ICB: cleaning icbutl interpolation routines

[mathiot]

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Context

• icbutl contains 4 routines for interpolation:
  • icb_utl_bilin_h to manage array with extra halo
  • icb_utl_bilin as _h but with the extra halo
  • icb_utl_bilin_x as icb_utl_bilin but for longitude only
  • icb_utl_bilin_e for scale factor

All of these subroutines have the same structure:

• Firstly, we find the variable ii and ij (indices) for T point or T/U/V/F point depending of the variables and first estimates of interpolation weights zi,zj
• Then, we apply a sanity check on the bound very similar for all 4 subroutines (it should never go in otherwise it break reproducibility)
• Finally we compute the weights depending on the case (mask needed, longitude case, scale factor, normal case)

There are, per RK4 step, about 15 calls of icb_utl_bilin* subroutines gather in icb_utl_interp and the first 2 steps (positions+pre weights and sanity checks) are only dependent of the position and the target point T/U/V/F (which don’t change between the various calls).

Suggestion for simplification

• Get rid of icb_utl_bilin and icb_utl_bilin_x by using icb_utl_bilin_h
  • Integrate in icb_utl_bilin_h an option to deal with longitude (Similar to how we deals with coastline in the interpolation)
  • Create a lon and lat array with extra halo
• Compute ii,ij and zi,zj for each point T/U/V/F before call all the interpolation routine icb_utl_bilin_h
  • Move the corresponding bloc from icb_utl_bilin_h and icb_ult_bilin_e into a new subroutine icb_utl_pos
  • Move the sanity check on position (on land ?, out bound ?) into the new subroutine icb_utl_pos. Maybe run this test if a new flag ln_icbdbg = .TRUE. as NEMO should never go trough.
• Add after the call the icb_lnk (maybe need to move it into icb_dyn instead of in the middle of icb_stp), a check if all the icb are inside the inner domain (under debug flag).

Why doing this ?

• Make icb_utl by limiting the number of interpolation routine.
• Make icb_utl_interp computing position and pre weight only once per T/U/V/F point (ie 4 times instead of about 15 times).
• Skip debug test (icb inside the inner domain, …) only if debug flag is true.

• Nacho work available in a tarballs (#1900) adds a lot of icb_utl_bilin_h calls (1 per level between the surface and the deepest level affected by an iceberg (could be at a depth of 250m). So making icb_utl_bilin simpler and faster could be useful now and much more useful once Nacho work included.

Testing

• We should have an idealized test case with a lot of icebergs in it, running for long time (offline iceberg ?). Nothing in mind yet, I have to see if something already exist in the literature.
• These changes should not changes NEMO results (identical run.stat).

[mathiot]

In 13243:

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[mathiot]

In 13265:

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[mathiot]

In 13273:

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[mathiot]

In 13276:

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[mathiot]

included in the trunk as part of 2020 merge (revision r14030)