sens   : W/(m**2) = J /(m**2 s)
                     evap   : Kg/(m**2 s)
                     tau_u  : Kg/(m s**2) = (Kg m/s)/(m**2 s) 
                               = (Kg/(m**3))((m/s)**2) = Pascals
                     tau_v  : as above

             Flux should be evaluated with atmospheric fields 
                     from time i   for two-time level schemes
                     from time i-1 for leapgrog

             If dflux_datmos is non-zero, input flux is modified by 
             the implicit c