Internal wave example

In this example, we initialize an internal wave packet in two-dimensions and watch it propagate. This example illustrates how to set up a two-dimensional model, set initial conditions, and how to use BackgroundFields.

Install dependencies

First let's make sure we have all required packages installed.

using Pkg
pkg"add Oceananigans, CairoMakie"

The physical domain

First, we pick a resolution and domain size. We use a two-dimensional domain that's periodic in $(x, z)$ and is Flat in $y$:

using Oceananigans

grid = RectilinearGrid(size=(128, 128), x=(-π, π), z=(-π, π), topology=(Periodic, Flat, Periodic))
128×1×128 RectilinearGrid{Float64, Periodic, Flat, Periodic} on CPU with 3×0×3 halo
├── Periodic x ∈ [-3.14159, 3.14159) regularly spaced with Δx=0.0490874
├── Flat y
└── Periodic z ∈ [-3.14159, 3.14159) regularly spaced with Δz=0.0490874

Internal wave parameters

Inertia-gravity waves propagate in fluids that are both (i) rotating, and (ii) density-stratified. We use Oceananigans' Coriolis abstraction to implement a background rotation rate:

coriolis = FPlane(f=0.2)

On an FPlane, the domain is idealized as rotating at a constant rate with rotation period 2π/f. coriolis is passed to NonhydrostaticModel below. Our units are arbitrary.

We use Oceananigans' background_fields abstraction to define a background buoyancy field B(z) = N^2 * z, where z is the vertical coordinate and N is the "buoyancy frequency". This means that the modeled buoyancy field perturbs the basic state B(z).

# Background fields are functions of `x, y, z, t`, and optional parameters.
# Here we have one parameter, the buoyancy frequency

N = 1       # buoyancy frequency [s⁻¹]
B_func(x, z, t, N) = N^2 * z
B = BackgroundField(B_func, parameters=N)
BackgroundField{typeof(Main.var"##257".B_func), Int64}
├── func: B_func (generic function with 1 method)
└── parameters: 1

We are now ready to instantiate our model. We pass grid, coriolis, and B to the NonhydrostaticModel constructor. We add a small amount of IsotropicDiffusivity to keep the model stable during time-stepping, and specify that we're using a single tracer called b that we identify as buoyancy by setting buoyancy=BuoyancyTracer().

model = NonhydrostaticModel(; grid, coriolis,
                            advection = CenteredFourthOrder(),
                            timestepper = :RungeKutta3,
                            closure = ScalarDiffusivity(ν=1e-6, κ=1e-6),
                            tracers = :b,
                            buoyancy = BuoyancyTracer(),
                            background_fields = (; b=B)) # `background_fields` is a `NamedTuple`
NonhydrostaticModel{CPU, RectilinearGrid}(time = 0 seconds, iteration = 0)
├── grid: 128×1×128 RectilinearGrid{Float64, Periodic, Flat, Periodic} on CPU with 3×0×3 halo
├── timestepper: RungeKutta3TimeStepper
├── advection scheme: Centered reconstruction order 4
├── tracers: b
├── closure: ScalarDiffusivity{ExplicitTimeDiscretization}(ν=1.0e-6, κ=(b=1.0e-6,))
├── buoyancy: BuoyancyTracer with ĝ = NegativeZDirection()
└── coriolis: FPlane{Float64}(f=0.2)

A Gaussian wavepacket

Next, we set up an initial condition that excites an internal wave that propates through our rotating, stratified fluid. This internal wave has the pressure field

\[p(x, z, t) = a(x, z) \, \cos(k x + m z - ω t) \, .\]

where $m$ is the vertical wavenumber, $k$ is the horizontal wavenumber, $ω$ is the wave frequncy, and $a(x, z)$ is a Gaussian envelope. The internal wave dispersion relation links the wave numbers $k$ and $m$, the Coriolis parameter $f$, and the buoyancy frequency $N$:

Non-dimensional internal wave parameters

m = 16      # vertical wavenumber
k = 8       # horizontal wavenumber
f = coriolis.f

# Dispersion relation for inertia-gravity waves
ω² = (N^2 * k^2 + f^2 * m^2) / (k^2 + m^2)

ω = sqrt(ω²)

We define a Gaussian envelope for the wave packet so that we can observe wave propagation.

# Some Gaussian parameters
gaussian_amplitude = 1e-9
gaussian_width = grid.Lx / 15

# A Gaussian envelope centered at `(x, z) = (0, 0)`
a(x, z) = gaussian_amplitude * exp( -( x^2 + z^2 ) / 2gaussian_width^2 )

An inertia-gravity wave is a linear solution to the Boussinesq equations. In order that our initial condition excites an inertia-gravity wave, we initialize the velocity and buoyancy perturbation fields to be consistent with the pressure field $p = a \, \cos(kx + mx - ωt)$ at $t=0$. These relations are sometimes called the "polarization relations". At $t=0$, the polarization relations yield

u₀(x, z) = a(x, z) * k * ω   / (ω^2 - f^2) * cos(k * x + m * z)
v₀(x, z) = a(x, z) * k * f   / (ω^2 - f^2) * sin(k * x + m * z)
w₀(x, z) = a(x, z) * m * ω   / (ω^2 - N^2) * cos(k * x + m * z)
b₀(x, z) = a(x, z) * m * N^2 / (ω^2 - N^2) * sin(k * x + m * z)

set!(model, u=u₀, v=v₀, w=w₀, b=b₀)

Recall that the buoyancy b is a perturbation, so that the total buoyancy field is $N^2 z + b$.

A wave packet on the loose

We're ready to release the packet. We build a simulation with a constant time-step,

simulation = Simulation(model, Δt = 0.1 * 2π/ω, stop_iteration = 20)
Simulation of NonhydrostaticModel{CPU, RectilinearGrid}(time = 0 seconds, iteration = 0)
├── Next time step: 1.304 seconds
├── Elapsed wall time: 0 seconds
├── Wall time per iteration: NaN days
├── Stop time: Inf days
├── Stop iteration : 20.0
├── Wall time limit: Inf
├── Callbacks: OrderedDict with 4 entries:
│   ├── stop_time_exceeded => Callback of stop_time_exceeded on IterationInterval(1)
│   ├── stop_iteration_exceeded => Callback of stop_iteration_exceeded on IterationInterval(1)
│   ├── wall_time_limit_exceeded => Callback of wall_time_limit_exceeded on IterationInterval(1)
│   └── nan_checker => Callback of NaNChecker for u on IterationInterval(100)
├── Output writers: OrderedDict with no entries
└── Diagnostics: OrderedDict with no entries

and add an output writer that saves the vertical velocity field every two iterations:

filename = "internal_wave.jld2"
simulation.output_writers[:velocities] = JLD2OutputWriter(model, model.velocities; filename,
                                                          schedule = IterationInterval(1),
                                                          overwrite_existing = true)
JLD2OutputWriter scheduled on IterationInterval(1):
├── filepath: ./internal_wave.jld2
├── 3 outputs: (u, v, w)
├── array type: Array{Float64}
├── including: [:grid, :coriolis, :buoyancy, :closure]
├── file_splitting: NoFileSplitting
└── file size: 32.8 KiB

With initial conditions set and an output writer at the ready, we run the simulation

[ Info: Initializing simulation...
[ Info:     ... simulation initialization complete (1.013 seconds)
[ Info: Executing initial time step...
[ Info:     ... initial time step complete (15.083 seconds).
[ Info: Simulation is stopping after running for 16.530 seconds.
[ Info: Model iteration 20 equals or exceeds stop iteration 20.

Animating a propagating packet

To animate a the propagating wavepacket we just simulated, we load CairoMakie and make a Figure and an Axis for the animation,

using CairoMakie
set_theme!(Theme(fontsize = 24))

fig = Figure(size = (600, 600))

ax = Axis(fig[2, 1]; xlabel = "x", ylabel = "z",
          limits = ((-π, π), (-π, π)), aspect = AxisAspect(1))

Next, we load w data with FieldTimeSeries of w and make contour plots of vertical velocity. We use Makie's Observable to animate the data. To dive into how Observables work, refer to Makie.jl's Documentation.

n = Observable(1)

w_timeseries = FieldTimeSeries(filename, "w")
x, y, z = nodes(w_timeseries)

w = @lift interior(w_timeseries[$n], :, 1, :)
w_lim = 1e-8

contourf!(ax, x, z, w;
          levels = range(-w_lim, stop=w_lim, length=10),
          colormap = :balance,
          colorrange = (-w_lim, w_lim),
          extendlow = :auto,
          extendhigh = :auto)

title = @lift "ωt = " * string(round(w_timeseries.times[$n] * ω, digits=2))
fig[1, 1] = Label(fig, title, fontsize=24, tellwidth=false)

And, finally, we record a movie.

using Printf

frames = 1:length(w_timeseries.times)

@info "Animating a propagating internal wave..."

record(fig, "internal_wave.mp4", frames, framerate=8) do i
    n[] = i
[ Info: Animating a propagating internal wave...

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