# Langmuir turbulence example

This example implements a Langmuir turbulence simulation reported in section 4 of

This example demonstrates

How to run large eddy simulations with surface wave effects via the Craik-Leibovich approximation.

How to specify time- and horizontally-averaged output.

## Install dependencies

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

```
using Pkg
pkg"add Oceananigans, CairoMakie"
```

```
using Oceananigans
using Oceananigans.Units: minute, minutes, hours
```

## Model set-up

To build the model, we specify the grid, Stokes drift, boundary conditions, and Coriolis parameter.

### Domain and numerical grid specification

We use a modest resolution and the same total extent as Wagner et al. 2021,

`grid = RectilinearGrid(size=(32, 32, 32), extent=(128, 128, 64))`

```
32×32×32 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── Periodic x ∈ [0.0, 128.0) regularly spaced with Δx=4.0
├── Periodic y ∈ [0.0, 128.0) regularly spaced with Δy=4.0
└── Bounded z ∈ [-64.0, 0.0] regularly spaced with Δz=2.0
```

### The Stokes Drift profile

The surface wave Stokes drift profile prescribed in Wagner et al. 2021, corresponds to a 'monochromatic' (that is, single-frequency) wave field.

A monochromatic wave field is characterized by its wavelength and amplitude (half the distance from wave crest to wave trough), which determine the wave frequency and the vertical scale of the Stokes drift profile.

```
using Oceananigans.BuoyancyModels: g_Earth
amplitude = 0.8 # m
wavelength = 60 # m
wavenumber = 2π / wavelength # m⁻¹
frequency = sqrt(g_Earth * wavenumber) # s⁻¹
# The vertical scale over which the Stokes drift of a monochromatic surface wave
# decays away from the surface is `1/2wavenumber`, or
const vertical_scale = wavelength / 4π
# Stokes drift velocity at the surface
const Uˢ = amplitude^2 * wavenumber * frequency # m s⁻¹
```

`0.06791774197745354`

The `const`

declarations ensure that Stokes drift functions compile on the GPU. To run this example on the GPU, write `architecture = GPU()`

in the constructor for `NonhydrostaticModel`

below.

The Stokes drift profile is

`uˢ(z) = Uˢ * exp(z / vertical_scale)`

`uˢ (generic function with 1 method)`

which we'll need for the initial condition.

Oceananigans implements the Craik-Leibovich approximation for surface wave effects using the *Lagrangian-mean* velocity field as its prognostic momentum variable. In other words, `model.velocities.u`

is the Lagrangian-mean $x$-velocity beneath surface waves. This differs from models that use the *Eulerian-mean* velocity field as a prognostic variable, but has the advantage that $u$ accounts for the total advection of tracers and momentum, and that $u = v = w = 0$ is a steady solution even when Coriolis forces are present. See the physics documentation for more information.

The vertical derivative of the Stokes drift is

`∂z_uˢ(z, t) = 1 / vertical_scale * Uˢ * exp(z / vertical_scale)`

`∂z_uˢ (generic function with 1 method)`

Finally, we note that the time-derivative of the Stokes drift must be provided if the Stokes drift and surface wave field undergoes *forced* changes in time. In this example, the Stokes drift is constant and thus the time-derivative of the Stokes drift is 0.

### Boundary conditions

At the surface at $z=0$, Wagner et al. 2021 impose

```
Qᵘ = -3.72e-5 # m² s⁻², surface kinematic momentum flux
u_boundary_conditions = FieldBoundaryConditions(top = FluxBoundaryCondition(Qᵘ))
```

```
Oceananigans.FieldBoundaryConditions, with boundary conditions
├── west: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── east: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── south: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── north: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── bottom: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── top: FluxBoundaryCondition: -3.72e-5
└── immersed: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
```

Wagner et al. 2021 impose a linear buoyancy gradient `N²`

at the bottom along with a weak, destabilizing flux of buoyancy at the surface to faciliate spin-up from rest.

```
Qᵇ = 2.307e-8 # m² s⁻³, surface buoyancy flux
N² = 1.936e-5 # s⁻², initial and bottom buoyancy gradient
b_boundary_conditions = FieldBoundaryConditions(top = FluxBoundaryCondition(Qᵇ),
bottom = GradientBoundaryCondition(N²))
```

```
Oceananigans.FieldBoundaryConditions, with boundary conditions
├── west: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── east: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── south: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── north: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
├── bottom: GradientBoundaryCondition: 1.936e-5
├── top: FluxBoundaryCondition: 2.307e-8
└── immersed: DefaultBoundaryCondition (FluxBoundaryCondition: Nothing)
```

Note that Oceananigans uses "positive upward" conventions for all fluxes. In consequence, a negative flux at the surface drives positive velocities, and a positive flux of buoyancy drives cooling.

### Coriolis parameter

Wagner et al. (2021) use

`coriolis = FPlane(f=1e-4) # s⁻¹`

`FPlane{Float64}(f=0.0001)`

which is typical for mid-latitudes on Earth.

## Model instantiation

We are ready to build the model. We use a fifth-order Weighted Essentially Non-Oscillatory (WENO) advection scheme and the `AnisotropicMinimumDissipation`

model for large eddy simulation. Because our Stokes drift does not vary in $x, y$, we use `UniformStokesDrift`

, which expects Stokes drift functions of $z, t$ only.

```
model = NonhydrostaticModel(; grid, coriolis,
advection = WENO(),
timestepper = :RungeKutta3,
tracers = :b,
buoyancy = BuoyancyTracer(),
closure = AnisotropicMinimumDissipation(),
stokes_drift = UniformStokesDrift(∂z_uˢ=∂z_uˢ),
boundary_conditions = (u=u_boundary_conditions, b=b_boundary_conditions))
```

```
NonhydrostaticModel{CPU, RectilinearGrid}(time = 0 seconds, iteration = 0)
├── grid: 32×32×32 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── timestepper: RungeKutta3TimeStepper
├── tracers: b
├── closure: AnisotropicMinimumDissipation{ExplicitTimeDiscretization, NamedTuple{(:b,), Tuple{Float64}}, Float64, Nothing}
├── buoyancy: BuoyancyTracer with -ĝ = ZDirection
└── coriolis: FPlane{Float64}(f=0.0001)
```

## Initial conditions

We make use of random noise concentrated in the upper 4 meters for buoyancy and velocity initial conditions,

`Ξ(z) = randn() * exp(z / 4)`

Our initial condition for buoyancy consists of a surface mixed layer 33 m deep, a deep linear stratification, plus noise,

```
initial_mixed_layer_depth = 33 # m
stratification(z) = z < - initial_mixed_layer_depth ? N² * z : N² * (-initial_mixed_layer_depth)
bᵢ(x, y, z) = stratification(z) + 1e-1 * Ξ(z) * N² * model.grid.Lz
```

`bᵢ (generic function with 1 method)`

The simulation we reproduce from Wagner et al. (2021) is zero Lagrangian-mean velocity. This initial condition is consistent with a wavy, quiescent ocean suddenly impacted by winds. To this quiescent state we add noise scaled by the friction velocity to $u$ and $w$.

```
u★ = sqrt(abs(Qᵘ))
uᵢ(x, y, z) = u★ * 1e-1 * Ξ(z)
wᵢ(x, y, z) = u★ * 1e-1 * Ξ(z)
set!(model, u=uᵢ, w=wᵢ, b=bᵢ)
```

## Setting up the simulation

`simulation = Simulation(model, Δt=45.0, stop_time=4hours)`

```
Simulation of NonhydrostaticModel{CPU, RectilinearGrid}(time = 0 seconds, iteration = 0)
├── Next time step: 45 seconds
├── Elapsed wall time: 0 seconds
├── Wall time per iteration: NaN years
├── Stop time: 4 hours
├── Stop iteration : Inf
├── 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
```

We use the `TimeStepWizard`

for adaptive time-stepping with a Courant-Freidrichs-Lewy (CFL) number of 1.0,

```
wizard = TimeStepWizard(cfl=1.0, max_change=1.1, max_Δt=1minute)
simulation.callbacks[:wizard] = Callback(wizard, IterationInterval(10))
```

`Callback of TimeStepWizard(cfl=1.0, max_Δt=60.0, min_Δt=0.0) on IterationInterval(10)`

### Nice progress messaging

We define a function that prints a helpful message with maximum absolute value of $u, v, w$ and the current wall clock time.

```
using Printf
function progress(simulation)
u, v, w = simulation.model.velocities
# Print a progress message
msg = @sprintf("i: %04d, t: %s, Δt: %s, umax = (%.1e, %.1e, %.1e) ms⁻¹, wall time: %s\n",
iteration(simulation),
prettytime(time(simulation)),
prettytime(simulation.Δt),
maximum(abs, u), maximum(abs, v), maximum(abs, w),
prettytime(simulation.run_wall_time))
@info msg
return nothing
end
simulation.callbacks[:progress] = Callback(progress, IterationInterval(20))
```

`Callback of progress on IterationInterval(20)`

## Output

### A field writer

We set up an output writer for the simulation that saves all velocity fields, tracer fields, and the subgrid turbulent diffusivity.

```
output_interval = 5minutes
fields_to_output = merge(model.velocities, model.tracers, (; νₑ=model.diffusivity_fields.νₑ))
simulation.output_writers[:fields] =
JLD2OutputWriter(model, fields_to_output,
schedule = TimeInterval(output_interval),
filename = "langmuir_turbulence_fields.jld2",
overwrite_existing = true)
```

```
JLD2OutputWriter scheduled on TimeInterval(5 minutes):
├── filepath: ./langmuir_turbulence_fields.jld2
├── 5 outputs: (u, v, w, b, νₑ)
├── array type: Array{Float32}
├── including: [:grid, :coriolis, :buoyancy, :closure]
└── max filesize: Inf YiB
```

### An "averages" writer

We also set up output of time- and horizontally-averaged velocity field and momentum fluxes,

```
u, v, w = model.velocities
b = model.tracers.b
U = Average(u, dims=(1, 2))
V = Average(v, dims=(1, 2))
B = Average(b, dims=(1, 2))
wu = Average(w * u, dims=(1, 2))
wv = Average(w * v, dims=(1, 2))
simulation.output_writers[:averages] =
JLD2OutputWriter(model, (; U, V, B, wu, wv),
schedule = AveragedTimeInterval(output_interval, window=2minutes),
filename = "langmuir_turbulence_averages.jld2",
overwrite_existing = true)
```

```
JLD2OutputWriter scheduled on TimeInterval(5 minutes):
├── filepath: ./langmuir_turbulence_averages.jld2
├── 5 outputs: (U, V, B, wu, wv) averaged on AveragedTimeInterval(window=2 minutes, stride=1, interval=5 minutes)
├── array type: Array{Float32}
├── including: [:grid, :coriolis, :buoyancy, :closure]
└── max filesize: Inf YiB
```

## Running the simulation

This part is easy,

`run!(simulation)`

```
[ Info: Initializing simulation...
[ Info: i: 0000, t: 0 seconds, Δt: 49.500 seconds, umax = (1.5e-03, 5.0e-04, 6.3e-04) ms⁻¹, wall time: 0 seconds
[ Info: ... simulation initialization complete (13.028 seconds)
[ Info: Executing initial time step...
[ Info: ... initial time step complete (2.752 minutes).
[ Info: i: 0020, t: 15.907 minutes, Δt: 59.895 seconds, umax = (2.5e-02, 9.6e-03, 1.6e-02) ms⁻¹, wall time: 3.156 minutes
[ Info: i: 0040, t: 33 minutes, Δt: 1 minute, umax = (3.4e-02, 8.7e-03, 1.5e-02) ms⁻¹, wall time: 3.344 minutes
[ Info: i: 0060, t: 53 minutes, Δt: 1 minute, umax = (4.8e-02, 1.3e-02, 1.5e-02) ms⁻¹, wall time: 3.533 minutes
[ Info: i: 0080, t: 1.217 hours, Δt: 1 minute, umax = (5.8e-02, 2.0e-02, 1.9e-02) ms⁻¹, wall time: 3.729 minutes
[ Info: i: 0100, t: 1.550 hours, Δt: 1 minute, umax = (6.1e-02, 2.7e-02, 2.1e-02) ms⁻¹, wall time: 3.921 minutes
[ Info: i: 0120, t: 1.883 hours, Δt: 1 minute, umax = (6.6e-02, 3.2e-02, 2.2e-02) ms⁻¹, wall time: 4.120 minutes
[ Info: i: 0140, t: 2.217 hours, Δt: 59.177 seconds, umax = (6.8e-02, 3.8e-02, 2.8e-02) ms⁻¹, wall time: 4.318 minutes
[ Info: i: 0160, t: 2.482 hours, Δt: 57.966 seconds, umax = (6.9e-02, 3.6e-02, 3.1e-02) ms⁻¹, wall time: 4.572 minutes
[ Info: i: 0180, t: 2.750 hours, Δt: 55.615 seconds, umax = (7.2e-02, 4.2e-02, 3.2e-02) ms⁻¹, wall time: 4.781 minutes
[ Info: i: 0200, t: 3.015 hours, Δt: 51.273 seconds, umax = (7.8e-02, 4.8e-02, 3.5e-02) ms⁻¹, wall time: 4.976 minutes
[ Info: i: 0220, t: 3.263 hours, Δt: 52.970 seconds, umax = (7.4e-02, 5.0e-02, 3.8e-02) ms⁻¹, wall time: 5.173 minutes
[ Info: i: 0240, t: 3.545 hours, Δt: 49.992 seconds, umax = (7.6e-02, 5.1e-02, 4.0e-02) ms⁻¹, wall time: 5.360 minutes
[ Info: i: 0260, t: 3.808 hours, Δt: 56.896 seconds, umax = (7.0e-02, 5.0e-02, 3.1e-02) ms⁻¹, wall time: 5.555 minutes
[ Info: Simulation is stopping. Model time 4.000 hours has hit or exceeded simulation stop time 4 hours.
```

# Making a neat movie

We look at the results by loading data from file with FieldTimeSeries, and plotting vertical slices of $u$ and $w$, and a horizontal slice of $w$ to look for Langmuir cells.

```
using CairoMakie
time_series = (;
w = FieldTimeSeries("langmuir_turbulence_fields.jld2", "w"),
u = FieldTimeSeries("langmuir_turbulence_fields.jld2", "u"),
B = FieldTimeSeries("langmuir_turbulence_averages.jld2", "B"),
U = FieldTimeSeries("langmuir_turbulence_averages.jld2", "U"),
V = FieldTimeSeries("langmuir_turbulence_averages.jld2", "V"),
wu = FieldTimeSeries("langmuir_turbulence_averages.jld2", "wu"),
wv = FieldTimeSeries("langmuir_turbulence_averages.jld2", "wv"))
times = time_series.w.times
xw, yw, zw = nodes(time_series.w)
xu, yu, zu = nodes(time_series.u)
```

We are now ready to animate using Makie. We use Makie's `Observable`

to animate the data. To dive into how `Observable`

s work we refer to Makie.jl's Documentation.

```
n = Observable(1)
wxy_title = @lift string("w(x, y, t) at z=-8 m and t = ", prettytime(times[$n]))
wxz_title = @lift string("w(x, z, t) at y=0 m and t = ", prettytime(times[$n]))
uxz_title = @lift string("u(x, z, t) at y=0 m and t = ", prettytime(times[$n]))
fig = Figure(resolution = (850, 850))
ax_B = Axis(fig[1, 4];
xlabel = "Buoyancy (m s⁻²)",
ylabel = "z (m)")
ax_U = Axis(fig[2, 4];
xlabel = "Velocities (m s⁻¹)",
ylabel = "z (m)",
limits = ((-0.07, 0.07), nothing))
ax_fluxes = Axis(fig[3, 4];
xlabel = "Momentum fluxes (m² s⁻²)",
ylabel = "z (m)",
limits = ((-3.5e-5, 3.5e-5), nothing))
ax_wxy = Axis(fig[1, 1:2];
xlabel = "x (m)",
ylabel = "y (m)",
aspect = DataAspect(),
limits = ((0, grid.Lx), (0, grid.Ly)),
title = wxy_title)
ax_wxz = Axis(fig[2, 1:2];
xlabel = "x (m)",
ylabel = "z (m)",
aspect = AxisAspect(2),
limits = ((0, grid.Lx), (-grid.Lz, 0)),
title = wxz_title)
ax_uxz = Axis(fig[3, 1:2];
xlabel = "x (m)",
ylabel = "z (m)",
aspect = AxisAspect(2),
limits = ((0, grid.Lx), (-grid.Lz, 0)),
title = uxz_title)
wₙ = @lift time_series.w[$n]
uₙ = @lift time_series.u[$n]
Bₙ = @lift time_series.B[$n][1, 1, :]
Uₙ = @lift time_series.U[$n][1, 1, :]
Vₙ = @lift time_series.V[$n][1, 1, :]
wuₙ = @lift time_series.wu[$n][1, 1, :]
wvₙ = @lift time_series.wv[$n][1, 1, :]
k = searchsortedfirst(grid.zᵃᵃᶠ[:], -8)
wxyₙ = @lift interior(time_series.w[$n], :, :, k)
wxzₙ = @lift interior(time_series.w[$n], :, 1, :)
uxzₙ = @lift interior(time_series.u[$n], :, 1, :)
wlims = (-0.03, 0.03)
ulims = (-0.05, 0.05)
lines!(ax_B, Bₙ, zu)
lines!(ax_U, Uₙ, zu; label = L"\bar{u}")
lines!(ax_U, Vₙ, zu; label = L"\bar{v}")
axislegend(ax_U; position = :rb)
lines!(ax_fluxes, wuₙ, zw; label = L"mean $wu$")
lines!(ax_fluxes, wvₙ, zw; label = L"mean $wv$")
axislegend(ax_fluxes; position = :rb)
hm_wxy = heatmap!(ax_wxy, xw, yw, wxyₙ;
colorrange = wlims,
colormap = :balance)
Colorbar(fig[1, 3], hm_wxy; label = "m s⁻¹")
hm_wxz = heatmap!(ax_wxz, xw, zw, wxzₙ;
colorrange = wlims,
colormap = :balance)
Colorbar(fig[2, 3], hm_wxz; label = "m s⁻¹")
ax_uxz = heatmap!(ax_uxz, xu, zu, uxzₙ;
colorrange = ulims,
colormap = :balance)
Colorbar(fig[3, 3], ax_uxz; label = "m s⁻¹")
```

`Colorbar()`

And, finally, we record a movie.

```
frames = 1:length(times)
record(fig, "langmuir_turbulence.mp4", frames, framerate=8) do i
msg = string("Plotting frame ", i, " of ", frames[end])
print(msg * " \r")
n[] = i
end
```

```
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```

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