Applying spaceborne reflectivity measurements for calculation of the solar ultraviolet radiation at ground level

Abstract

Abstract. Long-term analysis of cloud effects on ultraviolet (UV) radiation on the ground using spaceborne observations requires the use of instruments that have operated consecutively. The longest data record can be built from the reflectivity measurements produced by the instruments Total Ozone Mapping Spectrometers (TOMS) flown on Nimbus 7 from 1979 to 1992, TOMS on Earth Probe from 1996 to 2005, and the Ozone Monitoring Instrument (OMI) flown on EOS Aura since 2004. The reflectivity data produced by TOMS on Earth Probe is only included until 2002. A comparison is made with cloud effects inferred from ground-based pyranometer measurements at over 83 World Radiation Data Centre stations. Modelled UV irradiances utilizing the standard reflectivity are compared with measurements of UV irradiances at eight European low-elevation stations. The reflectivity data of the two TOMS instruments shows a consistent agreement, and the required corrections are of low percentage, i.e. 2-3 %. In contrast, the reflectivity product of OMI requires correction of 7-10 %, and a solar angle dependency therein is more pronounced. These corrections were inferred from a comparison with pyranometer data, and tested using the UV measurements. The average reduction of UV radiation due to clouds for all sites together indicates a small trend: a diminishing cloudiness, in line with ground-based UV observations. Uncorrected implementation of the reflectivity data would have indicated the opposite.An optimal area was established for reflectivity data for the calculation of daily sums of UV radiation. It measures approximately 1.25 • in latitudinal direction for square-shaped areas overhead the ground-based UV stations. Such an area can be traversed within 5 to 7 h at the average wind speeds found for the West European continent.

Annual anomalies and trends for TOMS reflectivities (1978–2005) in the Southern Hemisphere

“…The data's geographical spacing for the monthly means matched the one for daily data. Some issues related to the degradation of the scan mirror (EP) and to photomultiplier detector hysteresis effects (N7) (Herman et al 2013) do not seem to compromise the use of LER values as a cloudiness proxy, since the two databases are consistent with groundbased measurements (Den Outer et al 2012;Damiani et al 2014).…”

“…LER is a quantitative measure of the total amount of energy reflected from the Earth back to space in sunlight. It represents the average reflectivity of a combination scene that includes the contribution from clouds, the Earth's surface and airborne aerosols, neglecting multiple Rayleigh scattering effects, and under the assumption that all the reflecting surfaces behave as an ideal Lambertian reflector (Labow et al 2011;Den Outer et al 2012).…”