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 satel lite-derived fractional cloud cover as an estimator of cloud effects o n UV radiation measured at Lauder, New Zealand. Although there is a go od correlation between measures of cloud cover from satellite and from the ground, their correlation with surface irradiance in either the v isible or UV parts of the spectrum is found to be weaker. Measurement of fractional cloud cover alone is inadequate to parameterize cloud ef fects on incoming radiation. In particular, knowledge of whether or no t the Sun is obscured is required, while estimates of cloud optical de pth also appear to be important. The strong correlation between UV and visible radiation suggests that the use of ground-based pyranometer d ata to infer cloud effects on UV radiation is more accurate than the u se of presently available satellite data for process studies of the re lationship between cloud cover and instantaneous UV measurements at a particular location.