Surface Boundary Condition (SBC, ISF, ICB)

$$

!-----------------------------------------------------------------------
&namsbc        !   Surface Boundary Condition (surface module)
!-----------------------------------------------------------------------
   nn_fsbc     = 5         !  frequency of surface boundary condition computation
                           !     (also = the frequency of sea-ice model call)
   ln_ana      = .false.   !  analytical formulation                    (T => fill namsbc_ana )
   ln_flx      = .false.   !  flux formulation                          (T => fill namsbc_flx )
   ln_blk_clio = .false.   !  CLIO bulk formulation                     (T => fill namsbc_clio)
   ln_blk_core = .true.    !  CORE bulk formulation                     (T => fill namsbc_core)
   ln_blk_mfs  = .false.   !  MFS bulk formulation                      (T => fill namsbc_mfs )
   ln_cpl      = .false.   !  atmosphere coupled   formulation          ( requires key_oasis3 )
   ln_mixcpl   = .false.   !  forced-coupled mixed formulation          ( requires key_oasis3 )
   nn_components = 0       !  configuration of the opa-sas OASIS coupling
                           !  =0 no opa-sas OASIS coupling: default single executable configuration
                           !  =1 opa-sas OASIS coupling: multi executable configuration, OPA component
                           !  =2 opa-sas OASIS coupling: multi executable configuration, SAS component 
   ln_apr_dyn  = .false.   !  Patm gradient added in ocean & ice Eqs.   (T => fill namsbc_apr )
   nn_ice      = 2         !  =0 no ice boundary condition   ,
                           !  =1 use observed ice-cover      ,
                           !  =2 ice-model used                         ("key_lim3" or "key_lim2")
   nn_ice_embd = 1         !  =0 levitating ice (no mass exchange, concentration/dilution effect)
                           !  =1 levitating ice with mass and salt exchange but no presure effect
                           !  =2 embedded sea-ice (full salt and mass exchanges and pressure)
   ln_dm2dc    = .false.   !  daily mean to diurnal cycle on short wave
   ln_rnf      = .true.    !  runoffs                                   (T   => fill namsbc_rnf)
   nn_isf      = 0         !  ice shelf melting/freezing                (/=0 => fill namsbc_isf)
                           !  0 =no isf                  1 = presence of ISF
                           !  2 = bg03 parametrisation   3 = rnf file for isf
                           !  4 = ISF fwf specified
                           !  option 1 and 4 need ln_isfcav = .true. (domzgr)
   ln_ssr      = .true.    !  Sea Surface Restoring on T and/or S       (T => fill namsbc_ssr)
   nn_fwb      = 2         !  FreshWater Budget: =0 unchecked
                           !     =1 global mean of e-p-r set to zero at each time step
                           !     =2 annual global mean of e-p-r set to zero
   ln_wave = .false.       !  Activate coupling with wave (either Stokes Drift or Drag coefficient, or both)  (T => fill namsbc_wave)
   ln_cdgw = .false.       !  Neutral drag coefficient read from wave model (T => fill namsbc_wave)
   ln_sdw  = .false.       !  Computation of 3D stokes drift                (T => fill namsbc_wave)
   nn_lsm  = 0             !  =0 land/sea mask for input fields is not applied (keep empty land/sea mask filename field) ,
                           !  =1:n number of iterations of land/sea mask application for input fields (fill land/sea mask filename field)
   nn_limflx = -1          !  LIM3 Multi-category heat flux formulation (use -1 if LIM3 is not used)
                           !  =-1  Use per-category fluxes, bypass redistributor, forced mode only, not yet implemented coupled
                           !  = 0  Average per-category fluxes (forced and coupled mode)
                           !  = 1  Average and redistribute per-category fluxes, forced mode only, not yet implemented coupled
                           !  = 2  Redistribute a single flux over categories (coupled mode only)
/

$$

The ocean needs six fields as surface boundary condition:

• the two components of the surface ocean stress $\left( {\tau _u \;,\;\tau _v} \right)$
• the incoming solar and non solar heat fluxes $\left( {Q_{ns} \;,\;Q_{sr} } \right)$
• the surface freshwater budget $\left( {\textit{emp}} \right)$
• the surface salt flux associated with freezing/melting of seawater $\left( {\textit{sfx}} \right)$

plus an optional field:

• the atmospheric pressure at the ocean surface $\left( p_a \right)$

Five different ways to provide the first six fields to the ocean are available which are controlled by namelist namsbc variables: an analytical formulation (ln_ana = true), a flux formulation (ln_flx = true), a bulk formulae formulation (CORE (ln_blk_core = true), CLIO (ln_blk_clio = true) or MFS ^7.1(ln_blk_mfs = true) bulk formulae) and a coupled or mixed forced/coupled formulation (exchanges with a atmospheric model via the OASIS coupler) (ln_cpl or ln_mixcpl = true). When used ($i.e.$ ln_apr_dyn = true), the atmospheric pressure forces both ocean and ice dynamics.

The frequency at which the forcing fields have to be updated is given by the nn_fsbc namelist parameter. When the fields are supplied from data files (flux and bulk formulations), the input fields need not be supplied on the model grid. Instead a file of coordinates and weights can be supplied which maps the data from the supplied grid to the model points (so called "Interpolation on the Fly", see §7.2.2). If the Interpolation on the Fly option is used, input data belonging to land points (in the native grid), can be masked to avoid spurious results in proximity of the coasts as large sea-land gradients characterize most of the atmospheric variables.

In addition, the resulting fields can be further modified using several namelist options. These options control

• the rotation of vector components supplied relative to an east-north coordinate system onto the local grid directions in the model ;
• the addition of a surface restoring term to observed SST and/or SSS (ln_ssr = true) ;
• the modification of fluxes below ice-covered areas (using observed ice-cover or a sea-ice model) (nn_ice = 0,1, 2 or 3) ;
• the addition of river runoffs as surface freshwater fluxes or lateral inflow (ln_rnf = true) ;
• the addition of isf melting as lateral inflow (parameterisation) (nn_isf = 2 or 3 and ln_isfcav = false) or as fluxes applied at the land-ice ocean interface (nn_isf = 1 or 4 and ln_isfcav = true) ;
• the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift (nn_fwb = 0, 1 or 2) ;
• the transformation of the solar radiation (if provided as daily mean) into a diurnal cycle (ln_dm2dc = true) ; and a neutral drag coefficient can be read from an external wave model (ln_cdgw = true).

The latter option is possible only in case core or mfs bulk formulas are selected.

In this chapter, we first discuss where the surface boundary condition appears in the model equations. Then we present the five ways of providing the surface boundary condition, followed by the description of the atmospheric pressure and the river runoff. Next the scheme for interpolation on the fly is described. Finally, the different options that further modify the fluxes applied to the ocean are discussed. One of these is modification by icebergs (see §7.11), which act as drifting sources of fresh water. Another example of modification is that due to the ice shelf melting/freezing (see §7.10), which provides additional sources of fresh water.

Footnotes

... MFS^7.1

Note that MFS bulk formulae compute fluxes only for the ocean component