The amount of solar radiation transmitted through Arctic sea ice is determined by the thickness and physical properties of snow and sea ice. Light transmittance is highly variable in space and time since thickness and physical properties of snow and sea ice are highly heterogeneous on variable time and length scales. We present field measurements of under-ice irradiance along transects under undeformed land-fast sea ice at Barrow, Alaska (March, May, and June 2010). The measurements were performed with a spectral radiometer mounted on a floating under-ice sled. The objective was to quantify the spatial variability of light transmittance through snow and sea ice, and to compare this variability along its seasonal evolution. Along with optical measurements, snow depth, sea ice thickness, and freeboard were recorded, and ice cores were analyzed for chlorophyll a and particulate matter. Our results show that snow cover variability prior to onset of snow melt causes as much relative spatial variability of light transmittance as the contrast of ponded and white ice during summer. Both before and after melt onset, measured transmittances fell in a range from one third to three times the mean value. In addition, we found a twentyfold increase of light transmittance as a result of partial snowmelt, showing the seasonal evolution of transmittance through sea ice far exceeds the spatial variability. However, prior melt onset, light transmittance was time invariant and differences in under-ice irradiance were directly related to the spatial variability of the snow cover.

Spectral measurements of solar irradiance over and under seasonal land-fast sea ice have been performed off Barrow, Alaska. Main goal was to quantify spatial variability of under-ice irradiance as a function of snow and sea-ice properties. All radiation measurements have been performed with calibrated Ramses ACC radiometers (Trios, Rastede, Germany), covering a wavelength range from 320 to 950 nm at 3.3 nm spectral resolution. Surface measurements of solar irradiance have been performed on a stationary rack. The radiometer for under-ice measurements was mounted on a floating sled, that was pulled along transects under sea ice. The data set contains three profiles on the same site, one in March, one in May, and one in June 2010. Along with the radiation measurements, sea-ice and snow thickness as well as freeboard were measured. All times are given in UTC, the position along the profiles is given as an x-coordinate in distance [m].This project was based on the initiative and support of Sebastian Gerland (Norwegian Polar Institute, Tromsø) and Hajo Eicken (University of Alaska Fairbanks, UAF). We are most grateful for the exceptional support of field measurements by Dirk Kalmbach and Polona Itkin (both Alfred Wegener Institute, Bremerhaven, Germany, AWI), and Joshua Jones (UAF). Jennifer Peussner and Christian Katlein (both AWI) helped with the analysis of spectral data. Mette Kaufmann (UAF) kindly performed laboratory analyses of field samples. Data acquisition of the sea ice observatory at Barrow was funded under National Science Foundation grant no. OPP-0856867 (SIZONET). Data set preparation was supported by a German-Norwegian exchange program financed by the German-Norwegian bilateral research program of the Norwegian Research Council (NFR) (DAADppp program, grant 199844, and NFR grants 196143/S30 and 197236/V30).

Supplement to: Nicolaus, Marcel; Petrich, Chris; Hudson, Stephen R; Granskog, Mats A (2013): Variability of light transmission through Arctic land-fast sea ice during spring. The Cryosphere, 7(3), 977-986