[MITgcm-support] Production of spurious negative tracer values by GM

[Christopher L. Wolfe]

Hi,

I'm doing some tracer release experiments in a low resolution model  
using the ptracers package. The initial condition is a strip of  
positive tracer on the surface and zero everywhere else. The tracers  
are advected by scheme 33 and diffused by a very low diffusion  
(1e-7). If I set PTRACERS_useGMRedi(1)=.TRUE., I get rather  
significant (>30% RMS tracer value) negative tracer values pretty  
quickly (less than a year). If I turn GM off, the negative values are  
at the level of single precision numerical noise (see attached  
figure). I'm using the plain vanilla skew flux GM scheme with all the  
default values.

My understanding is that the skew flux GM is mathematically  
equivalent to advection by a non-divergent bolus velocity---which  
shouldn't be able to produce negative values from positive data---and  
that the skew flux form is preferred because it is numerically more  
accurate. In the present case, however, the skew flux form seems to  
be significantly in error.

I'll run a test using the advective form of GM, but until then, does  
anyone have any idea what's going on?

Thanks,
Christopher

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Dr. Christopher L. Wolfe                   858-534-4560
Physical Oceanography Research Division    OAR 357
Scripps Institution of Oceanography, UCSD  clwolfe at ucsd.edu
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