Coastline Antarctica

[jhelmert]

This modification from ICON (G. Zängl) transfered to EXTPAR checks if a grid cell at (i,j,k) has >1% snow/ice cover, is south of -60° latitude, has <50% land fraction, and is below 100m elevation.
If true, it:
Sets snow/ice fraction to 60% and bare soil to 40% of the land fraction.
Assigns a constant albedo of 0.15 to broadband, near-infrared, and UV/visible fields for all 12 months.

[jhelmert]

launch jenkins(debug)

[jenkins-apn]

The testsuite you launched for this PR has failed. You can inspect the results at this link.

[jhelmert]

Test results summary after Antarctica coastal glacier fix

The change intentionally modifies output in Antarctic coastal regions to remove unrealistic high-albedo artefacts.

All tests pass except for one:

• mpim/icon_r2b4 (GLOBE input) → FAIL in tolerance_check_cdo.py
  • Affected fields: ALB, ALNID, ALUVD, LU_CLASS_FRACTION
  • ~87–90 grid points differ out of 20480 (~0.43%)
  • Typical absolute differences: 14.9 – 22.2 (albedo fields), some small fraction differences (0.196–1.0)
  • The differences are expected and directly caused by the new Antarctica coastal correction:
    • Reduced excessive snow_ice fractions
    • Forced albedo = 0.15 on low-elevation (<100 m), low land-fraction (<50%) points south of 60°S

The differences are expected and directly caused by this new correction

! enforce that the lu_class_fractions sum up to 1  (existing code above)

IF (lu_class_fraction(i,j,k,ilu_snow_ice) > 0.01_wp .AND. lat_geo(i,j,k) < -60._wp &
     .AND. fr_land_lu(i,j,k) < 0.5_wp .AND. hh_topo(i,j,k) < 100._wp ) THEN
   count_frland_ice = count_frland_ice + 1
   lu_class_fraction(i,j,k,ilu_snow_ice) = 0.6_wp * fr_land_lu(i,j,k)
   lu_class_fraction(i,j,k,ilu_bare_soil) = 0.4_wp * fr_land_lu(i,j,k)
   alb_field_mom (i,j,k,1:12) = 0.15_wp
   alnid_field_mom (i,j,k,1:12) = 0.15_wp
   aluvd_field_mom (i,j,k,1:12) = 0.15_wp
ENDIF



**Other tests unaffected** (as expected):
- All COSMO-7/1, CCLM 12km, ICON-D2(+caching), Ecoclimap-SG, ECCI, Corine → green / bit-identical or within tolerance

**Recommendation**:  
Since this is a deliberate improvement for physical realism (especially relevant for global/coarse grids), I suggest  merge this PR afterwards