Coastline interaction

Example to illustrate stranding options using an artificial east-west oscillating current field

from datetime import datetime, timedelta
from opendrift.readers import reader_oscillating
from opendrift.models.oceandrift import OceanDrift
duration = timedelta(hours=36)
time_step = timedelta(hours=.5)
number = 500

# Oscillating east-west current to illustrate stranding options
reader_osc = reader_oscillating.Reader('x_sea_water_velocity', amplitude=1,
                                       zero_time=datetime.utcnow())

Coastline option “stranding”

Particles are moved forward with forcing velocity (e.g. current and wind drift) times the time step. If the next position is on land, the particle is deactivated. Note that particles may “jump” a distance onto land, depending on the calculation timestep.

o = OceanDrift(loglevel=50)
o.add_reader([reader_osc])
o.set_config('environment:fallback:y_sea_water_velocity', .2)
o.set_config('general:coastline_action', 'stranding')
o.set_config('general:coastline_approximation_precision', None)
o.seed_elements(lon=12.2, lat=67.7, radius=5000, number=number, time=reader_osc.zero_time)
o.run(duration=duration, time_step=time_step)
print(f'Calculation time: {o.timing["total time"]}')
o.animation()

Calculation time: 0:00:09.391139

Coastline option “stranding” with higher precision

By setting config “general:coastline_approximation_precision” to desired accuracy in degrees (default is 0.01 as in this example), a more exact coastline crossing is calculated by the deactivated particles. Note that with a (too) large compuation time step, particles may still “jump” over islands. An alternative to avoid this possibility is to use a smaller timestep for the simulation, though at a larger computational cost.

o = OceanDrift(loglevel=50)
o.add_reader([reader_osc])
o.set_config('environment:fallback:y_sea_water_velocity', .2)
o.set_config('general:coastline_action', 'stranding')
o.set_config('general:coastline_approximation_precision', .001)  # approx 100m
o.seed_elements(lon=12.2, lat=67.7, radius=5000, number=number, time=reader_osc.zero_time)
o.run(duration=duration, time_step=time_step)
print(f'Calculation time: {o.timing["total time"]}')
o.animation()

Calculation time: 0:00:04.498571

Coastline option “previous”

Particles hitting land are moved back to previous position in water, and may move offshore if currents/winds/forcing change direction. Here we see that several particles are jumping over small island. Reducing timestep to e.g. 15 minutes will reduce this problem.

o = OceanDrift(loglevel=50)
o.add_reader([reader_osc])
o.set_config('environment:fallback:y_sea_water_velocity', .2)
o.set_config('general:coastline_action', 'previous')
o.seed_elements(lon=12.2, lat=67.7, radius=5000, number=number, time=reader_osc.zero_time)
o.run(duration=duration, time_step=time_step)
print(f'Calculation time: {o.timing["total time"]}')
o.animation()

Calculation time: 0:00:02.263362

Coastline option “none”

No interaction with land, as used by e.g. WindBlow model (atmospheric drift)

o = OceanDrift(loglevel=50)
o.add_reader([reader_osc])
o.set_config('environment:fallback:y_sea_water_velocity', .2)
o.set_config('general:coastline_action', 'none')
o.seed_elements(lon=12.2, lat=67.7, radius=5000, number=number, time=reader_osc.zero_time)
o.run(duration=duration, time_step=time_step)
print(f'Calculation time: {o.timing["total time"]}')
o.animation()

Calculation time: 0:00:02.590779