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## Surface Albedo

The article discusses the calculation of surface albedo, which is the reflectivity of a surface, for lakes.

For direct radiation on unfrozen lakes, the albedo is given by the equation:
a = 0.5 / (cos(z) + 0.15)
where z is the zenith angle. For diffuse radiation, the albedo is 0.10.

For frozen lakes without resolved snow layers, the initial albedo (a0) is 0.60 for visible and 0.40 for near-infrared radiation. As the surface temperature (Tg) approaches the freezing temperature (Tf), the albedo is relaxed towards 0.10 using the equation:
a = a0 * (1-x) + 0.10 * x
where x = exp(-95 * (Tf - Tg) / Tf)

For frozen lakes with resolved snow layers, the reflectance of the ice surface is fixed at a0, and the snow reflectance is calculated as over non-vegetated surfaces. These two reflectances are combined to obtain the snow-fraction-weighted albedo.