The space-time variability of solar radiation maximum in Baltic Region

The study of the space-time variability of solar radiation incident on the surface of the Earth is of particular importance due to the climate change. 

This study provides analysis results of the maximum daily totals of global (Q) and diffuse (D) solar radiation, observed at 30 radiometric stations in the Baltic region: in Denmark, Germany, Latvia, Poland, Russia, Finland, Sweden and Estonia. To define the maximum values of solar radiation incident on the surface of the Earth the climatological series of global radiation for 1964-2023, as well as diffuse solar radiation for 1988-2023 from the World Radiation Data Center (WRDC) were used.

The long-term distribution of maximum values of solar radiation at the stations showed the presence of two periods of the highest recurrence: from 1964 to the early 1980s and from 2000s up to the present time.

To estimate the long-term changes of maximum values of solar radiation statistical methods of analysis were used. The 95 percent probability of the ratios of maximum daily values of Q and D ​​to solar irradiance at the top of the atmosphere was calculated for two periods: global radiation for 1964-2000 and 1964-2023, as well as diffuse radiation for 1988-2004 and 1988-2023.

The comparative analysis of the estimates of maximum solar radiation values ​​obtained for two periods shows their increase from the late 1990s or the early 2000s at most radiometric stations. The highest increase of maximum values of Q is observed from November to March - by 3-6%, at stations in Poland and in the south of Sweden up to 8%. From October to March maximum values of D increased by 5-10%, at some stations in Finland up to 15%. From May to September the changes of maximum values of Q and D are not significant in the Baltic region, a slight increase of 1-3%.

The presented results are consistent with the previously conducted studies of changes of average monthly values ​​of solar radiation measured at European stations. The observed changes of solar radiation incident on the surface of the Earth result in the conclusion that its increase in the last 20-25 years is associated not only with an increase of the direct solar radiation, but also with an increase of the diffuse component.

The obtained estimates of the maximum values ​​of Q and D used as climatic limits when conducting quality control of radiometric data in the WRDC. The solar radiation data received recently by the WRDC made it possible to refine the quality criteria.