Solar irradiance in the heterogeneous albedo environment of the Arctic coast: measurements and a 3-D model study

Abstract. We present a unique case study of the solar global irradiance in a highly heterogeneous albedo environment at the Arctic coast. Diodearray spectroradiometers were deployed at three sites around Ny Ålesund, Svalbard, and spectral irradiances were simultaneously measured under clear-sky conditions during a 24 h period. The 3-D radiative transfer model MYSTIC is applied to simulate the measurements in various model scenarios. First, we model the effective albedos of ocean and snow and consequently around each measurement site. The effective albedos at 340 nm increase from 0.57 to 0.75, from the coastal site in the west towards the site 20 km east, away from the coast. The observed ratios of the global irradiance indicate a 15% higher average irradiance, at 340 nm east relative to west, due to the higher albedo. The comparison of our model scenarios suggest a snow albedo of > 0.9 and confirm the observation that drift ice has moved into the Fjord during the day. The local time shift between the locations causes a hysteresis-like behavior of these east–west ratios with solar zenith angle (SZA). The observed hysteresis, however, is larger and, at 340 nm, can be explained by the drift ice. At 500 nm, a plausible explanation is a detector tilt of about 1°. The ratios between afternoon and morning irradiances at the same SZA are investigated, which confirm the above conclusions. At the coastal site, the measured irradiance is significantly higher in the afternoon than in the morning. Besides the effect of changing drift ice and detector tilt, the small variations of the aerosol optical depth have to be considered also at the other stations to reduce the discrepancies between model and observations. Remaining discrepancies are possibly due to distant high clouds.