Cloud Coverage Based on All-Sky Imaging and Its Impact on Surface Solar Irradiance

In Lauder, Central Otago, New Zealand, two all-sky imaging systems have been in operation for more than 1 yr, measuring the total, opaque, and thin cloud fraction, as well as indicating whether the sun is obscured by clouds. The data provide a basis for investigating the impact of clouds on the surface radiation field. The all-sky cloud parameters were combined with measurements of global, direct, and diffuse surface solar irradiance over the spectral interval from 0.3 to 3 μm. Here, the results of ongoing analysis of this dataset are described. As a reference for the magnitude of the cloud influence, clear-sky irradiance values are estimated as a simple function of solar zenith angle and the earth–sun distance. The function is derived from a least squares fit to measurements taken when available cloud images show clear-sky situations. Averaged over a longer time period, such as 1 month, cloud fraction and surface irradiance are clearly negatively correlated. Monthly means in the ratio of the measured surface irradiance to the clear-sky value had a correlation coefficient of about −0.9 with means of cloud fraction for the months from July 2000 to June 2001. In the present work reductions in the surface irradiance and situations in which clouds cause radiation values to exceed the expected clear-sky amount are analyzed. Over 1 yr of observations, 1-min-averaged radiation measurements exceeding the expected clear-sky value by more than 10% were observed with a frequency of 5%. In contrast, a reduction of more than 10% below estimated clear-sky values occurred in 66% of the cases, while clear-sky irradiances (measured irradiance within ±10% of estimated clear-sky value) were observed 29% of the time. Low cloud fractions frequently lead to moderate enhancement, because the sun is often unobscured and the clouds are brighter than the sky that they hide. As cloud fraction increases the sun is likely to be obscured, causing irradiance values to fall well below clear-sky values. However, in the case of unobscured sun, there is a tendency for strongest enhancements when cloud fractions are highest. Enhancements, especially at high solar zenith angle, are also often observed in association with thin clouds.