Physics-side sponge layer diffusion coefficients are not being applied

[adamrher]

What happened?

The sponge layer diffusion coefficients for momentum set in src/physics/cam/vertical_diffusion.F90 are not being applied to the sponge layer. This bug impacts mid-top, WACCM and WACCM-X configurations. In the following loop adding kvm_sponge to kvm:

  !
  ! add sponge layer vertical diffusion
  !
  if (allocated(kvm_sponge)) then
     do k=1,size(kvm_sponge(:),1)
        kvm(:ncol,1) = kvm(:ncol,1)+kvm_sponge(k)
     end do
  end if

kvm has a hard coded level index of 1, where it should instead be k.

A separate issue with this code is that the kvm array is on the pverp grid, meaning level 1 is the top of the model, and are therefore of no consequence because the fin_vol_lu_decomp diffusion solver code uses a fixed or zero flux boundary condition. This implies the values of kvm_sponge(1) are also not of any consequence, even after fixing the bug. Running two simulations and varying kvm(:ncol,1), the answers are bfb, confirming that the model top kvm values are not used.

What are the steps to reproduce the bug?

Run FMTHIST or a WACCM/-X compset

What CAM tag were you using?

cam6_4_038

What machine were you running CAM on?

CISL machine (e.g. cheyenne)

What compiler were you using?

Intel

Path to a case directory, if applicable

No response

Will you be addressing this bug yourself?

Yes

Extra info

@PeterHjortLauritzen

[dan800]

@PeterHjortLauritzen @adamrher Is this a bug that needs correcting in prior versions of the code, or did it creep in recently?

[adamrher]

Hi Dan, this bug came in when implementing sponge layer diffusion on the physics side, in cam6_3_151 #968.