Influences on the reflectance of Arctic sea ice and the impact of anthropogenic impurities on the surface shortwave radiation balance

hannes.schulz [ at ]


In order to investigate influences on the reflectance of snow covered Arctic sea ice, a discrete ordinate method and Mie-Theory based radiative transfer model has been set up. This model, the Snow on Sea Ice Model (SoSIM), is able to investigate changes in spectral and spectrally integrated (broadband) albedo of a multi-layer snow cover on sea ice due to varying snow microphysical parameters, atmospheric composition and incoming solar radiation. For typical conditions in the Arctic sea-ice area, it was found that the size of spherical snow grains and the angle of the sun above the horizon ultimately determine both spectral and broadband albedo of a thick snow layer. At 1300 nm, doubling the snow grain size decreases the albedo by about 20%, while a lower incident angle of solar light can offset this effect. The light absorbing impurity black carbon (soot) has a distinct influence on the albedo in the ultra-violet and visible range of the solar spectrum. However, it likely only lowers the albedo by less than 2%, for present concentrations of black carbon in Arctic snow. The spectral signature of black carbon is very similar to a thinning snow cover on top of a darker surface. SoSIM was also tested against other models and parametrisations for the spectral and broadband albedo of snowpacks as well as against field measurements of the spectral albedo of snow covered sea ice. The test proved the plausibility of the model results. Further, broadband albedo data from an airborne measurement campaign has been evaluated together with accompanying data such as sea ice thickness. This data demonstrated that sea-ice dynamics cause strong local surface heterogeneities. As a consequence, a strong variation is found in the spatially averaged surface albedo. To quantify this surface heterogeneity, an algorithm has been developed that automatically classifies typical freeze-up season surface covers of the Arctic ocean from photographs. As a combination of the findings from model study and evaluation of the campaign data, a model could be developed able to re-analyse the spatial distribution of broadband surface albedo of the Arctic ocean. The model utilises spatial information based on satellite observation of sea-ice concentration and thickness as well as climatological data of snow thickness. It is an alternative approach to derive the surface albedo based on SoSIM and not relying on satellite measurements in the visible range of the spectrum. First validations with air and satellite borne measurements of albedo distributions showed that the modelled albedo is plausible, yet has no better accuracy than ±5%. The model was used to predict the surface forcing of changes in the albedo as caused by the deposition of anthropogenic light absorbing substances onto the snow cover. It was found that depositing 40 ppbw black carbon into a pure snowpack causes an extra absorption of 1.58±0.83 W/m2 on average for the sea ice covered Arctic. The high relative uncertainty is caused by the uncertainty involved in the enhancement of light absorption by BC particles due to ageing processes.

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Thesis (Master)
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Schulz, H. , Herber, A. , Birnbaum, G. and Seckmeyer, G. (2014): Influences on the reflectance of Arctic sea ice and the impact of anthropogenic impurities on the surface shortwave radiation balance , Master thesis, Leibniz Universität Hannover.

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