Next: The ``cooking'' stage Up: The transformations and interpolations Previous: The pre-processing transformations

The interpolation

The following interpolations, controlled by interp.f, are available in OASIS3.

BLASOLD:
BLASOLD (routine blasold.f) performs a linear combination of the current coupling field with other coupling fields or with a constant before the interpolation per se.
This transformation requires at least one configuring line with two parameters:
```
 # BLASOLD operation 
     $XMULT     $NBFIELDS
```
where $XMULT is the multiplicative coefficient of the current field, and $NBFIELDS the number of additional fields to be combined with the current field. For each additional field, an additional input line is required:
```
# nbfields lines
     $CNAME  $AMULT
```
where $CNAME and $AMULT are the symbolic name and the multiplicative coefficient for the additional field. To add a constant value to the original field, put $XMULT = 1, $NBFIELDS = 1, $CNAME = CONSTANT, $AMULT = value to add.
SCRIPR:
SCRIPR is new in OASIS3 and gathers the interpolation techniques offered by Los Alamos National Laboratory SCRIP 1.4 library(1). SCRIPR routines are in prism/src/lib/scrip. See also the SCRIP 1.4 documentation in prism/src/mod/oasis3/doc/SCRIPusers.pdf. Linking with NetCDF library is mandatory when using SCRIPR interpolations.
The following types of interpolations are available:
- DISTWGT performs N nearest-neighbour interpolation (N neighbours). All grid types are supported. If the N nearest neighbours of a target grid point are masked, no value is calculated for that point; transformations MASK and EXTRAP should be used to avoid problems for those points. No values are calculated for masked target grid points.
  The configuring line is:
```
 # SCRIPR/DISWGT 
  $CMETH $CGRS $CFTYP $REST $NBIN $NV $ASSCMP $PROJCART
```
  where:
  - $CMETH = DISTWGT.
  - $CGRS is the source grid type (LR, D or U)- see annexe A.
  - $CFTYP is the field type: SCALAR if the field is a scalar one, or VECTOR_I or VECTOR_J whether the field represents respectively the first or the second component of a vector field (see paragraph Support of vector fields below). Note that VECTOR, which is fact leads to a scalar treatment of the field (as in the previous versions), is still supported.
  - $REST is the search restriction type: LATLON or LATITUDE (see SCRIP 1.4 documentation). Note that for D or U grid, the restriction may influence sligthly the result near the borders of the restriction bins.
  - $NBIN the number of restriction bins (see SCRIP 1.4 documentation).
  - $NV is the number of neighbours used.
  - $ASSCMP: optional, for VECTOR_I or VECTOR_J vector fields only; the source symbolic name of the associated vector component.
  - $PROJCART: optional, for vector fields only; should be PROJCART if the user wants the vector components to be projected in a Cartesian coordinate system before interpolation (see paragraph Support of vector fields below).
- GAUSWGT performs N nearest-neighbour interpolation weighted by a gaussian function. All grid types are supported. Behaviour for source and target masked points is the same than for DISTWGT. The configuring line is:
```
 # SCRIPR/GAUSWGT
$CMETH  $CGRS  $CFTYP  $REST  $NBIN  $NV $VAR $ASSCMP $PROJCART
```
  where all entries are as for DISTWGT, except that:
  - $CMETH = GAUSWGT
  - $VAR, which must be given as a REAL value (e.g 2.0 and not 2), defines the weight given to a neighbour source grid point as inversely proportional to $exp(-1/2 \cdot d^2/\sigma^2)$ where is the distance between the source and target grid points, and $\sigma^2 = \$VAR \cdot \overline{d}^2$ where $\overline{d}^2$ is the average distance between two source grid points (calculated automatically by OASIS3).
- BILINEAR performs bilinear interpolation.
- BICUBIC performs a bicubic interpolation.
  For BILINEAR and BICUBIC, Logically-Rectangular (LR) and Reduced (D) source grid types are supported. If the some of the source grid points NORMlly used in the bilinear or bicubic interpolation are masked, another algorithm is applied; at least, the nearest non-masked source neighbour is used. No values are calculated for masked target grid points.
  The configuring line is:
```
 # SCRIPR/BILINEAR or SCRIPR/BICUBIC
     $CMETH  $CGRS  $CFTYP  $REST  $NBIN $ASSCMP $PROJCART
```
  where:
  - $CMETH = BILINEAR or BICUBIC
  - $CGRS is the source grid type (LR or D)
  - $CFTYP, $NBIN, $ASSCMP $PROJCART are as for DISTWGT.
  - $REST is as for DISTWGT, except that only LATITUDE is possible for a Reduced (D) source grid.
- CONSERV performs 1st or 2nd order conservative remapping, which means that the weight of a source cell is proportional to area intersected by target cell. Values are possibly calculated for all target grid points whether they are masked or not. The source grid mask is taken into account in the NORMlisation (see below).
  The configuring line is:
```
 # SCRIPR/CONSERV
$CMETH  $CGRS  $CFTYP  $REST  $NBIN  $NORM  $ORDER $ASSCMP $PROJCART
```
  where:
  - $CMETH = CONSERV
  - $CGRS is the source grid type: LR, D and U are supported for 1st-order remapping if the grid corners are given by the user in the grid data file which is, in this case, necessarily a netCDF file (grids.nc, see section 7.2); only LR is supported if the grid corners are not available in the grid data file and therefore have to be calculated automatically by OASIS3. For second-order remapping, only LR is supported because the gradient of the coupling field used in the transformation has to be calculated automatically by OASIS3.
  - $CFTYP, $REST, $NBIN, $ASSCMP,and $PROJCART are as for DISTWGT. Note that for CONSERV the restriction does not influence the result.
  - $NORM is the NORMlization option:
    - FRACAREA: The sum of the source cell intersected areas is used to NORMlise each target cell field value: the flux is not locally conserved, but the flux value itself is reasonable.
    - DESTAREA: The total target cell area is used to NORMlise each target cell field value even if it only partly intersects the source grid cells: local flux conservation is ensured, but unreasonable flux values may result.
    - FRACNNEI: as FRACAREA, except that the source nearest neighbour is used for target cells that do not intersect any unmasked source cells.
  - $ORDER: FIRST or SECOND for first or second order remapping respectively (see SCRIP 1.4 documentation).
Support of vector fields
SCRIPR supports 2D vector interpolation. The two vector components have to be identified by replacing $CFTYP by VECTOR_I or VECTOR_J and have to be associated by replacing $ASSCMP, for each component field, by the source symbolic name of the associated vector component in (see above). A proper example of vector interpolation is given in the interpolator-only mode example (see details in prism/util/running/testinterp/README_testinterp). The details of the vector treatment, performed by the routines scriprmp_vector.F90 and rotations.F90 in prism/src/lib/scrip/src are the following:
- If the angles of the source grid local coordinate system are defined in the grids.nc data file (see section 7.2), an automatic rotation from the local to the geographic spherical coordinate system is performed.
- If the two source vector components are not defined on the same source grid, one component is automatically interpolated on the grid of the other component.
- If the user put the PROJCART keyword at the end of the SCRIPR configuring line (see above), projection of the two vector components in a Cartesian coordinate system, interpolation of the resulting 3 Cartesian components, and projection back in the spherical coordinate system are performed. In debug mode (compilation with DEBUG pre-compiling key), the resulting vertical component in the spherical coordinate system after interpolation is written to a file projection.nc; as the source vector is horizontal, this component should be very close to 0.
- If the user did not put the PROJCART keyword at the end of the SCRIPR configuring line, the two spherical coordinate system components are interpolated.
- If the angles of the target grid local coordinate system are defined in the grids.nc data file (see section 7.2), an automatic rotation from the geographic spherical to the local coordinate system is performed.
- The first and second components of the interpolated vector field are then present in the target fields associated respectively to the first and second source vector component. The target grids for the two vector components can be different.
INTERP:
INTERP gathers different techniques of interpolation controlled by routine fiasco.f. The following interpolations are available:
- BILINEAR performs a bilinear interpolation using 4 neighbours.
- BICUBIC performs a bicubic interpolation.
- NNEIBOR performs a nearest-neighbour interpolation.
  These three interpolations are performed by routines in /prism/src/lib/fscint and support only A, B, G, L, Y, or Z grids (see annexe A). All sources grid points, masked or not, are used in the calculation. To avoid the `contamination' by masked source grid points, transformations MASK and EXTRAP should be used. Values are calculated for all target grid points, masked or not.
  The configuring line is as follows:
```
 # BILINEAR or BICUBIC or NNEIBOR interpolation 
    $CMETH $CGRS $CFTYP
```
  where
  - $CMETH = BILINEAR, BICUBIC or NNEIBOR
  - $CGRS is the source grid type (A, B, G, L, Y, or Z, see annexe A)
  - $CFTYP the field type (SCALAR or VECTOR). VECTOR has an effect for target grid points located near the pole: the sign of a source value located on the other side of the pole will be reversed.
- SURFMESH (routines in /prism/src/lib/anaism) is a first-order conservative remapping from a fine to a coarse grid (the source grid must be finer over the whole domain) and supports only Lat-Lon grids. For a target grid cell, all the underlying not masked source grid cells are found and the target grid field value is the sum of the source grid field values weighted by the overlapped surfaces. No value is assigned to masked cells. Note that it is not recommended to use this interpolation anymore, as the more general SCRIPR/CONSERV remapping is now available. The configuring line is as follows:
```
 # SURFMESH remapping
     $CMETH $CGRS $CFTYP $NID $NV $NIO
```
  where
  - $CMETH = SURFMESH
  - $CGRS and $CFTYP are as for BILINEAR
  - $NID is the identificator for the weight-address dataset in all the different INTERP/SURFMESH datasets in the present coupling. This dataset will be calculated by OASIS3 if $NIO= 1, or only read if $NIO= 0.
  - $NV is the maximum number of source grid meshes used in the remapping.
- GAUSSIAN (routines in /prism/src/lib/anaisg) is a gaussian weighted nearest-neighbour interpolation technique. The user can choose the variance of the function and the number of neighbours considered. The masked source grid points are not used and no value are calculated for masked target grid points.
  The configuring line is:
```
    # GAUSSIAN interpolation
    $CMETH $CGRS $CFTYP $NID $NV $VAR  $NIO
```
  where
  - $CMETH = GAUSSIAN
  - $CGRS is the source grid type (LR, D or U) and $CFTYP is as for the DISTWGT
  - $NID is the identificator for the weight-address dataset in all the different INTERP/GAUSSIAN datasets in the present coupling. This weight-address dataset will be calculated by OASIS3 if $NIO= 1, or only read if $NIO= 0.
  - $NV is the number of neighbours used in the interpolation.
  - $VAR is as for SCRIPR/GAUSWGT (see above).
MOZAIC:
MOZAIC performs the mapping of a field from a source to a target grid. The grid-mapping dataset, i.e. the weights and addresses of the source grid points used to calculate the value of each target grid point are defined by the user in a file (see section 7.5). The configuring line is:
```
# MOZAIC operation
     $CFILE  $NUMLU  $NID  $NV
```
where
- $CFILE and $NUMLU are the grid-mapping file name and associated logical unit on which the grid-mapping dataset is going to be read),
- $NID the identificator for this grid-mapping dataset in all MOZAIC grid-mapping datasets in the present coupling
- $NV is the maximum number of target grid points use in the mapping.
NOINTERP:
NOINTERP is the analysis that has to be chosen when no other transformation from the interpolation class is chosen. There is no configuring line.
FILLING:
FILLING (routine /prism/src/mod/oasis3/src/filling.f) performs the blending of a regional data set with a climatological global one for a Sea Surface Temperature (SST) or a Sea Ice Extent field. This occurs when coupling a non-global ocean model with a global atmospheric model. FILLING can only handle fields on Logically Rectangular grid (LR, but also A, B, G, L, Y, and Z grids, see section A.
The global data set has to be a set of SST given in Celsius degrees (for the filling of a Sea Ice Extent field, the presence or absence of ice is deduced from the value of the SST). The frequency of the global set can be interannual monthly, climatological monthly or yearly.
The blending can be smooth or abrupt. If the blending is abrupt, only model values are used within the model domain, and only the global data set values are used outside. If the blending is smooth, a linear interpolation is performed between the two fields within the model domain over narrow bands along the boundaries. The linear interpolation can also be performed giving a different weight to the regional or and global fields.
The smoothing is defined by parameters in /prism/src/mod/oasis3/src/ mod_smooth.F90. The lower smoothing band in the global model second dimension is defined by nsltb (outermost point) and nslte (innermost point); the upper smoothing band in the global model second dimension is defined by nnltb (outermost point) and nnlte (innermost point). The parameter qalfa controls the weights given to the regional and to the global fields in the linear interpolation. qalfa has to be or . For the outermost points (nsltb or nnltb) in the smoothing band, the weight given to the regional and global fields will respectively be 0 and 1; for the innermost points (nslte or nnlte) in the smoothing band, the weight given to the regional and global fields will respectively be 1 and 0; within the smoothing band, the weights will be a linear interpolation of the outermost and innermost weights.
The smoothing band in the global model first dimension will be a band of nliss points following the coastline. To calculate this band, OASIS3 needs nwlgmx, the greater first dimension index of the lower coastline and nelgmx, the smaller first dimension index on the upper coastline. The parameter qbeta controls the weights given to the regional and to the global fields in the linear interpolation. qbeta has to be . The weights given to the regional and global fields in the global model first dimension smoothing bands will be calculated as for the second dimension.
The user must provide the climatological data file with a specific format described in 7.5. When one uses FILLING for SST with smooth blending, thermodynamics consistency requires to modify the heat fluxes over the blending regions. The correction term is proportional to the difference between the blended SST and the original SST interpolated on the atmospheric grid and can be written out on a file to be read later, for analysis CORRECT for example. In that case, the symbolic name of the flux correction term read through the input file namcouple must correspond in FILLING and CORRECT analyses.
In case the regional ocean model includes a coastal part or islands, a sea-land mask mismatch may occur and a coastal point correction can be performed if the field has been previously interpolated with INTER/SURFMESH. In fact, the mismatch could result in the atmosphere undesirably ``seeing'' climatological SST's directly adjacent to ocean model SST's. Where this situation arises, the coastal correction consists in identifying the suitable ocean model grid points that can be used to extrapolate the field, excluding climatological grid points.
This analysis requires one configuring line with 3, 4 or 6 arguments.
1. If FILLING performs the blending of a regional data set with a global one for the Sea Ice Extent, the 3-argument input line is:
```
# Sea Ice Extent FILLING operation
     $CFILE  $NUMLU  $CMETH
```
  where $CFILE is the file name for the global data set, $NUMLU the associated logical unit. $CMETH, the FILLING technique, is a CHARACTER*8 variable: the first 3 characters are either SMO, smooth filling, or RAW, no smoothing ; the next three characters must be SIE for a Sea Ice Extent filling operation; the last two define the time characteristics of the global data file, respectively MO, SE and AN for interannual monthly, climatological monthly and yearly. Note that in all cases, the global data file has to be a Sea Surface Temperature field in Celsius degrees.
2. If FILLING performs the blending of a regional data set with a global one for the Sea Surface Temperature without any smoothing, the 4-argument input line is:
```
#Sea Surface Temperature FILLING operation without smoothing
     $CFILE  $NUMLU  $CMETH  $NFCOAST
```
  where $CFILE, $NUMLU are as for the SIE filling. In this case however, $CMETH(1:3) = RAW, $CMETH(4:6) = SST, and the last two characters define the time characteristics of the global data file, as for the SIE filling. $NFCOAST is the flag for the calculation of the coastal correction ( 0 no, 1 yes).
3. If FILLING performs the blending of a regional data set with a global one for the Sea Surface Temperature with smoothing, the 6-argument input line is:
```
#Sea Surface Temperature FILLING operation with smoothing
     $CFILE  $NUMLU  $CMETH  $NFCOAST  $CNAME  $NUNIT
```
  where $CFILE, $NUMLU and $NFCOAST are as for the SST filling without smoothing. In this case, $CMETH(1:3) = SMO, $CMETH(4:6) = SST, and the last two characters define the time characteristics of the global data file, as for the SIE filling. $CNAME is the symbolic name for the correction term that is calculated by OASIS3 and $NUNIT the logical unit on which it is going to be written.

Next: The ``cooking'' stage Up: The transformations and interpolations Previous: The pre-processing transformations

Reinhard Budich 2004-12-22