Skip to content

ClimaCoupler Lessons Learned

Jump to bottom
Julia Sloan edited this page Oct 7, 2022 · 23 revisions

Remapping Approach

Background

We’ve tried two different approaches to implement remappings:

  • Non-conservative clean_mask function applied after non-monotone remapping
    • Cuts off values outside of desired range
    • Advantages: fast, does not reduce spatial resolution
    • Disadvantages: does not conserve global total of quantity being remapped
  • Conservative monotone remapping function
    • A monotone remapping has the quality that no new minima nor maxima are introduced (i.e. all weights used in the mapping are between [0,1])
    • Advantage: remapping is both conservative and monotone
    • Disadvantages: decreases spatial resolution (i.e., it's a lower order method), slower approach

TempestRemap has multiple functions that generate remappings. The currently-used function in ClimaCoupler is GenerateOfflineMap, and the alternative is GenerateTransposeMap. Note that GenerateTransposeMap reverses a previous remapping and can only map FV griddings to CGLL (not vice versa). Thus, to apply this function, a minimum of 2 and potentially 3 remappings must be applied, which is quite costly.

Tests

To compare GenerateOfflineMap and GenerateTransposeMap, as well as monotone vs non-monotone remappings, we performed a number of remappings using the seamask.nc dataset.

We find that at a spatial resolution of h_elem=6, monotone and non-monotone remappings produce qualitatively similar results when applying the map created by either GenerateOfflineMap or GenerateTransposeMap. However, at a spatial resolution of h_elem=4, the monotonicity has a substantial effect. This suggests that considerations of enforcing monotonicity of the mapping may be important at lower resolutions, but is not as essential for slightly higher resolutions (for which monotone remapping is always recommended).

Main Takeaways

  • Monotone remappings should be used when being applied to quantities where global conservation is important (i.e. for fluxes), but are not strictly speaking necessary for values where this is not required (i.e. for land cover) or when spatial resolution is sufficiently high.
  • GenerateOfflineMap is the currently used method to create remappings in ClimaCoupler, and performs about as well as GenerateTransposeMap. In addition, it is easier to use as it doesn’t require a previous mapping, so we will continue to use it moving forward.

Relevant Plots

Remapping using GenerateTransposeMap with h_elem = 4: monotone (left) vs non-monotone (right)


Remapping using GenerateTransposeMap with h_elem = 6: monotone (left) vs non-monotone (right)


Remapping using GenerateOfflineMap with h_elem = 4: monotone (left) vs non-monotone (right)


Remapping using GenerateOfflineMap with h_elem = 6: monotone (left) vs non-monotone (right)

Note that while it may appear that the monotone plots contain negative values, this is merely a consequence of the plotting method used. The values for these plots are contained in [0, 1].

Also note that these boundary conditions cause numerical instability when used in an atmospheric simulation with h_elem=4 and n_poly=3, whether monotonous or not. The same resolution for the aquaplanet setup is stable.

Clone this wiki locally