4 The post-processing stage

• CONSERV:

  CONSERV performs a global modification of the coupling field. This analysis requires the source and target grid mesh areas to be defined in the areas.nc file or transferred to the coupler with oasis_write_area (see section 2.2.4). For a correct CONSERV operation, overlapping grid cells on the source grid or on the target grid must be masked. In the namcouple, CONSERV requires one input line with one argument and one optional argument:

  # CONSERV operation
       $CMETH  $CONSOPT
  

  
  where:
  • $CMETH is the method desired with the following choices
    • with $CMETH = GLOBAL, the field is integrated on both source and target grids, without considering values of masked points, and the residual (target - source) is uniformly distributed on the target grid; this option ensures global conservation of the field;
    • with $CMETH = GLBPOS, the same operation is performed except that the residual is distributed proportionally to the value of the original field; this option ensures the global conservation of the field and does not change the sign of the field;
    • with $CMETH = BASBAL, the operation is analogous to GLOBAL except that the non masked surface of the source and the target grids are taken into account in the calculation of the residual; this option does not ensure global conservation of the field but ensures that the energy received is proportional to the non masked surface of the target grid;
    • with $CMETH = BASPOS, the non masked surface of the source and the target grids are taken into account and the residual is distributed proportionally to the value of the original field; therefore, this option does not ensure global conservation of the field but ensures that the energy received is proportional to the non masked surface of the target grid and it does not change the sign of the field.
  • $CONSOPT is an optional argument specifying the algorithm. $CONSOPT can be bfb, gather, lsum16, lsum8, ddpdd, reprosum or opt. Details on the perfromance of these different options can also be found in (Craig et al 2017) and references there in.
    • The bfb option is the default for CONSOPT and uses the reprosum option (see below).
    • The gather option computes global sums by gathering a decomposed array onto the root process before doing an index ordered sum. This is guaranteed to produce identical results for different numbers of processors and decompositions but is expensive both with respect to performance and memory use. This is equivalent to the bfb option in previous OASIS versions.
    • The lsum16 option computes a local sum at quadruple precision before doing an MPI reduction on the local sums at quadruple precision. This is likely to be bit-for-bit for different numbers of processors and decompostions but that's not guaranteed. This is just like lsum8 but at quadruple precision and a little slower.
    • The lsum8 option computes a local sum at double precision before doing an MPI reduction on the local sums at double precision. This is NOT likely to be bit-for-bit for different numbers of processors and decompostions. This is just like lsum16 but at double precision and faster.
    • The ddpdd option is a parallel double-double algorithm using a single scalar reduction. It should behave between lsum8 and lsum16 with respect to performance and reproducibility. See (He and Ding 2001).
    • The reprosum option is a fixed point method based on ordered double integer sums that requires two scalar reductions per global sum. The cost of reprosum will be higher than some of the other methods but it will be bit-for-bit for different processor counts or different decompostions except in extremely rare cases and the cost is significantly less than the gather option. See (Mirin and Worley 2012).
    • The opt option carries out the global sum using the fastest algorithm generally available. Currently, this is set to lsum8.
      

• SUBGRID: UNUSED

• BLASNEW:

  BLASNEW performs a scalar multiply or scalar add to any destination field. This is the equivalent of BLASOLD on the destination side. The prior feature that supported linear combinations of the current coupling field with any other fields after the interpolation has been deprecated.

  This analysis requires the same input line(s) as BLASOLD.

• MASKP: UNUSED

• REVERSE: UNUSED

• CHECKOUT:

  CHECKOUT calculates the global minimum, the maximum and the sum of the target field values (not weighted by the grid cell area) and prints them to the OASIS3-MCT debug file (for the master process of the target component model only). These informations are found in the debug file of the master process of the target model under the attribute “diags”. This operation does not transform the field. CHECKOUT operation can significantly slow down the simulation. It should not be used in production mode. The generic input line is as for CHECKIN (see above).

• GLORED: UNUSED