# Background fields

`BackgroundField`

s are velocity and tracer fields around which the resolved velocity and tracer fields evolve. In `Oceananigans`

, only the *advective* terms associated with the interaction between background and resolved fields are included. For example, tracer advection is described by

\[\boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{v} c \right ) \, ,\]

where $\boldsymbol{v}$ is the resolved velocity field and $c$ is the resolved tracer field corresponding to `model.tracers.c`

.

When a background field $C$ is provided, the tracer advection term becomes

\[\boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{v} c \right ) + \boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{v} C \right ) \, .\]

When both a background field velocity field $\boldsymbol{U}$ and a background tracer field $C$ are provided, then the tracer advection term becomes

\[\boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{v} c \right ) + \boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{v} C \right ) + \boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{U} c \right ) \, .\]

Notice that the term $\boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{U} C \right )$ is neglected: only the terms describing the advection of resolved tracer by the background velocity field and the advection of background tracer by the resolved velocity field are included. An analogous statement holds for the advection of background momentum by the resolved velocity field. Other possible terms associated with the Coriolis force, buoyancy, turbulence closures, and surface waves acting on background fields are neglected.

## Specifying background fields

`BackgroundField`

s are defined by functions of $(x, y, z, t)$ and optional parameters. A simple example is

```
using Oceananigans
U(x, y, z, t) = 0.2 * z
grid = RectilinearGrid(size=(1, 1, 1), extent=(1, 1, 1))
model = NonhydrostaticModel(grid = grid, background_fields = (u=U,))
model.background_fields.velocities.u
# output
FunctionField located at (Face, Center, Center)
├── func: U (generic function with 1 method)
├── grid: 1×1×1 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── clock: Clock(time=0 seconds, iteration=0)
└── parameters: nothing
```

`BackgroundField`

s are specified by passing them to the kwarg `background_fields`

in the `NonhydrostaticModel`

constructor. The kwarg `background_fields`

expects a `NamedTuple`

of fields, which are internally sorted into `velocities`

and `tracers`

, wrapped in `FunctionField`

s, and assigned their appropriate locations.

`BackgroundField`

s with parameters require using the `BackgroundField`

wrapper:

```
using Oceananigans
parameters = (α=3.14, N=1.0, f=0.1)
# Background fields are defined via function of x, y, z, t, and optional parameters
U(x, y, z, t, α) = α * z
B(x, y, z, t, p) = - p.α * p.f * y + p.N^2 * z
U_field = BackgroundField(U, parameters=parameters.α)
B_field = BackgroundField(B, parameters=parameters)
# output
BackgroundField{typeof(B), NamedTuple{(:α, :N, :f), Tuple{Float64, Float64, Float64}}}
├── func: B (generic function with 1 method)
└── parameters: (α = 3.14, N = 1.0, f = 0.1)
```

When inserted into `NonhydrostaticModel`

, we get

```
grid = RectilinearGrid(size=(1, 1, 1), extent=(1, 1, 1))
model = NonhydrostaticModel(grid = grid, background_fields = (u=U_field, b=B_field),
tracers=:b, buoyancy=BuoyancyTracer())
model.background_fields.tracers.b
# output
FunctionField located at (Center, Center, Center)
├── func: B (generic function with 1 method)
├── grid: 1×1×1 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 3×3×3 halo
├── clock: Clock(time=0 seconds, iteration=0)
└── parameters: (α = 3.14, N = 1.0, f = 0.1)
```