Is Evaporation acting as a sink of mass?

[balbin]

Dear all,

I am starting to play with roms trying to get a Mediterranean Sea climatology.
The model encloses the whole basin with 384x176x30 grid points and has a west open boundary at 9ºW (4º West of Strait of Gibraltar). Open boundary uses FS, T, M2, M3 RADIATION and TNUDGING with SPONGE and WEST_VOLCONS.
T,S initial fields are from MEDAR and forcing fields are from COADS. I am not calculating bulk fluxes but providing the shflux, swrad, swflux, sustr, svstr, dQdSST, SST and SSS for QCORRECTION and SCORRECTION.
Running the model for ten years I get a volume averaged trend in tracers (I still did not try to correct surface fluxes) but most of the features of termohaline circulation are defined (some of them not at all).

As you know, the Mediterranean sea has a negative water budget: the loss in the atmosphere by evaporation is larger than the gain by precipitation, runoff from the rivers and input from the Black sea. Water loss to the atmosphere and riverine inputs combined lead to an estimated Mediterranean freshwater deficit of about 500 mm/yr. However, the mediterranean sea level and salinity remain mostly unchanged, since a fresher surface flow enters from the Atlantic ocean, driven by the sea surface height difference.

When I compute tracer budgets through a section at 5.5 º W there is good agreement between the integrated surface tracer flux plus tracer flux at the Gibraltar strait and the inter-annual volume averaged tracer derivatives. But, while volume is keep constant along time, there is no net volume influx to compensate surface evaporation.

I tried to check the code but I could not find any line regarding this E-P effect as a sink of mass or volume. Anyway, I am not an expert.
Is there any way to activate this kind of calculation?

I also have doubts about the way VOLCONS activate the volume conservation. If it only cancels the sum of fluxes through the open boundaries it is not appropriate for my problem. I have tried to switch it off (a first to check to include evaporation as a next step) but the model blows up after the first month. I am now trying to understand why.

Thanks for your comments and best regards

[kate]

ROMS does not change the volume when there's evaporation. It's not something we've ever needed. A friend of mine was working on it many years ago with another model and said it's tricky, so it's not a quick and easy change.

[mathieu]

The key code to look at is bulk_flux and there you see that the evaporation is modeled as a salt flux:

Code: Select all

          stflx(i,j,isalt)=cff*(evap(i,j)-rain(i,j))

Thus the model does not lose water but increases its total quantity of salt. SELFE do the same thing as ROMS but SHYFEM does it correctly as a water flux.

[balbin]

Thanks a lot for your answers.
Kate, would it be possible that I contact this friend of yours to ask him directly and to know his experience? Do you think he would mind?
Thanks in advance

[kate]

I haven't seen that friend since Ocean Sciences some years ago. Here's her contact info from a publication:

Ray-Qing Lin, David Taylor Model Basin, Division of Seakeeping,
9500 MacArthur Blvd, West Bethesda, NSWCCD, MD 20817-5700

I don't think she would mind.