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Correcting spaceborne reflectivity measurements for application in solar ultraviolet radiation levels calculations at ground level

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Author(s): P. N. den Outer | A. van Dijk | H. Slaper | A. V. Lindfors | H. De Backer | A. F. Bais | U. Feister | T. Koskela | W. Josefsson

Journal: Atmospheric Measurement Techniques Discussions
ISSN 1867-8610

Volume: 5;
Issue: 1;
Start page: 61;
Date: 2012;
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ABSTRACT
The Lambertian Equivalent Reflection (LER) produced by satellite-carried instruments is used to determine cloud effects on ground level UltraViolet (UV) radiation. The focus is on data use from consecutive operating instruments: the 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 Aura since 2004. The LER data produced by TOMS on Earth Probe is only included until 2002. The possibility to use the Radiative Cloud Fraction (RCF)-product of OMI is also investigated. 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 LER data are compared with measurements of UV irradiances at eight European low elevation stations. The LER data set of the two TOMS instruments shows a consistent agreement, and the required corrections are of low percentage i.e. 2–3%. In contrast, the LER data 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 RCF product of OMI requires a large correction but can then be implemented as a cloud effect proxy. However, a major drawback of RCF is the large number of clipped data, i.e. 18%, and results are not better than those obtained with the corrected LER product of OMI. The average reduction of UV radiation due to clouds for all sites together indicate a small trend: a diminishing cloudiness, in line with ground-based UV observations. Uncorrected implementation of LER would have indicated the opposite. An optimal field of view of 1.25° was established for LER data to calculate UV radiations levels. The corresponding area can be traversed within 5–7 h at the average wind speeds found for the West European continent.
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