Enhanced spectral UV irradiance: a 1 year preliminary study

The ultraviolet (UV) spectra on cloudy days were compared to those on cloud free days to determine which part of the UV spectrum has the greatest enhancement due to the cloud compared to both corresponding measured clear sky spectra as well as other enhanced spectra. In this preliminary study, cloud enhanced UV spectra selected for maximum UVA enhancement compared to a clear sky UV spectrum at similar solar zenith angle (SZA) and ozone values, showed that the ratio of the two sets of spectral irradiances was approximately wavelength independent (approximately 1.1) above the cut-off wavelength of approximately 306 nm. Similarly, above 306 nm the average ratio of the spectral irradiances of a maximum UVB enhanced UV spectrum compared to a clear sky spectrum was 1.2 with maximum values generally above this average between 316 and 344 nm and generally below 1.2 above the wavelength of 344 nm. The UVA and UVB enhanced spectra were separated into five SZA ranges and the irradiance at each wavelength averaged for each range and compared to clear sky spectra in each of the ranges. Above approximately 306 nm, the ratios are wavelength independent for all SZA. However, with the exception of the SZA range centred on 20°, there is an increasing dependency with shorter wavelengths below the 306 nm. Also there appears to be two distinct groupings of the average irradiance ratios, corresponding to the SZA range centred on 20°, 37° and 49° (ratio of 1.2) and 32° and 42° (ratio 1.0), the latter cases suggesting that on average there is no enhancement for these SZA, except for wavelengths less than 306 nm.