Time-varying depths

Some hydrodynamic models (such as SWASH) have time-evolving depth dimensions, for example because they follow the waves on the free surface. Parcels can work with these types of models, but it is a bit involved to set up. That is why we explain here how to run Parcels on FieldSets with time-evolving depth dimensions

from datetime import timedelta

import matplotlib.pyplot as plt
import xarray as xr

import parcels

Here, we use sample data from the SWASH model. We first set the filenames and variables

example_dataset_folder = parcels.download_example_dataset("SWASH_data")
filenames = f"{example_dataset_folder}/field_*.nc"
variables = {
    "U": "cross-shore velocity",
    "V": "along-shore velocity",
    "depth_u": "time varying depth_u",
}

Now, the first key step when reading time-evolving depth dimensions is that we specify depth as 'not_yet_set' in the dimensions dictionary

dimensions = {
    "U": {"lon": "x", "lat": "y", "depth": "not_yet_set", "time": "t"},
    "V": {"lon": "x", "lat": "y", "depth": "not_yet_set", "time": "t"},
    "depth_u": {"lon": "x", "lat": "y", "depth": "not_yet_set", "time": "t"},
}

Then, after we create the FieldSet object, we set the depth dimension of the relevant Fields to fieldset.depth_u and fieldset.depth_w, using the Field.set_depth_from_field() method

fieldset = parcels.FieldSet.from_netcdf(
    filenames, variables, dimensions, mesh="flat", allow_time_extrapolation=True
)
fieldset.U.set_depth_from_field(fieldset.depth_u)
fieldset.V.set_depth_from_field(fieldset.depth_u)

WARNING: Flipping lat data from North-South to South-North. Note that this may lead to wrong sign for meridional velocity, so tread very carefully

Now, we can create a ParticleSet, run those and plot them

pset = parcels.ParticleSet(fieldset, parcels.JITParticle, lon=9.5, lat=12.5, depth=-0.1)

pfile = pset.ParticleFile("SwashParticles", outputdt=timedelta(seconds=0.05))

pset.execute(parcels.AdvectionRK4, dt=timedelta(seconds=0.005), output_file=pfile)

INFO: Output files are stored in SwashParticles.zarr.
100%|██████████| 0.25/0.25 [00:00<00:00,  1.61it/s]

ds = xr.open_zarr("SwashParticles.zarr")

plt.plot(ds.lon.T, ds.lat.T, ".-")
plt.show()

Note that, even though we use 2-dimensional AdvectionRK4, the particle still moves down, because the grid itself moves down