Delayed starts

In many applications, it is needed to ‘delay’ the start of particle advection. For example because particles need to be released at different times throughout an experiment. Or because particles need to be released at a constant rate from the same set of locations.

This tutorial will show how this can be done. We start with importing the relevant modules.

from datetime import timedelta as delta

import matplotlib.pyplot as plt
import numpy as np
import xarray as xr
from IPython.display import HTML
from matplotlib.animation import FuncAnimation

from parcels import (
    AdvectionRK4,
    FieldSet,
    JITParticle,
    ParticleFile,
    ParticleSet,
    Variable,
    download_example_dataset,
)

First import a FieldSet (from the Peninsula example, in this case)

example_dataset_folder = download_example_dataset("Peninsula_data")
fieldset = FieldSet.from_parcels(
    f"{example_dataset_folder}/peninsula", allow_time_extrapolation=True
)

Now, there are two ways to delay the start of particles. Either by defining the whole ParticleSet at initialisation and giving each particle its own time. Or by using the repeatdt argument. We will show both options here

Assigning each particle its own time

The simplest way to delaye the start of a particle is to use the time argument for each particle

npart = 10  # number of particles to be released
lon = 3e3 * np.ones(npart)
lat = np.linspace(3e3, 45e3, npart, dtype=np.float32)

# release every particle one hour later
time = np.arange(0, npart) * delta(hours=1).total_seconds()

pset = ParticleSet(fieldset=fieldset, pclass=JITParticle, lon=lon, lat=lat, time=time)

Then we can execute the pset as usual

output_file = pset.ParticleFile(name="DelayParticle_time.zarr", outputdt=delta(hours=1))

pset.execute(
    AdvectionRK4, runtime=delta(hours=24), dt=delta(minutes=5), output_file=output_file
)

INFO: Compiled ArrayJITParticleAdvectionRK4 ==> /var/folders/x0/1qxj679n55zcybylvdsb4bxh0000gq/T/parcels-503/f998c2c7f013a30b5e42fcd160d9b2fa_0.c

And then finally, we can show a movie of the particles. Note that the southern-most particles start to move first.

%%capture

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

fig = plt.figure(figsize=(7, 5), constrained_layout=True)
ax = fig.add_subplot()

ax.set_ylabel("Meridional distance [m]")
ax.set_xlabel("Zonal distance [m]")
ax.set_xlim(0, 9e4)
ax.set_ylim(0, 5e4)

timerange = np.unique(ds["time"].values[np.isfinite(ds["time"])])

# Indices of the data where time = 0
time_id = np.where(ds["time"] == timerange[0])

sc = ax.scatter(ds["lon"].values[time_id], ds["lat"].values[time_id])

t = str(timerange[0].astype("timedelta64[h]"))
title = ax.set_title(f"Particles at t = {t}")


def animate(i):
    t = str(timerange[i].astype("timedelta64[h]"))
    title.set_text(f"Particles at t = {t}")

    time_id = np.where(ds["time"] == timerange[i])
    sc.set_offsets(np.c_[ds["lon"].values[time_id], ds["lat"].values[time_id]])


anim = FuncAnimation(fig, animate, frames=len(timerange), interval=100)

HTML(anim.to_jshtml())

Once Loop Reflect

Using the repeatdt argument

The second method to delay the start of particle releases is to use the repeatdt argument when constructing a ParticleSet. This is especially useful if you want to repeatedly release particles from the same set of locations.

npart = 10  # number of particles to be released
lon = 3e3 * np.ones(npart)
lat = np.linspace(3e3, 45e3, npart, dtype=np.float32)
repeatdt = delta(hours=3)  # release from the same set of locations every 3h

pset = ParticleSet(
    fieldset=fieldset, pclass=JITParticle, lon=lon, lat=lat, repeatdt=repeatdt
)

Now we again define an output file and execute the pset as usual.

output_file = pset.ParticleFile(name="DelayParticle_releasedt", outputdt=delta(hours=1))

pset.execute(
    AdvectionRK4, runtime=delta(hours=24), dt=delta(minutes=5), output_file=output_file
)

INFO: Compiled ArrayJITParticleAdvectionRK4 ==> /var/folders/x0/1qxj679n55zcybylvdsb4bxh0000gq/T/parcels-503/c93783213e198edf2cb1d5c0e5130e7e_0.c

And we get an animation where a new particle is released every 3 hours from each start location

%%capture

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

fig = plt.figure(figsize=(7, 5), constrained_layout=True)
ax = fig.add_subplot()

ax.set_ylabel("Meridional distance [m]")
ax.set_xlabel("Zonal distance [m]")
ax.set_xlim(0, 9e4)
ax.set_ylim(0, 5e4)

timerange = np.unique(ds["time"].values[np.isfinite(ds["time"])])

# Indices of the data where time = 0
time_id = np.where(ds["time"] == timerange[0])

sc = ax.scatter(ds["lon"].values[time_id], ds["lat"].values[time_id])

t = str(timerange[0].astype("timedelta64[h]"))
title = ax.set_title(f"Particles at t = {t}")


def animate(i):
    t = str(timerange[i].astype("timedelta64[h]"))
    title.set_text(f"Particles at t = {t}")

    time_id = np.where(ds["time"] == timerange[i])
    sc.set_offsets(np.c_[ds["lon"].values[time_id], ds["lat"].values[time_id]])


anim = FuncAnimation(fig, animate, frames=len(timerange), interval=100)

HTML(anim.to_jshtml())

Once Loop Reflect