The effects of surface roughness on the calculated, spectral, conical–conical reflectance factor as an alternative to the bidirectional reflectance distribution function of bare sea ice

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he conical–conical reflectance factor (CCRF) has been calculated as an alternative to the bidirectional re- flectance distribution function (BRDF) for three types of bare sea ice with varying surface roughness (σ = 0.1–10) and ice thicknesses (50–2000 cm) over an incident solar irradi- ance wavelength range of 300–1400 nm. The comprehensive study of the CCRF of sea ice presented here is paramount for interpreting sea ice measurements from satellite imagery and inter-calibrating space-borne sensors that derive albedo from multiple multi-angular measurements. The calculations per- formed by a radiative-transfer code (PlanarRad) show that the CCRF of sea ice is sensitive to realistic values of sur- face roughness. The results presented here show that sur- face roughness cannot be considered independently of sea ice thickness, solar zenith angle and wavelength. A typical CCRF of sea ice has a quasi-isotropic reflectance over the hemisphere, associated with a strong forward-scattering peak of photons. Surface roughness is crucial for the location, size and intensity of the forward-scattering peak. As the surface roughness increases, a spreading of the CCRF peak is ob- served. The hemisphere was split in to 216 quadrangular re- gions or quads. The peak remains specular for the smaller surface roughnesses (σ = 0.001 to σ = 0.01), whereas for larger surface roughness features (above σ = 0.05), the peak spreads out over multiple quads with a lower intensity than for smaller roughness features, and the highest value is dis- placed further out on the solar principal plane. Different types of sea ice have a similar pattern with wavelength: the CCRF increases by 30 % from first-year sea ice to multi-year sea ice at 400nm and up to 631% at 1100nm, 32% from melting sea ice to multi-year sea ice at 400 nm and a max-
imum of 98% at 900nm, and 11% from melting sea ice to first-year sea ice at 400 nm and up to 86 % at 800 nm. The CCRF calculations presented in this study form the first set of complete CCRF values as an approximation of the BRDF for bare sea ice with a wide range of configurations
Original languageEnglish
Pages (from-to)737–751
JournalThe Cryopshere
Publication statusPublished - 2023

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