Cloud climatology from visual observations at Sao Paulo, Brazil

摘要

Clouds remain as the main uncertainty source in climate studies. Specifically, over urban and other polluted regions, where their properties are subject to anthropic influence, more studies are needed. This study focuses on clouds at the southeast of Brazil, especially on the diurnal and annual cycles, trends of cloud amount and cloud occurrence in one of the biggest metropolitan areas of the world, the Metropolitan Area of Sao Paulo (MASP). For this purpose, 59 years of visual observations of cloud amount and type, made from surface at MASP, are analysed. In addition, trends of total cloud amount in the state of Sao Paulo, Brazil, are estimated using 30 years of total cloud amount, adding nine synoptic meteorological stations. To assess the reliability of the visual observations, annual-average time series of cloud amount are compared with ones from the International Satellite Cloud Climatology Project (ISCCP) and solar irradiation reaching the surface. The results show that the diurnal cycle of total cloud amount at MASP is highly influenced by low clouds, especially stratiform clouds, which are the predominant cloud type. Moreover, the seasonality of the diurnal cycles was also analysed, with higher amplitude and late-afternoon occurrence in spring-summer. In winter-autumn, the maxima of cloud amount diurnal cycles tends to occur at early morning with lower amplitude. The low cloud amount at MASP has undergone an increase, especially in the last 30 years, evidenced when compared annual-average time series of visual observations and solar irradiation. In contrast, a decrease of total cloud amount in the state of Sao Paulo, mainly since the beginning of 1990s, is observed in ISCCP and visual observations databases.