Abstract
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.
Highlights
The reflectivity of the Earth’s surface, the albedo, is a significant factor in the global radiation budget
Since some of the input parameters are associated with some uncertainty, we model multiple scenarios considering the range of plausible values for these parameters
Global irradiance spectra were recorded over the complete day from 00:00 to 24:00 UTC simultaneously at the three locations west, center and east
Summary
The reflectivity of the Earth’s surface, the albedo, is a significant factor in the global radiation budget. Global irradiances were measured with three broadband UV radiometers at the Antarctic coast line under variable overcast conditions, which obscures conclusions about the coastal effect on the irradiance (Lubin et al, 2002) Another relevant measurement campaign was conducted around the Salar de Uyuni on the Bolivian Altiplano where a reflective dry salt lake creates a high-albedo transition. In a detailed 3-D modeling study, Ricchiazzi and Gautier (1998) have investigated the Antarctic coast and have shown how the irradiances are affected by the albedo of distant regions and how this “region of interest” depends on atmospheric conditions. The motivation for a field campaign at the Arctic coast was to contribute a new multidimensional data set: simultaneous spectra at three locations over the course a cloudless day on the peninsula around Ny Ålesund, Svalbard, to compare the spectral, temporal and spatial variation of the global irradiance with 3-D model simulations. By considering the direct contribution to the global irradiance (given below in section 3.2), the expected maximum uncertainty for each instrument is 1.5 and 1 % for the global irradiance at 70◦and 80◦ SZA, respectively, for 500 nm and < 1 % for 340 nm
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