is used by most of the verification examples, with mom_vecinv for momentum. I tried pTRACERS_advScheme=33 but it made little difference. Also PTRACERS_ImplVertAdv=.FALSE.; with it set to TRUE it is worse (in that case the negative concentration can be of comparable magnitude to the initial postive one).
> 
> I'm wondering why values in pTracers can go negative at all - surely this is unphysical?
> 
> Best wishes,
> 
> Roland
> 
> 
> 
> 
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