If you are using a bottom boundary layer model you should not have UV_QDRAG defined. The bottom boundary layer model is computing the stress itself so you don't want to override that with a quadratic drag law.

Whatever your choice of bottom boundary layer model (which affects the .h file included into bbl.F) the bblm subroutine concludes by calculating bustr,bvstr.

Look in set_vbc.F and you'll see that provided neither UV_LDRAG or UV_QDRAG are defined, ROMS simply applies the stresses from bblm (which is called in main3d immediately prior to the call to set_vbc).

However, if either UV_LDRAG or UV_QDRAG are defined the stresses from bblm will be replaced with the simple drag law in set_vbc.

I suspect your model has used UV_QDRAG and ignored everything from the bottom boundary layer model.

To answer your question about drag coefficient ... what you what is the equivalent quadratic drag coefficient as if there were a quadratic drag law. I think you can diagnose this from the saved bottom stresses (bustr,bvstr) which you can output, and the saved bottom velocity ub,vb (u,v at k=1) which you can output.

Then CD_effective = mag(sustr,svstr)/(rho0*mag(ub,vb)

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John Wilkin: DMCS Rutgers University

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