Trends in solar radiation in NCEP/NCAR database and measurements in northeastern Brazil
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The database from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis project available for the period from 1948 to 2009 was used for obtaining long-term solar radiation for northeastern Brazil. Measurements of global solar radiation (R s ) from data collection platform (DCP) for four climatic zones of northeastern Brazil were compared to the re-analysis data. Applying cluster analysis to R s from database, homogeneous sub-regions in northeastern Brazil were determined. Long times series of R s and sunshine duration measurements data for two sites, Petrolina (0909'S, 4022'W) and Juazeiro (0924'S, 4026'W), exceeding 30. years, were analyzed. In order to exclude the decadal variations which are linked to the Pacific Decadal Oscillation, high-frequency cycles in the solar radiation and sunshine duration time series were eliminated by using a 14-year moving average, and the Mann-Kendall test was employed to assess the long-term variability of re-analysis and measured solar radiation. This study provides an overview of the decrease in solar radiation in a large area, which can be attributed to the global dimming effect. The global solar radiation obtained from the NCEP/NCAR re-analysis data overestimate that obtained from DCP measurements by 1.6% to 18.6%. Results show that there is a notable symmetry between R s from the re-analysis data and sunshine duration measurements. 2010 Elsevier Ltd.