Abstract
The directional reflection of solar radiation by the Arctic Ocean is dominated by two main surface types: sea ice (often snow-covered) and ice-free (open) ocean. However, in the transitional marginal sea ice zone (MIZ), the reflection properties of both surface types are mixed, which might cause uncertainties in the results of retrieval methods of atmospheric parameters over the MIZ using airborne and satellite measurements. To quantify these uncertainties, respective measurements of reflection properties of the MIZ are needed. Therefore, in this study, an averaged hemispherical-directional reflectance factor (HDRF) of the inhomogeneous surface (mixture of sea ice and open ocean surfaces) in the MIZ is derived using airborne measurements collected with a digital fish-eye camera. For this purpose, a sea ice mask was constructed to separate the reflectivity measurements from sea ice and open ocean pixels. The separated data sets were accumulated and averaged to provide separate HDRFs for sea ice and open ocean surfaces. The respective results were compared with simulations and independent measurements available from the literature. Using the sea ice fraction derived in parallel from the digital camera images, the mixed HDRF describing the directional reflectivity of the inhomogeneous surface of the MIZ was reconstructed by a linear weighting procedure. The result was compared with the original measurements of directional reflectivity over the MIZ. It is concluded that the HDRF of the MIZ can be well reconstructed by linear combination of the HDRFs of homogeneous sea ice and open ocean surfaces, accounting for the special conditions present in the MIZ compared to homogeneous surfaces.
Highlights
The Arctic Ocean is a key element of the complex Arctic climate system
Using the sea ice fraction derived in parallel from the digital camera images, the mixed hemispherical-directional reflectance factor (HDRF) describing the directional reflectivity of the inhomogeneous surface of the marginal sea ice zone (MIZ) was reconstructed by a linear weighting procedure
375 The number of pixels used for the separated open ocean HDRF was rather low, which led to a higher variability compared to the separated sea ice HDRF
Summary
The Arctic Ocean is a key element of the complex Arctic climate system. From a solar radiative point of view it is characterized by a strong contrast of bright sea ice surfaces and dark areas of open ocean, which both determine the surface solar radiative energy budget. The reflection of solar radiation by snow-covered sea ice and open ocean surfaces was extensively studied and characterized by ground-based, airborne, and satellite observations Bourgeois et al (2006) and Marks et al (2015) retrieved the HDRF of snow using a ground-based goniospectrometer, which measures the spectral radiance reflected from a small surface area from different directions. For homogeneous snow-covered areas in the Antarctic, Carlsen et al (2020) used airborne 180◦ fish-eye camera observations to quantify the anisotropy of the snow HDRF with changing surface roughness, snow grain size, and SZA. In our study, the spatial coverage of airborne observations with a 180◦ fish-eye camera is used to characterize the HDRF of a mixture of open 80 ocean and snow-covered sea ice surfaces in the MIZ.
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