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Fix cvmix_KPP ice scaling: apply (1-Aice) to ustar and Stokes wind velocity #945

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Fix cvmix_KPP ice scaling: apply (1-Aice) to ustar and Stokes wind velocity #945

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06b6ee1
Fix KPP ice scaling: apply (1-Aice) to ustar and Stokes wind velocity
patrickscholz Jun 26, 2026
cd0966a
Merge branch 'main' into workbench_fix_kpp_ice_wind_scaling
JanStreffing Jun 26, 2026
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49 changes: 33 additions & 16 deletions src/cvmix_driver/gen_modules_cvmix_kpp.F90
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Original file line number Diff line number Diff line change
Expand Up @@ -447,6 +447,7 @@ subroutine calc_cvmix_kpp(ice, dynamics, tracers, partit, mesh)
real(kind=WP) :: vshear2, dz2, aux, aux_wm(mesh%nl), aux_ws(mesh%nl)
real(kind=WP) :: aux_coeff, sigma, stable
real(kind=WP) :: ustar, aux_surfbuoyflx_nl(mesh%nl), wind_norm
real(kind=WP) :: u_wind_eff, v_wind_eff

integer :: nzsfc, nztmp, nz_buoyflx_idx
real(kind=WP) :: sldepth, sfc_temp, sfc_salt, sfc_u, sfc_v, htot, delh, rho_sfc, rho_nz, oblguess
Expand Down Expand Up @@ -528,11 +529,27 @@ subroutine calc_cvmix_kpp(ice, dynamics, tracers, partit, mesh)
! calculate friction velocity (ustar) at surface (m/s)
ustar = sqrt( sqrt( stress_node_surf(1,node)**2 + stress_node_surf(2,node)**2 )*density_0_r ) ! @ the surface (eqn. 2)

! reduce friction velocity under ice --> approach take from the mpiom
! interface --> haven't been tested before with FESOM
! reduce friction velocity under ice --> approach taken from the mpiom
! interface. Physical basis: effective wind speed on the ocean is reduced
! by (1-a_ice) due to ice sheltering, so stress ∝ u_eff^2 ∝ (1-a_ice)^2,
! giving ustar = sqrt(tau/rho) ∝ (1-a_ice).
if (kpp_reduce_tauuice) then
ustar = ustar*(1.0_WP-a_ice(node))**2
end if
ustar = ustar*(1.0_WP-a_ice(node))
!ustar = ustar*(1.0_WP-a_ice(node))**2 ! original MPIOM-ported scaling
end if

! reduce effective wind velocity for Stokes drift / Langmuir turbulence
! under sea ice. The Pierson-Moskowitz parameterisation assumes a locally
! generated wind-sea; sea ice suppresses local wave growth. Using the same
! (1-a_ice) factor as ustar keeps LaSL = sqrt(ustar/us) finite and prevents
! spurious EFactor blow-up (EFactor → ∞ when ustar→0 but us stays finite).
if (kpp_reduce_tauuice) then
u_wind_eff = u_wind(node) * (1.0_WP - a_ice(node))
v_wind_eff = v_wind(node) * (1.0_WP - a_ice(node))
else
u_wind_eff = u_wind(node)
v_wind_eff = v_wind(node)
end if

!___3D Quantities___________________________________________________
!!PS if (flag_debug .and. mype==0) print *, achar(27)//'[35m'//' --> call shear variables'//achar(27)//'[0m'
Expand Down Expand Up @@ -588,19 +605,19 @@ subroutine calc_cvmix_kpp(ice, dynamics, tracers, partit, mesh)
zbar_3d_n(nz,node), &
zbar_3d_n(nz+1,node), &
kpp_A_stokes, &
u_wind(node), v_wind(node) , &
u_wind_eff, v_wind_eff , &
kpp_uS_t(nz), kpp_vS_t(nz) , &
kpp_uS_c(nz), kpp_vS_c(nz) , &
kpp_uS_m(nz), kpp_vS_m(nz))
call compute_stokes_velocities_MOM6style(&
kpp_uS_m(nz), kpp_vS_m(nz))

call compute_stokes_velocities_MOM6style(&
zbar_3d_n(nzsfc,node), &
sldepth, &
kpp_A_stokes, &
u_wind(node), v_wind(node), &
u_wind_eff, v_wind_eff, &
uS_sld_t, vS_sld_t , &
uS_sld_c, vS_sld_c , &
uS_sld_m, vS_sld_m)
uS_sld_m, vS_sld_m)


call cvmix_kpp_compute_StokesXi (&
Expand Down Expand Up @@ -798,7 +815,7 @@ subroutine calc_cvmix_kpp(ice, dynamics, tracers, partit, mesh)
OBL_depth = abs(Z_3d_n(nun:nln,node)), & ! (in) Assume OBL depth (m) = mid-depth level
surf_buoy_force = kpp_buoyflx_nl(nun:nln), & ! (in) surfce buoyancy flux (m2/s3) consider sw_pene
surf_fric_vel = ustar, & ! (in) turbulent friction velocity at surface (m/s)
xi = kpp_stokesVt_z(nun:nln), & ! (in) Stokes parameter xi= Ps/(PU+PS+PB)
xi = kpp_stokesVt_z(nun:nln), & ! (in) Stokes parameter xi= Ps/(PU+PS+PB)
w_s = kpp_ws_cntr(nun:nln) & ! (out) Turbulent velocity scale profile (m/s) for skalars
)
! --> need w_s to compute cvmix_kpp_compute_bulk_Richardson(...)
Expand All @@ -813,8 +830,8 @@ subroutine calc_cvmix_kpp(ice, dynamics, tracers, partit, mesh)
! is known
oblguess = max(kpp_obldepth(node), kpp_minOBLdepth)

! compute norm of wind velocity
wind_norm = sqrt(u_wind(node)**2 + v_wind(node)**2)
! compute norm of effective wind velocity (ice-scaled when kpp_reduce_tauuice)
wind_norm = sqrt(u_wind_eff**2 + v_wind_eff**2)

! computes the surface layer averaged Stokes drift, given
! the 10-meter wind (m/s) and the boundary layer depth (m).
Expand Down Expand Up @@ -939,14 +956,14 @@ subroutine calc_cvmix_kpp(ice, dynamics, tracers, partit, mesh)
end if
end do

call compute_stokes_velocities_MOM6style(&
call compute_stokes_velocities_MOM6style(&
zbar_3d_n(nzsfc,node), &
sldepth, &
kpp_A_stokes, &
u_wind(node), v_wind(node), &
u_wind_eff, v_wind_eff, &
uS_sld_t, vS_sld_t , &
uS_sld_c, vS_sld_c , &
uS_sld_m, vS_sld_m)
uS_sld_m, vS_sld_m)

! Cray compiler workaround: store subscript as scalar integer
nz_buoyflx_idx = kpp_nzobldepth(node)
Expand Down
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