Generating model bathymetry from alternative datasets

anton · 12 January 2026 21:45

Yeah - this seems likely, it looks like there is hidden assumption that the topography source file is larger than the model grid:

github.com/COSIMA/bathymetry-tools

src/gen_topo.f90

a29d07a28


      
          do j = 1, ylen
            jstart = jstart + 1
            if (ytopo(jstart) >= y_c(1, 1)) exit
          end do
          
          write(output_unit,*) 'Mosaic grid starts at ', y_c(1, 1), ' topography jstart = ', jstart,' lat = ', ytopo(jstart)
          
          yt_delta = (ytopo(ylen) - ytopo(1))/(ylen - 1)
          xt_delta = (xtopo(xlen) - xtopo(1))/(xlen - 1)
          
          jstart = nint((y_c(1, 1) - ytopo(1))/yt_delta) + 1
          write(output_unit,*) 'Mosaic grid starts at ', y_c(1, 1), ' topography jstart = ', jstart,' lat = ', ytopo(jstart)
          
          ! Shift topography grid if requested
          ishift = nint(offset/xt_delta)
          if (ishift /= 0) then
            xtopo = xtopo + offset
            write(output_unit,*) "Shifted topography longitude grid by ", offset, "(ishift = ", ishift, ")"
          end if
          
          !

This doesn’t make sense, this line shouldn’t by impacted by changing the topo file. It’s just writing the model grid coordinates to the new output file.

github.com/COSIMA/bathymetry-tools

src/gen_topo.f90

a29d07a28


      
          call handle_error(nf90_def_var(ncid_topo, 'geolon_t', nf90_float, dids, geolon_id, deflate_level=1, shuffle=.true.))
          call handle_error(nf90_put_att(ncid_topo, geolon_id, 'long_name', 'tracer longitude'))
          call handle_error(nf90_put_att(ncid_topo, geolon_id, 'units', 'degrees_E'))
          call handle_error(nf90_put_var(ncid_topo, geolon_id, x_t))

I think line 218 might have a bug - it should use dids_topo not dids , although how it ever works is a fluke/mystery!

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Generating model bathymetry from alternative datasets

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