Add temperature/snow-dependent sea ice albedo

[simone-silvestri]

Summary

Add a state-dependent sea ice albedo parameterization following the CCSM3 scheme
(Briegleb et al. 2004; Briegleb & Light 2007), replacing the default constant albedo.
The SeaIceAlbedo struct stores references to sea ice model fields and is evaluated
pointwise via stateindex inside the atmosphere--sea-ice flux kernel.

Also adds Adapt.adapt_structure for InterfaceProperties so that nested structs
containing GPU field references (like SeaIceAlbedo) are properly adapted when
passed to GPU kernels.

Algorithm

The broadband albedo is computed in four steps:

1. Temperature-dependent melt reduction

Near the melting point, albedo is reduced to represent implicit melt-pond formation:

fT = clamp((Ts - Tm + ΔT) / ΔT, 0, 1)

αi = αi_cold - Δαi * fT
αs = αs_cold - Δαs * fT

where αi_cold = 0.54, αs_cold = 0.83, Δαi = 0.075, Δαs = 0.10, and ΔT = 1 K.

2. Thin-ice transition to ocean albedo

For ice thinner than h_min = 0.5 m, the bare-ice albedo blends toward ocean albedo αo = 0.06:

fh = clamp(hi / h_min, 0, 1)

αi = αo + (αi - αo) * fh

3. Snow cover blending

Snow depth interpolates between bare-ice and snow albedo, reaching full snow cover at hs_min = 0.02 m:

fs = clamp(hs / hs_min, 0, 1)

4. Final albedo

α = fs * αs + (1 - fs) * αi

Default parameters

Parameter	Default	Source
Cold ice albedo αi_cold	0.54	0.52 × 0.73 + 0.48 × 0.33 (vis + nir weighted)
Cold snow albedo αs_cold	0.83	0.52 × 0.96 + 0.48 × 0.68
Ice melt reduction Δαi	0.075	Briegleb et al. (2004)
Snow melt reduction Δαs	0.10	Briegleb et al. (2004)
Melting temperature Tm	0 °C
Temperature range ΔT	1 K
Ocean albedo αo	0.06
Min ice thickness h_min	0.5 m
Min snow depth hs_min	0.02 m

Changes

• New file: sea_ice_albedo.jl — SeaIceAlbedo struct, constructor, stateindex, Adapt.adapt_structure
• InterfaceComputations.jl — export SeaIceAlbedo, include sea_ice_albedo.jl
• interface_states.jl — Adapt.adapt_structure for InterfaceProperties
• atmosphere_sea_ice_fluxes.jl — evaluate radiation properties via stateindex before the similarity-theory iteration; pass local_interface_properties with scalar albedo/emissivity

References

• Briegleb, B.P., C.M. Bitz, E.C. Hunke, W.H. Lipscomb, and M.M. Schramm (2004):
  Scientific description of the sea ice component in CCSM3. NCAR Tech Note NCAR/TN-463+STR.
• Briegleb, B.P. and B. Light (2007): NCAR/TN-472+STR.

Tests

test/test_sea_ice_albedo.jl — 8 test sets:

• Constructor (Float64/Float32)
• Nothing snow thickness
• Cold thick ice, no snow → α = 0.54
• Cold thick ice, deep snow → α = 0.83
• Melting ice, no snow → α = 0.465
• Zero ice → ocean albedo α = 0.06
• Partial snow cover → weighted blend
• Adapt.adapt_structure for SeaIceAlbedo and InterfaceProperties

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/Radiations/sea_ice_albedo.jl	81.81%	4 Missing ⚠️

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