Land and coast#

Every seagrid model (glorys, ecco2, scrippsgog) has a land mask and a distance-to-land field. Both return a NumPy-backed xarray.DataArray on the model’s (y, x) T-cells, with lon and lat coordinates:

from tracmass.seagrid import glorys

land = glorys.landmask()             # bool, True on land (kmt == 0)
dist = glorys.distance_to_land()     # float32 km, 0 on land
deep = glorys.distance_to_land(k=20) # at level 20 the sea floor counts as land

  • k is the 1-based model level. At level k, every column with kmt < k counts as land.

  • Distance is the great-circle distance between cell centres, so a cell next to the coast is about one cell width from land. It is computed with a KD-tree on unit-sphere coordinates, which handles curvilinear grids and the date line. Full GLORYS takes about 5 s.

  • On a regional grid such as GoG, land outside the domain is unknown, so distances near the open boundaries can be too large.

  • A model only resolves the islands its grid resolves. In the open South Pacific, ECCO2 (1/4°) gives distances up to about 4900 km, where GLORYS (1/12°) gives about 3100 km.

For another grid, pass any dataset with kmt, lon and lat to the generic functions:

from tracmass.seagrid import coast

mask = coast.landmask(grid_ds, k=1)
dist = coast.distance_to_land(mask)

Typical uses are seeding only near the coast, or removing particles that start too close to it:

near = dist < 50                                   # within 50 km of land
state = seed.mask(near.values & ~land.values, kpos=1)