Cloud cover measured by satellite and from the ground: relationship to UV radiation at the surface

Clouds are important modulators of UV radiation, but quantification of their effects has been limited. This preliminary study assesses satellite-derived fractional cloud cover as an estimator of cloud effects on UV radiation measured at Lauder, New Zealand. Although there is a good correlation between measures of cloud cover from satellite and from the ground, their correlation with surface irradiance in either the visible or UV parts of the spectrum is found to be weaker. Measurement of fractional cloud cover alone is inadequate to parameterize cloud effects on incoming radiation. In particular, knowledge of whether or not the Sun is obscured is required, while estimates of cloud optical depth also appear to be important. The strong correlation between UV and visible radiation suggests that the use of ground-based pyranometer data to infer cloud effects on UV radiation is more accurate than the use of presently available satellite data for process studies of the relationship between cloud cover and instantaneous UV measurements at a particular location.