G. Exceptions__and__Special__Cases____________________

I. Ice-Ocean Stress

      For the cavitating fluid ice model used in the NCAR CSM, the stress between the ice
and the ocean is defined by

                                     h                                                i
                            "o4 = Cw     uGH  - uC     cosOE + k x {uGH  - uC  } sinOE  :                                    (G1)



In equation (G1), uGH  is the geostrophic surface velocity of the ocean, uC  is the ice velocity,
Cw   is  a  constant  coefficient,  and  OE  is  the  turning  angle  taken  to  be  25O  in  the  Arctic
and Antarctic.  Although the surface ocean velocity is passed to the coupler, it is not used
in the calculation of the ice-ocean stress; instead the ocean surface geostrophic velocity is
calculated from the ocean surface slope rj, which is also passed to the coupler, by



                                                      uGH  =  g__fk x rj;                                                    (G2)



where  g  is  the  gravitational  acceleration  and  f  is  the  Coriolis  frequency.   A  term  pro-
portional  to  rj  appears  as  a  forcing  term  in  the  right-hand-side  of  the  equation  for  ice
velocity.  The  ice  model  solves  for  the  ice  velocity  by  transferring  the  terms  proportional
to uC   in (G1) to the left-hand-side of the equation and adding them to the Coriolis terms.
Details can be found in the sea-ice model document.  However, the resulting stress needed
for the right-hand-side of the ice velocity equation is

                                                    h                            i
                                     "oc = "o1 + Cw   uGH  cosOE + k x uGH  sinOE  ;                                         (G3)



where "o1  is the stress on the ice from the atmosphere.  "oc is what is presently sent by the
coupler  to  the  ice  model  instead  of  the  vector  difference  between  the  wind  stress  on  the
ice minus the stress of the ice on the underlying water.  However, the ice-ocean stress that
the coupler sends to force the ocean model is "o4  given by equation (G1).  Thus, there is a
mismatch in the definition of the ice-ocean stress because of the method used by the ice
model to determine ice velocity.  In the coupler code there is one location where the names
of stress arrays contain the number 2; this is to address this situation where there are two
forms of the ice-ocean stress.

II. Balancing Precipitation and Evaporation

      In the Climate System Model, fresh water should be conserved.  Fresh water goes into
the  ocean  through  river  run-off,  as  well  as  by  precipitation.  The  present  CSM  does  not
have a river run-off component and, in addition, the marginal seas do not exchange water
with the rest of the global ocean domain.  So, a short-term fix has been implemented in the
coupler so that the net fresh water in the active domain of the ocean model is conserved.
The fix is to calculate the area averages over the active ocean domain of both precipitation,


                                                                G-1

< P  >, and the evaporation, < E >.  Then the precipitation coming from the atmosphere
model over the active ocean is multiplied by a factor, fp , given by



                                                         fp  =  <_E_>_____<;P  >                                             (G4)


Thus,  the  fresh  water  budget  over  the  active  ocean  is  balanced.  This  balancing  is  done
as  frequently  in  time  as  the  precipitation  and  evaporation  fields  are  sent  to  the  coupler.
Values  of  fp  at  different  times  can  vary  somewhat,  but  time-averaged  values  of  fp  are
usually between 1.02 and 1.03.  This is consistent with river run-off into the ocean being
between 2% and 3% of the globally averaged precipitation falling on the ocean surface.



                                                                G-2