Metamorphism of snow on Arctic sea ice during the melt season: impact on spectral albedo and radiative fluxes through snow

Abstract

The energy budget of Arctic sea ice is strongly affected by the snow cover. Intensive sampling of snow properties was conducted near Qikiqtarjuaq in Baffin Bay on typical landfast sea ice during two melt seasons in 2015 and 2016. The sampling included stratigraphy, vertical profiles of snow specific surface area (SSA), density and irradiance, and spectral albedo (300-1100 nm). Both years featured four main phases: (I) dry snow cover, (II) surface melting, (III) ripe snowpack, and (IV) melt pond formation. Each phase was characterized by distinctive physical and optical properties. A high SSA value of 49.3m2 kg-1 was measured during phase I on surface wind slabs together with a corresponding broadband albedo (300-3000 nm) of 0.87. Phase II was marked by alternating episodes of surface melting, which dramatically decreased the SSA below 3m2 kg-1, and episodes of snowfall re-establishing pre-melt conditions. Albedo was highly time-variable, with minimum broadband values around 0.70. In phase III, continued melting led to a fully ripe snowpack composed of clustered rounded grains. Albedo began to decrease in the visible as snow thickness decreased but remained steady at longer wavelengths. Moreover, significant spatial variability appeared for the first time following snow depth heterogeneity. Spectral albedo was simulated by radiative transfer using measured SSA and density vertical profiles and estimated impurity contents based on limited measurements. Simulations were most of the time within 1% of measurements in the visible and within 2% in the infrared. Simulations allowed the calculations of albedo and of the spectral flux at the snow-ice interface. These showed that photosynthetically active radiation fluxes at the bottom of the snowpack durably exceeded 5Wm-2 (_ 9:2 μmolm-2 s-1) only when the snowpack thickness started to decrease at the end of phase II.