An improved scheme for interpolating between an atmospheric model and underlying surface grids near orography and ocean boundaries

To take into account the strong nonlinearities of vertical fluxes due to sm all-scale heterogeneities of surface properties, more and more coupled gene ral circulation models compute part of their atmospheric physical parameter izations, either the surface fluxes or the whole package, on the finer grid of their ocean or land model. A modification of a traditional interpolatio n scheme is presented to calculate the values of atmospheric variables over surface model grid points. In addition to the desirable properties of flux conservation and preservation of a constant field, the new scheme allows d iscontinuities in the interpolated fields at the surface model's boundaries and orographic jumps, while remaining continuous elsewhere. It can also be tuned separately for each variable. The modified scheme is then evaluated using the circulation model of the La boratoire de Meteorologie Dynamique coupled to the Laboratoire d'Oceanograp hie Dynamique et de Climatologie tropical Pacific Ocean model using the del ocalized physics method. The results show a large improvement of heat and h umidity fluxes near the focus region of the South American coast in the sou theastern equatorial Pacific, and a subsequent westward propagation of sign ificant cold SST anomalies.