# How is nu_div (weighting of divergence component when applying hyperviscosity) used?

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jihwang@...
How is nu_div (weighting of divergence component when applying hyperviscosity) used?

Hello All:

In user_nl_cam, there is a parameter called "nu_div". It seems to me that CAM5 damps divergent flow using "nu_div" parameter, not "nu". Is this correct? Does anybody know exactly how "nu_div" and "nu" is used and how they differ?

Thank you very much.

Best Regards,
Jihwang (Aaron) Wang
NOAA/PSD & CIRES

jihwang@...

Hello All:

Does anybody know how "nu_div" is used in CESM SE grid?

Thanks a lot.

Best Regards,
Jihwang (Aaron) Wang
NOAA/PSD & CIRES

goldy

'nu_div' is the hyperviscosity applied to divergence component of winds [m^4/s]. Its use in CAM5 is documented in:

Dennis, J. M., J. Edwards, K. J. Evans, O. Guba, P. H. Lauritzen, A. A. Mirin, A. St-Cyr, M. A. Taylor, and P. H. Worley (2012), CAM-SE: A scalable spectral element dynamical core for the Community Atmosphere Model, Int. J. High. Perform. C., 26(1), 74–89, doi:10.1177/1094342011428142

Its default value is 1.5x10^16 for the ne16np4 grid, 2.5x10^13 for the ne120np4 grid, and 2.5x10^15 otherwise.
If you set the value of 'nu_div' to be less than zero, the value of 'nu' is used instead. The value of 'nu' (the hyperviscosity coefficient for u,v, T [m^4/s]) is dependent on the grid resolution.

jihwang@...

Thank you, Goldy.

I read Section 2.9 of that paper. Although it explained the use of "nu", it is still ambiguous how "nu_div" applies on Equation (17) and (18). There are two terms on RHS of Eq (18), which can be plugged into Eq (17) to form four terms in the big parenthesis on RHS of Eq (17). When you said "applied to divergence component of winds", are you referring to the 1st term on the RHS of Eq (18)?

Thanks again. I appreciate your help.

Best Regards,
Jihwang (Aaron) Wang
NOAA/PSD & CIRES

goldy

I can see why that is confusing. To really understand this, I strongly encourage you to contact the authors for their take.

The way I think of it, is that they apply the ratio, "nu_div" / "nu", directly to the divergence (\nabla \cdot \vec{u}) but I defer to the authors on that interpretation.

jihwang@...

Thank you.

I'll email the authors and keep you posted.

Best Regards,
Jihwang (Aaron) Wang
NOAA/PSD & CIRES