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- Lat/Lon → SE
- FV→SE "mono". Disabling the conservation constraint with --noconserve does not improve accuracy
- NOTE: For Lat/Lon grids, check the min/max of the weights - if the values are far outside the "[0,1]" interval, may need more coefficients for Vandermonde inverse near poles: edit LinearRemapFV.cpp: nRequiredFaceSetSize = 2*nCoefficients.
- SE → SE
- SE→SE "mono"
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Background
Motivation for Moving to TempestRemap
Most mapping files used in the E3SM coupled system and used to remap to lat/lon grids for analysis are based on the ESMF_Regrid package. The ESMF remap algorithms and underlying tools are quite good, but they are targeted at finite volume grids and assume cell centered data. The TR algorithms have native support for vertex centered finite element grids, such as used by the E3SM atmosphere dycore.
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- Some compsets use ESMF bilinear maps for state variables The bilinear algorithm introduces aliasing artifacts when mapping from a high-resolution to low-resolution region (downscaling). Some v1 compsets already use TR high-order for atm2ocn maps, which avoid this issue.
- We currently use ESMF conservative maps for fluxes. This algorithm uses piecewise-constant finite volume reconstruction. These maps are good for downscaling, but will produce blocky artifacts when upscaling.
- ESMF conservative maps for spectral element grids requires us to first generate a "dual grid" that puts a finite volume around each spectral element node. Constructing this dual grid requires running a Matlab tool that uses Newton iteration to find the dual grid. For a high-resolution grid, this procedure can take several days. (For information on the dual grid, see CAM-SE Grid Overview). The dual grid is contained in the SCRIP format metadata file.
- TR algorithms natively support finite element grids, do not require the SCRIP dual grid, and give more accurate results when remapping to/from a spectral element atmosphere grid.
- TR algorithm recommended for state variables is automatically downscaling, so can always be used when mapping from high-res to low-res grids.
- Inline mapping: TempestRemap algorithms are part of the MOAB coupler, making it possible to eliminate mapping files and have them computed as needed.
- Land/Ocean Mask consistency: The flux ocn2atm map is used to define the land/ocean mask on the atmosphere grid. All other ocn2atm maps (state and vectors) must have the same range as the map uses for fluxes. That is, if a point on the atmosphere grid receives ocean data from the flux map, it must also receive ocean data from all other maps. The aave and bilinear maps do not have the same range, and thus if an aave map is used for fluxes, it must be used for all other ocn2atm maps. We speculate that TR maps all have the same range and thus we can use high order maps for ocn2atm state and vectors.
Notation and Abbreviations
SE = Atmosphere spectral element dycore mesh ( CAM-SE Grid Overview ). Usually in "np4" configuration where each element has a 4x4 array of GLL nodes.
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See Mapping file algorithms and naming convention page.
The Overlap Mesh
IMPORTANT!!!: MAPS grids often have holes where there are no mesh points. When computing the TempestRemap overlap grid (step 1 below), the grid with less coverage must be the "a" grid, while the global grid should be the "b" grid. To generate the overlap grid:
./GenerateOverlapMesh --a $ocngrid --b $atmgrid --out overlap_mesh.g
Generic TR Commands to Generate Mapping Files
Update to TR options 2020/2/21: to fix precision issues, with row sums and conservation errors not close to machine precision at high resolution meshes, added the "–correct_areas" option to the recommendations.
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Using TempestRemap
Generic TR Commands to Generate Mapping Files
Update to TR options 2020/2/21: to fix precision issues, with row sums and conservation errors not close to machine precision at high resolution meshes, added the "–correct_areas" option to the recommendations.
- SE→FV mono (conservative, monotone, 1st order)
- ./GenerateOfflineMap --in_mesh $atmgrid --out_mesh $ocngrid --ov_mesh overlap_mesh.g --in_type cgll --in_np 4 --out_type fv --mono --correct_areas --out_map map_atm2ocn_mono.nc
- SE→FV intbilin (nonconservative, monotone, higher order)
- ./GenerateOfflineMap --in_mesh $atmgrid --out_mesh $ocngrid --ov_mesh overlap_mesh.g --in_type cgll --in_np 4 4 --out_type fv fv --mono3 --mono noconserve --correct_areas --out_map map_atm2ocn_monointbilin.nc
- SE→FV intbilin highorder. (nonconservative, monotone, higher orderconservative, nonmonotone)
- ./GenerateOfflineMap --in_mesh $atmgrid --out_mesh $ocngrid --ov_mesh overlap_mesh.g --in_type cgll --in_np 4 --out_type fv --mono3 --noconserve --correct_areas --out_map map_atm2ocn_intbilinhighorder.nc
- FV→SE monotr (conservative, monotone, nonmonotone)1st order)
- The transpose of the SE→FV mono map:
- ./GenerateOfflineMap GenerateTransposeMap --in _mesh $atmgrid map_atm2ocn_mono.nc --out map_ocn2atm_monotr.nc
- FV→SE highorder (conservative, non-monotone)FV→SE highorder (conservative, non-monotone
- ./GenerateOfflineMap --in_mesh $ocngrid --out_mesh $ocngrid $atmgrid --ov_mesh overlap_mesh.g --in_type cgll fv --in_np 4 2 --out_type fv cgll --correctout_areas np 4 --out_map map_atm2ocnocn2atm_highorder.nc
- FV→SE monotr (conservative, monotone, 1st order)
- The transpose of the SE→FV mono map:
- ./GenerateTransposeMap --in map_atm2ocn_mono.nc --out map_ocn2atm_monotr.nc
- Does these maps need "–correct_areas" ?
- FV→SE mono (conservative, monotone, 1st order)
- ./GenerateOfflineMap --in_mesh $ocngrid --out_mesh $atmgrid --ov_mesh overlap_mesh.g --in_type fv --in_np 2 1 --out_type cgll --out_np 4 4 --out_map map_ocn2atm_highordermono.nc
- Does these maps need "–correct_areas" ?
- FV→SE mono intbilintr (conservativenonconservative, monotonenonmonotone, 1st higher order)
- The transpose of the SE→FV intbilin map./GenerateOfflineMap No negative weights, but weights can be larger than 1.
- ./GenerateTransposeMap --in _mesh $ocngrid map_atm2ocn_intbilin.nc --out map_mesh $atmgrid --ov_mesh overlap_mesh.g --in_type fv --in_np 1 --out_type cgll --out_np 4 ocn2atm_intbilintr.nc
- FV→FV aave (conservative, monotone, 1st order)
- The classic cell integrated piecewise constant map. Equivalent to the ESMF "aave" map and NCO's built in map.
- ./GenerateOfflineMap --in_mesh $ocngrid --out_mesh $atmgrid --ov_mesh overlap_mesh.g --in_type fv --in_np 1 --out_type fv --out_np 1 -correct_areas --out_map map_ocn2atm_mono.nc
- Does these maps need "–correct_areas" ?
- FV→SE intbilintr (nonconservative, nonmonotone, higher order)
- The transpose of the SE→FV intbilin map. No negative weights, but weights can be larger than 1.
- ./GenerateTransposeMap --in map_atm2ocn_intbilin.nc --out map_ocn2atm_intbilintr.nc
- FV→FV aave (conservative, monotone, 1st order)
- The classic cell integrated piecewise constant map. Equivalent to the ESMF "aave" map and NCO's built in map. ./GenerateOfflineMap --in_mesh $ocngrid --out_mesh $atmgrid --ov_mesh overlap_mesh.g --in_type fv --in_np 1 --out_type fv --out_np 1 -correct_areas FV→FV highorder (conservative, non-monotone, high order)
- ./GenerateOfflineMap --in_mesh $atmgrid --out_mesh $ocngrid --ov_mesh overlap_mesh.g --in_np 2 --in_type fv --out_type fv --out_np 1 -correct_areas --out_map map_ocn2atmatm2ocn_monohighorder.nc
- ./GenerateOfflineMap
- --in_mesh $atmgrid --out_mesh $ocngrid --ov_mesh overlap_mesh.g --in_np 2 --in_type fv --out_type fv --out_np 1 -correct_areas --out_map map_atm2ocn_highorder.nc
- --in_np 2 means all the nearest neighbor cells will be used for a higher order reconstruction of the source data, which will then be integrated over the target cell. Unlike the SE case, no mass matrix is used for an FV target grid, and thus --out_np is ignored (and will always be 1 internally).
Testing
NCO as of 4.9.0-beta01 can test maps for conservation and consistency. See: Assess and Address Regridding Weights in Map-Files with ncks.
In addition, maps can be checked for accuracy. TempestRemap can automatically generate some standard test functions which can then be remapped and we can compute the mapping error, and check if it is at the expected level. In practice, all 1st order conservative maps give the same l2 and max error if they pass the conservation and consistency checks. For higher order maps, these errors can be used to compare different algorithms.
Check the quality of the mapping file with "ncks --chk_map map.nc"
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mapping weights are between 0 and 1. for higher order maps, it may be acceptable for small overshoots, but we need to test this.
Consistency: Row sum of weights = 1.
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- np 2 means all the nearest neighbor cells will be used for a higher order reconstruction of the source data, which will then be integrated over the target cell. Unlike the SE case, no mass matrix is used for an FV target grid, and thus --out_np is ignored (and will always be 1 internally).
The Overlap Mesh
IMPORTANT!!!: MAPS grids often have holes where there are no mesh points. When computing the TempestRemap overlap grid (step 1 below), the grid with less coverage must be the "a" grid, while the global grid should be the "b" grid. To generate the overlap grid:
./GenerateOverlapMesh --a $ocngrid --b $atmgrid --out overlap_mesh.g
TR support in NCO and ncremap
The TR maps discussed in "Recommended Settings" above include many options and requirements (transposes, ocean grid first) that might slow-down casual TR users. NCO's ncremap provides a "Make Weight Files" (MWF) mode that automatically generates and correctly names all recommended TR (or ESMF) maps. Instructions and examples for MWF mode are summarized on the ncremap page here, with more in-depth discussion in the NCO manual here.
Testing and Validating Mapping Files
NCO as of 4.9.0-beta01 can test maps for conservation and consistency. See: Assess and Address Regridding Weights in Map-Files with ncks.
In addition, maps can be checked for accuracy. TempestRemap can automatically generate some standard test functions which can then be remapped and we can compute the mapping error, and check if it is at the expected level. In practice, all 1st order conservative maps give the same l2 and max error if they pass the conservation and consistency checks. For higher order maps, these errors can be used to compare different algorithms.
Check the quality of the mapping file with "ncks --chk_map map.nc"
- sum of area weights is close to physical area
mapping weights are between 0 and 1. for higher order maps, it may be acceptable for small overshoots, but we need to test this.
Consistency: Row sum of weights = 1.
- Should be 1 to machine precision for all grids where a >= b. (global → regional)
- For grids with a<b (such as ocn→atm) row sums should be 1.0 at all ocean points, 0 at all land points, and between 0 and 1 at coastal points.
- Conservation: area_b weighted column sum of weights = area_a (exact for conservation, close otherwise)
- Should be equal to machine precision for all grids where a <= b. (regional → global)
- For grids with a>b (such as atm->ocn) should be equal to machine precision at all ocean points, 0 at all land points and between 0 and 1 at coastal points
- Non-conservatives maps are expected to have small errors. The traditional ESMF bilinear map does not include areas in the map file and this metric might not be accurate.
- no missing target points (unless source grid has holes)
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NOTE: The land mask, as used by the land model is computed by remaping the ocean mask (on the ocean grid) to the atmosphere grid. On the ocean grid, the ocean/land mask is all 0's or 1's (that is, every point is either ocean or land). Actually, on an MPAS grid, the land points are all removed, so the mask is all ocean. This field is then mapped on to the land grid (when creating domain files). This results in a land mask on the atmosphere/land grid with fractional land points ( 0< mask<1) near coastlines.
We had some issues with TempestRemap, where the initial land mask was not very good. These were resolved here: /wiki/spaces/CMDV/pages/128827525
TR support in NCO and ncremap
The TR maps discussed in "Recommended Settings" above include many options and requirements (transposes, ocean grid first) that might slow-down casual TR users. NCO's ncremap provides a "Make Weight Files" (MWF) mode that automatically generates and correctly names all recommended TR (or ESMF) maps. Instructions and examples for MWF mode are summarized on the ncremap page here, with more in-depth discussion in the NCO manual here.domain files). This results in a land mask on the atmosphere/land grid with fractional land points ( 0< mask<1) near coastlines.
We had some issues with TempestRemap, where the initial land mask was not very good. These were resolved here: /wiki/spaces/CMDV/pages/128827525