Fifty-six years of Surface Solar Radiation and Sunshine Duration at the Surface in São Paulo, Brazil: 1961–2016

Abstract

Abstract. Fifty-six years (1961–2016) of daily surface downward solar irradiation, sunshine duration, diurnal temperature range and the fraction of the sky covered by clouds in the city of São Paulo, Brazil, were analyzed. The main purpose was to contribute to the characterization and understanding of the dimming and brightening effects on solar global radiation in this part of South America. As observed in most of the previous studies worldwide, in this study, during the period between 1961 up to the early 1980's, more specifically up to 1983, a negative trend in surface solar irradiation was detected in São Paulo, characterizing the occurrence of a dimming effect. A similar behavior, a negative trend, was also observed for sunshine duration and the diurnal temperature range, the three variables in opposition to the trend in the sky cover fraction. However, a brightening effect, as observed in western industrialized countries in more recent years, was not observed. Instead, for surface downward irradiation, the negative trend persisted and still in consonance to the cloud cover fraction increasing trend. The trends for sunshine duration and the diurnal temperature range, by contrast, changed signal. Some possible causes for the discrepancy were discussed, such as the frequency of fog occurrence, urban heat island effects, aerosol changes and greenhouse gas concentration increase. Future studies on aerosol effect are encouraged, particularly with higher temporal resolution as well as modeling studies, to better analyze the contribution of each possible causes.