An Anisotropy Correction Method for All-Sky Measurements of Diffuse UV-B Erythemal Irradiance

Abstract

Occulting disk or shadowband arrangements are often applied to both spectral and broadband sensors in order to retrieve the total diffuse irradiance. However, there is difficulty in the application of a suitable diffuse correction due to the instrument cosine response. Generally, the sky radiance distribution is assumed isotropic during the correction, although it has been shown in most studies to be nearly always anisotropic. This study presents a method for using a measurement of the actual anisotropy of the UV-B sky in order to derive correction factors to account for the integral of this anisotropy and the nonideal cosine functions in the response of instruments; the method being applicable when the instruments carry out synchronous spectral or broadband measurements. In situ erythemal sky radiance distribution data are obtained from a variable sky-view platform (VSP). The VSP consists of a Solar Light Company 501-A UV biometer that moves up and down within a vertically aligned fixed shading cylinder, therefore causing the sky-view factor to vary in discrete steps. VSP output is used to correct for the cosine error of instruments that measure diffuse erythemal radiation data (less than 400 nm), for both clear-sky and cloudy conditions. Results show that an isotropic diffuse correction overestimates the real diffuse signal by a maximum of 4%-6% for clear skies, with this magnitude decreasing with an increase in solar zenith angle (SZA). This is in accordance with the increase in sky isotropy at large SZAs. In contrast, the overestimate at small to mid-SZAs for stratocumulus and cumulus cloud conditions ranges from approximately 2%-5% for moderate cloud cover to 0%-10% for high cloud cover. Here, the overestimate variability can be significant at large SZAs. Results under cloudy skies are affected by both variability in cloud sky distribution and VSP signal error resulting from loss of sensitivity. Overall, it is expected that cloud distribution masks the effect of increased sky isotropy.