Enhancement of solar and ultraviolet surface irradiance under partial cloudy conditions

Clouds are important for the transfer of solar radiation to the Earth's surface. In this study, the enhancements of the global irradiance due to the contributions of the direct and diffuse components of the solar radiation are investigated using the 10 seconds irradiance measurements of De Bilt (52.100 N, -5.183) during 1995 and 1996, which consist of the global, direct and diffuse surface irradiance of the total solar spectrum (300-3000 m) and the global and direct surface irradiance of the UV-A (367 nm with FWHM=10 m) and UV-B (306 nm with FWHM=3 m) spectral regions. The dataset is completed with hourly standard meteorological observations, including cloud coverage, cloud base height, cloud type, horizontal visibility etc. With use of a constructed cloud-free reference as a function of solar zenith angle, enhancements of 73 %, 50 % and 60 % are calculated for the total solar spectrum, UV-A and UV-B, respectively.

Most of the enhancements are observed when cumulus-like clouds are present. We find that enhancements are most likely within the cloud coverage (N) and the cloud base height (h) combinations: 0.125<N<0.625 with 100 m<h<900 m and 0.625<NsI with 900 m<h<1500 We show under what conditions the highest irradiances are found in terms of cloud coverage, cloud base height, cloud type and solar zenith angle.

Two models are constructed in which cloud coverage is derived in 10 minute intervals from the irradiance data. One model uses both direct and global irradiance of the total solar spectrum and the other uses only the global irradiance of the total solar spectrum. The derived cloud coverage data are succesfully compared with the hourly available observations of cloud coverage.