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Fixes for the RK45 Kernel #2397

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Merged
erikvansebille merged 6 commits into from
Nov 24, 2025
Merged

Fixes for the RK45 Kernel #2397

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3db13dd
Fixing wrong return code in RK45 Kernel
erikvansebille Nov 21, 2025
e9d136a
Fixing backward time in RK45
erikvansebille Nov 21, 2025
6a67411
Add next_dt in RK45
erikvansebille Nov 21, 2025
e15c46c
Revert "Add next_dt in RK45"
erikvansebille Nov 21, 2025
36ec784
Making unt tests more explicit by testing against endtime
erikvansebille Nov 21, 2025
eefda47
Merge branch 'v4-dev' into fix_RK45
erikvansebille Nov 24, 2025
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16 changes: 8 additions & 8 deletions src/parcels/kernels/advection.py
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Original file line number Diff line number Diff line change
Expand Up @@ -129,14 +129,14 @@ def AdvectionEE(particles, fieldset): # pragma: no cover
def AdvectionRK45(particles, fieldset): # pragma: no cover
"""Advection of particles using adaptive Runge-Kutta 4/5 integration.

Note that this kernel requires a Particle Class that has an extra Variable 'next_dt'
and a FieldSet with constants 'RK45_tol' (in meters), 'RK45_min_dt' (in seconds)
and 'RK45_max_dt' (in seconds).
Note that this kernel requires a FieldSet with constants 'RK45_tol' (in meters),
'RK45_min_dt' (in seconds) and 'RK45_max_dt' (in seconds).

Time-step dt is halved if error is larger than fieldset.RK45_tol,
and doubled if error is smaller than 1/10th of tolerance.
"""
dt = particles.dt / np.timedelta64(1, "s") # TODO: improve API for converting dt to seconds
sign_dt = np.sign(dt)

c = [1.0 / 4.0, 3.0 / 8.0, 12.0 / 13.0, 1.0, 1.0 / 2.0]
A = [
Expand Down Expand Up @@ -189,17 +189,17 @@ def AdvectionRK45(particles, fieldset): # pragma: no cover
)
particles.dt = np.where(increase_dt_particles, particles.dt * 2, particles.dt)
particles.dt = np.where(
particles.dt > fieldset.RK45_max_dt * np.timedelta64(1, "s"),
fieldset.RK45_max_dt * np.timedelta64(1, "s"),
np.abs(particles.dt) > np.abs(fieldset.RK45_max_dt * np.timedelta64(1, "s")),
fieldset.RK45_max_dt * np.timedelta64(1, "s") * sign_dt,
particles.dt,
)
particles.state = np.where(good_particles, StatusCode.Success, particles.state)
particles.state = np.where(good_particles, StatusCode.Evaluate, particles.state)

repeat_particles = np.invert(good_particles)
particles.dt = np.where(repeat_particles, particles.dt / 2, particles.dt)
particles.dt = np.where(
particles.dt < fieldset.RK45_min_dt * np.timedelta64(1, "s"),
fieldset.RK45_min_dt * np.timedelta64(1, "s"),
np.abs(particles.dt) < np.abs(fieldset.RK45_min_dt * np.timedelta64(1, "s")),
fieldset.RK45_min_dt * np.timedelta64(1, "s") * sign_dt,
particles.dt,
)
particles.state = np.where(repeat_particles, StatusCode.Repeat, particles.state)
Expand Down
15 changes: 9 additions & 6 deletions tests/test_advection.py
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Original file line number Diff line number Diff line change
Expand Up @@ -235,11 +235,12 @@ def test_radialrotation(npart=10):
lon = np.linspace(32, 50, npart)
lat = np.ones(npart) * 30
starttime = np.arange(np.timedelta64(0, "s"), npart * dt, dt)
endtime = np.timedelta64(10, "m")

pset = parcels.ParticleSet(fieldset, lon=lon, lat=lat, time=starttime)
pset.execute(parcels.kernels.AdvectionRK4, endtime=np.timedelta64(10, "m"), dt=dt)
pset.execute(parcels.kernels.AdvectionRK4, endtime=endtime, dt=dt)

theta = 2 * np.pi * (pset.time - starttime) / np.timedelta64(24 * 3600, "s")
theta = 2 * np.pi * (endtime - starttime) / np.timedelta64(24 * 3600, "s")
true_lon = (lon - 30.0) * np.cos(theta) + 30.0
true_lat = -(lon - 30.0) * np.sin(theta) + 30.0

Expand Down Expand Up @@ -282,20 +283,21 @@ def test_moving_eddy(kernel, rtol):

start_lon, start_lat, start_z = 12000, 12500, 12500
dt = np.timedelta64(30, "m")
endtime = np.timedelta64(1, "h")

if kernel == AdvectionRK45:
fieldset.add_constant("RK45_tol", rtol)

pset = ParticleSet(fieldset, lon=start_lon, lat=start_lat, z=start_z, time=np.timedelta64(0, "s"))
pset.execute(kernel, dt=dt, endtime=np.timedelta64(1, "h"))
pset.execute(kernel, dt=dt, endtime=endtime)

def truth_moving(x_0, y_0, t):
t /= np.timedelta64(1, "s")
lat = y_0 - (ds.u_0 - ds.u_g) / ds.f * (1 - np.cos(ds.f * t))
lon = x_0 + ds.u_g * t + (ds.u_0 - ds.u_g) / ds.f * np.sin(ds.f * t)
return lon, lat

exp_lon, exp_lat = truth_moving(start_lon, start_lat, pset.time[0])
exp_lon, exp_lat = truth_moving(start_lon, start_lat, endtime)
np.testing.assert_allclose(pset.lon, exp_lon, rtol=rtol)
np.testing.assert_allclose(pset.lat, exp_lat, rtol=rtol)
if kernel == AdvectionRK4_3D:
Expand All @@ -321,13 +323,14 @@ def test_decaying_moving_eddy(kernel, rtol):

start_lon, start_lat = 10000, 10000
dt = np.timedelta64(60, "m")
endtime = np.timedelta64(23, "h")

if kernel == AdvectionRK45:
fieldset.add_constant("RK45_tol", rtol)
fieldset.add_constant("RK45_min_dt", 10 * 60)

pset = ParticleSet(fieldset, lon=start_lon, lat=start_lat, time=np.timedelta64(0, "s"))
pset.execute(kernel, dt=dt, endtime=np.timedelta64(23, "h"))
pset.execute(kernel, dt=dt, endtime=endtime)

def truth_moving(x_0, y_0, t):
t /= np.timedelta64(1, "s")
Expand All @@ -343,7 +346,7 @@ def truth_moving(x_0, y_0, t):
)
return lon, lat

exp_lon, exp_lat = truth_moving(start_lon, start_lat, pset.time[0])
exp_lon, exp_lat = truth_moving(start_lon, start_lat, endtime)
np.testing.assert_allclose(pset.lon, exp_lon, rtol=rtol)
np.testing.assert_allclose(pset.lat, exp_lat, rtol=rtol)

Expand Down