Page Comparison

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1. fluxes:  FV→FV mono
2. state and vector maps:  
   1. ocn→atm: currently this map must be the same as used for fluxes (FV→FV mono)
   2. lnd↔atm and atm→ocn
      1. if source grid resolution >> target grid resolution (fine-to-coarse):  "FV→FV mono"  
      2. if source grid resolution <= target grid resoluitonresolution (coarse-to-fine):  use ESMF's bilin.
         (will be replaced by TR's FV→FV intbilin when it becomes available) 

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1. SE→FV  for fluxes:  use "SE→FV mono" 
2. SE→FV for state and vector maps:  SE->FV intbilin
3. FV→SE for fluxes:  FV→SE monotr
   1. The best map is the transpose of the SE->FV map computed above:
   2. This is the most important map, as it is used to compute the domain files and defines the ocean/land mask on the atm/land grid.  
4. FV→SE for state and vector maps:  
   1. ocn→atm: currently this map must be the same as used for fluxes (FV→SE monotr) 
   2. lnd→atm:   if land grid resolution >> atmosphere resolution (fine-to-coarse):  "FV→SE mono". 
   3. lnd→atm:   if land grid resolution <= atmosphere resolution (coarse-to-fine):   ESMF's bilin.  (replace by FV→SE intbilin if it becomes available)

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1. Some compsets use ESMF bilinear maps for state variables The bilinear algorithm introduces aliasing artifacts when mapping from a high-resolution to low-resolution regionfor fine-to-coarse maps.  Some v1 compsets already use TR high-order for atm2ocn maps, which avoid this issue.    
2. We currently use ESMF conservative maps for fluxes. This algorithm uses piecewise-constant finite volume reconstruction. These maps are good for fine-to-coarse mapping, but will produce blocky artifacts when upscaling.with coarse-to-fine maps.   
3. 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 SE Atmosphere Grid Overview (EAM & CAM)).  The dual grid is contained in the SCRIP format metadata file.  
4. 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.
5. TR algorithm recommended for state variables is cell integration based, so can always be used when mapping from high-res to low-res grids.
6. Inline mapping: TempestRemap algorithms are part of the MOAB coupler, making it possible to eliminate mapping files and have them computed as needed.
7. 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.   

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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.

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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.  

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