Cabo Verde FTLE¶

The minimal wiring between lcs_parcels and Parcels v4: seed a grid, advect it through CMEMS surface currents, ingest the final positions, map the forward FTLE. One stencil (NeighborSeedGrid); nothing tuned for speed.

The currents come from the local file data/cabo_verde_currents_hourly.nc; run get_data.ipynb once to produce it.

# Importing Parcels pulls in the holoviews/bokeh bootstrap and prints an
# alpha-version notice, neither of which belongs on the rendered page.
import numpy as np
import xarray as xr
from parcels import FieldSet, Particle, ParticleSet, StatusCode
from parcels.convert import copernicusmarine_to_sgrid
from parcels.kernels import AdvectionRK4

from lcs_parcels import NeighborSeedGrid

Parameters¶

Ten days is roughly one eddy turnover time at these latitudes: the FTLE map below comes out around 0.1/day over most of the box, an e-folding time of about ten days. That is long enough for the stretching to separate neighbouring particles by more than one seed-grid cell (\(1/25^\circ\), about 4.4 km), and short enough that the seed grid still resolves where they went. The seed spacing is finer than the \(1/12^\circ\) currents, so the differencing stencil is not what limits the FTLE.

t0 = np.datetime64("2025-08-01")
T = np.timedelta64(10, "D")  # signed window; sign(T) sets the direction
t1 = t0 + T

resolution_deg = 1 / 25  # seed-grid spacing
seed_lon, seed_lat = (-27.0, -21.0), (13.5, 18.5)  # release box

Parcels v4 field set¶

copernicusmarine_to_sgrid tags the CMEMS A-grid with SGRID metadata; from_sgrid_conventions wraps it as a spherical FieldSet.

sgrid = copernicusmarine_to_sgrid(fields={"U": currents["uo"], "V": currents["vo"]})
fieldset = FieldSet.from_sgrid_conventions(sgrid, mesh="spherical")
z_surface = float(currents["depth"].values[0])

Recovery kernel¶

Particles that leave the domain or hit land are turned into NaN in place (Parcels would otherwise abort the run), so losses propagate as NaN through the FTLE. StatusCode.EndofLoop rather than StatusCode.Delete: deleting shrinks the particle array and breaks the alignment with the seed order.

def set_lost_to_nan(particles, fieldset):
    lost = particles.state >= StatusCode.Error
    particles.x = np.where(lost, np.nan, particles.x)
    particles.y = np.where(lost, np.nan, particles.y)
    particles.state = np.where(lost, StatusCode.EndofLoop, particles.state)

Advect¶

to_parcels_pset() flattens the release positions to (lon, lat); Parcels calls those x/y.

lon, lat = seed.to_parcels_pset()
z = np.full(len(lon), z_surface)
pset = ParticleSet(fieldset, pclass=Particle, x=lon, y=lat, z=z, t=t0)

pset.execute(
    [AdvectionRK4, set_lost_to_nan],
    dt=np.timedelta64(1, "h"),
    runtime=T,
    verbose_progress=False,
)

Map¶

ftle() returns SI \(1/\mathrm{s}\), which over a 10-day window is a field of numbers around \(10^{-6}\); rescaling to \(1/\mathrm{day}\) makes the map readable. The scaling carries the name and long_name over, so only units has to be corrected. The grid points are 2-D coords on the logical dims (i, j), so x and y name the coords to use as axes; the labels come from the metadata.

ftle_per_day = (ftle * 86400.0).assign_attrs(units="1/day")

_ = ftle_per_day.plot.pcolormesh(x="lon_grid", y="lat_grid")

../_images/5c48e1a24ef57ba005f05b1b12f2c51ee76068865bae12b527d0535cda6a3179.png

Cabo Verde FTLE¶

Parameters¶

Currents¶

Parcels v4 field set¶

Recovery kernel¶

Seed grid¶

Advect¶

Flow map¶

FTLE¶

Map¶

LCS-Parcels

Navigation

Related Topics