The performance of the cosine error correction method used at the Finnish Meteorological Institute (FMI) for correcting spectral UV measurements of the Brewer spectroradiometer (Brewer) was studied. An instrument specific cosine error correction has to be applied due to the non ideal angular response of the Brewer. The correction depends on the actual sky radiation distribution, which can change even during one spectral scan due to rapid changes in cloudiness. The method has 5 been developed to take into account such changes and derive a correction coefficient for each measured wavelength.
Measurements of five Brewers were corrected using the method and the results were compared to a travel reference spectroradiometer (QASUME). Measurements were performed during the RBCC-E (Regional Brewer Calibration Center – Europe) X Campaign held at El Arenosillo, Huelva (37°N, 7°W), Spain, in 2015. In addition, results of site audits of FMI’s Brewers in Sodankylä (67°N, 23°W) and Jokioinen (61°N, 24°W) during 2002–2014 were studied. The results showed that the spectral cosine error 10 correction varied between 4 to 14%, and the differences between the QASUME and the Brewers diminished by up to 10%.
The study showed that the method, originally developed for measurements made at high latitudes, can be used at midlatitudes as well. It also showed that the method is applicable to other Brewers as far as required input parameters, i.e., total ozone, information from aerosols, albedo, instrument specific angular response and slit function, are known.
Lakkala, K., Arola, A., Gröbner, J., León-Luis, S.F., Redondas, A., Kazadzis, S., Karppinen, T., Karhu, J.M., Egli, L., Heikkilä, A. and Koskela, T., Performance of the FMI cosine error correction method for the Brewer spectral UV measurements.
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