[WIP] Initial attempt at basic Terrarium extension

ext/NumericalEarthTerrariumExt/NumericalEarthTerrariumExt.jl

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+module NumericalEarthTerrariumExt
+
+using OffsetArrays
+using KernelAbstractions
+using Statistics
+
+import Terrarium
+import Terrarium.RingGrids
+import NumericalEarth
+import Oceananigans
+
+include("terrarium_land_simulations.jl")
+include("terrarium_exchanger.jl")
+
+end # module NumericalEarthTerrariumExt

ext/NumericalEarthTerrariumExt/terrarium_exchanger.jl

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+using Oceananigans
+using Oceananigans.BoundaryConditions: fill_halo_regions!
+using Oceananigans.Grids: architecture
+using Oceananigans.Fields: set!, interior
+
+import NumericalEarth.EarthSystemModels: update_net_fluxes!, interpolate_state!
+import NumericalEarth.EarthSystemModels.InterfaceComputations: net_fluxes, ComponentExchanger
+
+net_fluxes(::LandSimulation) = nothing
+
+# Land exchanger constructor.
+# For now, no regridder is needed since the exchange grid is assumed to match the land grid.
+# The state holds a single surface temperature field that is communicated back to the atmosphere.
+# TODO: add regridder when exchange grid differs from land grid.
+function ComponentExchanger(land::LandSimulation, exchange_grid)
+    regridder = nothing
+    state = (; Ts = Field{Center, Center, Nothing}(exchange_grid))
+    return ComponentExchanger(state, regridder)
+end
+
+# Read the land surface state onto the exchange grid.
+# Currently: exchange grid == land grid → direct copy, no regridding.
+# TODO: regrid when exchange_grid differs from land grid.
+function interpolate_state!(exchanger, exchange_grid, land::LandSimulation, coupled_model)
+    Ts_land = land.state.skin_temperature  # °C
+    Ts_exchange = exchanger.state.Ts
+
+    # Convert skin temperature from °C to K and copy to exchange grid field
+    parent(Ts_exchange) .= parent(Ts_land) .+ 273.15
+
+    return nothing
+end
+
+# Update Terrarium land model inputs from the atmospheric state on the exchange grid.
+# This is the primary atmosphere → land coupling step.
+# TODO: regrid when land grid differs from the exchange grid.
+function update_net_fluxes!(coupled_model, land::LandSimulation)
+    atmos_state = coupled_model.interfaces.exchanger.atmosphere.state
+
+    # Air temperature: atmosphere provides K, Terrarium expects °C
+    set!(land.state.air_temperature, atmos_state.T - 273.15)
+
+    # Wind speed: compute magnitude from (u, v) components
+    # TODO: implement as a proper GPU-compatible kernel
+    set!(land.state.windspeed, sqrt(atmos_state.u^2 + atmos_state.v^2))
+
+    # Remaining atmospheric scalars: direct copy (same units)
+    set!(land.state.specific_humidity, atmos_state.q)
+    set!(land.state.air_pressure, atmos_state.p)
+    set!(land.state.surface_shortwave_down, atmos_state.ℐꜜˢʷ)
+    set!(land.state.surface_longwave_down, atmos_state.ℐꜜˡʷ)
+
+    # Total precipitation → rainfall; snowfall set to zero.
+    # TODO: partition rain/snow based on air temperature.
+    set!(land.state.rainfall, atmos_state.Jᶜ)
+    set!(land.state.snowfall, 0)
+
+    return nothing
+end

ext/NumericalEarthTerrariumExt/terrarium_land_simulations.jl

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+import NumericalEarth.Land: land_simulation
+
+const LandSimulation = Terrarium.ModelIntegrator
+const LandEarthSystemModel = NumericalEarth.EarthSystemModel{<:Any, <:Any, <:LandSimulation} #TODO: fix when updated
+
+Base.summary(::LandSimulation) = "Terrarium.ModelIntegrator"
+
+# Take one time-step or more depending on the global timestep
+function Oceananigans.TimeSteppers.time_step!(integrator::Terrarium.ModelIntegrator, Δt)
+    Δt_land = Terrarium.default_dt(integrator.timestepper)
+    nsteps = ceil(Int, Δt / Δt_land)
+
+    if (Δt / Δt_land) % 1 != 0
+        @warn "NumericalEarth only supports land timesteps that are integer divisors of the ESM timesteps"
+    end
+
+    for _ in 1:nsteps
+        Terrarium.timestep!(integrator, Δt_land)
+    end
+    return
+end
+
+"""
+    land_simulation(grid::Terrarium.AbstractLandGrid)
+
+Return a land simulation based on the given `AbstractLandGrid`.
+
+TODO: add more kwarg config options
+"""
+function land_simulation(grid::Terrarium.AbstractLandGrid)
+    # The land model
+    land_model = Terrarium.LandModel(grid)
+
+    # Construct the Terrarium integrator
+    integrator = Terrarium.initialize(land_model)
+
+    return integrator
+end