How to export bottom drag?

[jhdong2016]

I am running a simulation focusing on the bottom mixed layer. How can I export bottom drag if I am interested about that?

[glwagner]

Unfortunately there is no user interface to build Field from boundary conditions, though we do need one. It's still possible, but a bit of work. This code is fairly general I think:

using Oceananigans.BoundaryConditions: getbc
using Oceananigans: fields

# Boundary condition extractor in "kernel function form"
@inline kernel_getbc(i, j, k, grid, boundary_condition, clock, fields) =
    getbc(boundary_condition, i, j, grid, clock, fields)

# Kernel arguments
grid = model.grid
clock = model.clock
model_fields = merge(fields(model), model.auxiliary_fields)
u, v, w = model.velocities
u_bc = u.boundary_conditions.bottom
v_bc = v.boundary_conditions.bottom

# Build operations
u_bc_op = KernelFunctionOperation{Face, Center, Nothing}(kernel_getbc, grid, u_bc, clock, model_fields)
v_bc_op = KernelFunctionOperation{Center, Face, Nothing}(kernel_getbc, grid, v_bc, clock, model_fields)

u_bc_op and v_bc_op can be passed to OutputWriters. You can also build Fields from them:

# Build Fields
u_bc_field = Field(u_bc_op)
v_bc_field = Field(v_bc_op)

Note that this will compute the boundary condition directly. So if you specify a boundary condition as FluxBoundaryCondition, this outputs the flux. If you are using ValueBoundaryCondition, however, you would not get the momentum flux from this code. For that we need to work a little harder...

Another possibility is simply to write a function that computes the drag as it's been specified. However to do that correctly you need to be careful to handle the staggered grid correctly. If you share your code, we can help with that.

[jhdong2016]

Thanks for your reply! I will try that for my simulation!

[jhdong2016]

I use the code as you suggested, but it reported an error as follows. Do you have any idea for solving it?

[simone-silvestri]

Hello. That error shows that you are using an order version of Oceananigans where the signature for KernelFunctionOperation was different. You can use the old version and write:

u_bc_op=KernelFunctionOperation{Face, Center, Nothing}(kernel_getbc, grid; computed_dependencies=(u_bc, clock, model_fields))
v_bc_op=KernelFunctionOperation{Center, Face, Nothing}(kernel_getbc, grid; computed_dependencies=(v_bc, clock, model_fields))

Or you can update Oceananigans to the newer version

using Pkg
Pkg.update("Oceananigans")

[josuemtzmo]

Hello, I'm also looking into this, is there a new user interface to build the Field for the boundary conditions without the need of this cool brute force way?

[glwagner]

There's nothing automatic, but you could open an issue that outlines the feature you think would be helpful, then we can come up with what the syntax should look like, and then we can implement it.