Add Newton iterations for the ice-shelf velocity solution#1072
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Overall this PR makes sense to me (although I have not verified the mathematical correctness of each of the expressions), but there are a number of very minor comments that I think should be addressed for efficiency or style before this PR is accepted.
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The previous Shallow Shelf Approximation solution used an iteration-on-viscosity scheme with Picard iterations only. This commit adds the capability to change to Newton iterations once a certain tolerance is met, which should reduce the number of iterations needed to reach convergence. This tolerance to change to Newton is set by the new parameter NEWTON_AFTER_TOLERANCE. This parameter defaults to the same value of ICE_NONLINEAR_TOLERANCE so that Newton is not called by default. The Newton scheme is paired with the Eisenstat & Walker 1994 approach for adapting the CG tolerance to help the Newton scheme achieve quadratic convergence without over-tightening at later Newton steps. To use the adaptive CG tolerance, set NEWTON_ADAPT_CG_TOLERANCE=True (default is True).
Style changes, parentheses, removed string comparison from loops
Made some changes to pass_var 'complete' arguments for Newton-related variables. Also removed some Newton-related pass_vars and a CS%doing_newton=.false. that were made before entering the the SSA solution loop (at this point, Newton is already false, and there is no need to update Newton-related halos yet). Added parentheses for rotational symmetry with FMAs.
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I believe that these changes are now ready to be merged in. Thank you for these new capabilites.
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This PR has passed pipeline testing at https://gitlab.gfdl.noaa.gov/ogrp/mom6ci/MOM6/-/pipelines/30448. |
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The previous Shallow Shelf Approximation solution used an iteration-on-viscosity scheme with Picard iterations only. This commit adds the capability to change to Newton iterations once a certain tolerance is met, which should reduce the number of iterations needed to reach convergence. This tolerance to change to Newton is set by the new parameter NEWTON_AFTER_TOLERANCE. This parameter defaults to the same value of ICE_NONLINEAR_TOLERANCE so that Newton is not called by default. The Newton scheme is paired with the Eisenstat & Walker 1994 approach for adapting the CG tolerance to help the Newton scheme achieve quadratic convergence without over-tightening at later Newton steps. To use the adaptive CG tolerance, set NEWTON_ADAPT_CG_TOLERANCE=True. The adaptive CG tolerance has no effect unless Newton iterations are used.
-Tested on MISMIP+ and iOM4
-Generated documentation to explain changes:
MOM6_IS_Newton.pdf