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Remapping from ocean to atmosphere

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Remapping from ocean to atmosphere

Posted by Anonymous at March 19. 2019

Hi folks,

I wondered if you could help to answer a question about what happens when regridding a flux from an ocean to an atmosphere grid, with regard to how the things behave when the target (atmosphere) grid cell is fractionally land and how the resulting received value should be regarded? In our case we're regridding a CO2 flux (kg/m2/s), using 1st order conservative regridding with the FRACAREA normalization option.

So here's an example to illustrate what I think happens: Suppose we start with a uniform field value of 4.0 on all source (ocean) points, and a sea fraction of 0.75 (land fraction = 0.25) in the target (atmos) cell. That would ordinarily be translated to a value of 0.75 x 4.0 = 3.0 on the target cell. But because we use FRACAREA, then that value gets normalized by the sea fraction: 3.0 / 0.75 = 4.0. So we should get back to our desired 4.0 kg/m2/s again on the target cell.

I thought that was right but other people have their doubts and are causing me to worry that I'm wrong!? Could you clarify whether the above interpretation is correct?

Thanks for any thoughts, Richard

Re: Remapping from ocean to atmosphere

Posted by Anonymous at March 19. 2019

Hi Richard,

If you want to locally conserve the CO2 mass, one should use DESTAREA. In the DESTAREA case, normalisation is done with the target surface so the flux value would be 4.0/0.75 = 5.333 kg/m2/s. You would conserve the CO2 mass as what is leaving the ocean (i.e. 4.0 x 1) is equal to what enters the atmosphere (i.e. 5.33 x 0.75). In practice we don’t use the DESTAREA option because it may result in pretty unrealistic flux value if the target area is very small.

If you use the FRACAREA, then the normalisation is done using source surface, so indeed the flux value would be 4.0/1.0 = 4.0 kg/m2/s. In this case, the value is reasonable but the local conservation is not ensured as the mass of CO2 leaving the ocean (i.e. 4.0 x 1) is not equal to the mass of CO2 that enters the atmosphere (i.e. 4.0 x 0.75).

In summary, DESTAREA ensures local conservation but may lead to unrealistic values; FRACAREA does not ensure local conservation (in case of non matching land-sea masks) but gives reasonable values.

In CNRM-CM6 we use FRACAREA but we then apply a global CONSERV.

I hope this helps?

With best regards, Sophie

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