#5
Post
by **wilkin** » Wed Aug 15, 2007 7:27 pm

The COARE algorithm is clear that the wind vector needs to be referenced to the surface current. This is straightforward to do in bulk_fluxes because the wind is already expressed in local roms coordinate directions.

Schematically, we modify uwind,vwind:

Uwind(i,j) = Uwind(i,j) - 0.5*(u(i,j,N)+u(i+1,j,N))

Vwind(i,j) = Vwind(i,j) - 0.5*(v(i,j,N)+v(i,j+1,N))

The height at which the wind obs are made does not matter because similarity theory assumes the stress does not rotate through the boundary layer (regardless of whether the conditions are stable or unstable). This is readily seen when considering the stress calculation:

Drag coefficient Cd is computed from wind speed, then

cff=rhoAir*Cd*Wspeed

Taux=cff*Uwind ! plus adjustment for momentum of the falling rain

Tauy=cff*Vwind !

So in the absence of rain, abs(Taux,Tauy) is rhoair*Cd*Wspeed^2 and stress and wind vectors are in the same direction.

We accept simple C-grid coordinate averaging in the final step where Taux and Tauy are averaged in i,j to place sustr,svstr on the ROMS C-grid velocity locations. So pre-shifting Uwind,Vwind with surface u,v in the same way is a comparable approximation. We always assume conditions at z=zeta are represented by values at z_r(N) (in bulk flux we do this for SST for example) so there are no new approximations here.

The only issue in terms of the implementation that I wonder about is whether it is sensible to modify Uwind,Vwind globally (i.e. communicate this back to ROMS on exit from bulk_fluxes), or keep the modified Uwind,Vwind local to bulk_fluxes. I suspect the latter is safer, until other coupled models demand another strategy.

(As an aside: this issue of calculating the wind relative to the current came up for discussion in the recent Gordon Research Conference on Coastal Ocean Modeling. GRC rules preclude me repeating the details of the discussion here - but suffice to say nobody challenged the rationale of the published COARE algorithm in this respect).

John.

John Wilkin: DMCS Rutgers University

71 Dudley Rd, New Brunswick, NJ 08901-8521, USA. ph: 609-630-0559

jwilkin@rutgers.edu