PNNL

Abstract

In Lauder, Central Otago, two all-sky imaging systems have been operated for more than one year measuring the total, opaque, and thin cloud fraction as well as an indicator of whether the sun is obscured by clouds. The data provide a basis for investigating the impact of clouds on the surface radiation field. We aligned the all-sky cloud parameters with measurements of global, direct and diffuse surface solar irradiance over the spectral interval from 0.3 to 3 mm. Here we describe results of ongoing analysis of this data set. 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 Earth-Sun distance. The function is derived from a least-square fit to measurements taken when available cloud images show clear sky situations. Averaged over a longer time period, such as a 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 July 2000 to June 2001. In the present work we analyse reductions in the surface irradiance and also situations where clouds cause radiation values to exceed the expected clear sky amount. Over one year of observations, 1- minute-average 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, as 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 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.