No explicit lateral tracer diffusion in NEMOTAM

[sam]

When Laplacian isopycnal tracer diffusion (the default option for ORCA2) is selected, the addition of non-zero horizontal background diffusion (parameter 'ln_ahtb_0') causes the test 'tra_ldf_iso_ad' of the corresponding tangent-linear and adjoint subroutines to fail. Without additional background diffusion the test of this option for lateral diffusion passes.

Analysis of the corresponding code revealed that the lateral diffusion coefficients are scaled by the value of variable 'rldf' (in OPA_SRC/LDF/ldftra_substitute.h90) which remains undefined in NEMOTAM (for the non-linear model the value of 'rldf' is set to 1.0). Setting 'rldf' to 1.0 at the start of each of the subroutines 'tra_ldf_tan', 'tra_ldf_adj', and 'tra_ldf_adj_tst' in module 'traldf_tam' would provide the expected values of the diffusion coefficients.

The assignment of 'rldf' with a value of 1.0 results in a failure of the test of the OPATAM subroutines for some of the options for lateral diffusion (the tests of the code in modules 'tra_lap_tam' and 'tra_iso_tam' fails, the test of the code in module 'tra_bilap_tam' completes successfully). If without definition of 'rld' the lateral diffusion coefficients are set to zero, the test of the tangent-linear and adjoint code for lateral diffusion becomes ineffective, which seems to explain why the subroutine tests for the tangent-linear and adjoint code previously did not reveal any problem with the 'tra_lap_tam' and 'tra_iso_tam' code.