Ground-based measurements of the weather-driven sky radiance distribution in the Southern Hemisphere

The angular distribution of the sky radiance determines the energy generation of solar power technologies as well as the ultraviolet (UV) doses delivered to the biosphere. The sky-diffuse radiance distribution depends on the wavelength, the solar elevation, and the atmospheric conditions. Here, we report on ground-based measurements of the all-sky radiance at three sites in the Southern Hemisphere across a transect of about 5,000 km: Santiago (33°S, a mid-latitude city of 6 million inhabitants with endemic poor air quality), King George Island (62°S, at the northern tip of the Antarctic Peninsula, one of the cloudiest regions on Earth), and Union Glacier (79°S, a snow-covered glacier in the vast interior of Western Antarctica). The sites were strategically selected for studying the influence of urban aerosols, frequent and thick clouds, and extremely high albedo on the sky-diffuse radiance distribution. Our results show that, due to changing site-specific atmospheric conditions, the characterization of the weather-driven sky radiance distribution may require ground-based measurements.

Downwelling and upwelling radiance distributions sampled under cloudless conditions in Antarctica

We have sampled both the downwelling and upwelling radiance distributions at a camp located in the southern Ellsworth Mountains on the broad expanse of Union Glacier (700 m altitude, 79° 46' S, 82° 52' W). The measurements (at 320-440 nm wavelength range) were carried out under cloudless conditions by using a sky scanner system, during a campaign (in December, 2012) meant to assess the effects of the high albedo on the radiance distribution. The angular variations observed in both the downwelling and upwelling radiance distributions increase with the wavelength. However, these variations were considerably greater in the case of the downwelling radiance than in the case of the upwelling radiance. Indeed, we found that downwelling radiance tends to be less isotropic than the corresponding upwelling radiance. Regardless of the solar zenith angle and the wavelength, the minima of the downwelling and the upwelling radiance distributions were measured close to the zenith and to the nadir, respectively. The downwelling (upwelling) radiance increased nearly monotonically toward the horizon and peaked at zenith (nadir) angles that ranged from 75° to 90°. Comparisons with the UVSPEC radiative transfer model were used to weight up the response of the downwelling radiance distribution to changes in the albedo.

Spectral UV radiance measured at a coastal site: a case study

We have sampled the spatial distribution of the UV radiation (i.e. the UV radiance) at a station located on the southern pacific coastline (Valparaiso, Chile, 33.03°S-71.58°W). The site is characterized by the partial horizon obstruction (due to the surrounding topography). Our spectral measurements were carried out over the period January-March 2012 and were meant to weigh up the effects of the local cloudiness, the heterogeneous albedo, and the horizon obscuration. We found that a nearly translucent overcast sky affects the radiance distribution such that from its maximum (measured close to the solar zenith angle) the radiance is monotonically decreasing towards the horizon. Under cloudless conditions, the radiance distribution becomes less isotropic with the wavelength; we detected spatial variations in the distribution of radiation up to a factor of 5 at 320 nm, and up to a factor of 9 at 400 nm. We also observed that radiances measured at points over the sea are greater than those measured at the corresponding point over the land; we partially attributed this effect to the spatial variations in the albedo. Moreover, we found that the horizon obscuration leads to significant reductions in the radiance at points on the blocked horizon; these reductions range from 60% (at 400 nm) to 80% (at 300 nm). Methodological details are provided below.

Measuring high-resolution sky luminance distributions with a CCD camera

We describe how sky luminance can be derived from a newly developed hemispherical sky imager (HSI) system. The system contains a commercial compact charge coupled device (CCD) camera equipped with a fish-eye lens. The projection of the camera system has been found to be nearly equidistant. The luminance from the high dynamic range images has been calculated and then validated with luminance data measured by a CCD array spectroradiometer. The deviation between both datasets is less than 10% for cloudless and completely overcast skies, and differs by no more than 20% for all sky conditions. The global illuminance derived from the HSI pictures deviates by less than 5% and 20% under cloudless and cloudy skies for solar zenith angles less than 80°, respectively. This system is therefore capable of measuring sky luminance with the high spatial and temporal resolution of more than a million pixels and every 20 s respectively.