Ocean Modeling Discussion

ROMS/TOMS

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PostPosted: Wed Aug 09, 2017 7:47 pm 
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Joined: Mon Apr 28, 2003 5:44 pm
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Location: Rutgers University
Those of you working with large discharge river point sources have probably experienced over/under-shooting of the salt concentration due to poor performance of the advection near to the point source when using 3rd-order upwind, Akima, or other schemes (not MPDATA). These extremes usually mix away quickly and I generally put this in the category of things not to worry about.

But I've recently noticed some added fluctuations in regions of negative salinity in the upper reaches of small tributaries that occur over the entire patch of negative salinity and I think it might be due to the virtual salt flux surface condition.

When using BULK_FLUXES and allowing for EMINUSP to change sea surface salinity (SSS), we parameterize the freshwater flux (fwf) as a Virtual Salt Flux (VSF) calculated as fwf times SSS. If advection has taken SSS into the range SSS < 0, then net precipitation over evaporation EMINUSP < 0 turns into a negative salt flux: heavy rain makes the ocean more salty and strong evaporation makes it more fresh.

Moreover, since in the upper reaches of small tributaries I might have 1 to 2 m of water, and 40 levels, the dz is only a few cm and the impact could be significant.

I can't immediately think of an easy fix to this. In reality we should have the EMINUSP freshwater flux add a finite volume of zero salinity water and dilute the mass of salt. This will mess with the continuity equation.

I may experiment with a simple minded governor on the flux when there are negative salinities (i.e. it's already totally freshwater so more rain won't change that) ...

In set_vbc.F it would be this:

stflx(i,j,isalt)=stflx(i,j,isalt)*MAX(0.0_r8,t(i,j,N(ng),nrhs,isalt))

Anyone have any thoughts on this issue?

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John Wilkin: IMCS Rutgers University
71 Dudley Rd, New Brunswick, NJ 08901-8521, USA. ph: 609-630-0559 jwilkin@rutgers.edu


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