We evaluate the accuracy of the satellite-based surface solar radiation dataset called Surface Solar Radiation Data Set-Heliosat (SARAH-E) against in situ measurements over a variety of sites in India between 1999 and 2014. We primarily evaluate the daily means of surface solar radiation.The results indicate that SARAH-E consistently overestimates surface solar radiation, with a mean bias of 21.9 W/m 2 . The results are complicated by the fact that the estimation bias is stable between 1999 and 2009 with a mean of 19.6 W/m 2 but increases sharply thereafter as a result of rapidly decreasing (dimming) surface measurements of solar radiation. In addition, between 1999 and 2009, both in situ measurements and SARAH-E estimates described a statistically significant (at 95% confidence interval) trend of approximately −0.6 W/m 2 /year, but diverged strongly afterward. We investigated the cause of decreasing solar radiation at one site (Pune) by simulating clear-sky irradiance with local measurements of water vapor and aerosols as input to a radiative transfer model. The relationship between simulated and measured irradiance appeared to change post-2009, indicating that measured changes in the clear-sky aerosol loading are not sufficient to explain the rapid dimming in measured total irradiance. Besides instrumentation biases, possible explanations in the diverging measurements and retrievals of solar radiation may be found in the aerosol climatology used for SARAH-E generation. However, at present, we have insufficient data to conclusively identify the cause of the increasing retrieval bias. Users of the datasets are advised to be aware of the increasing bias when using the post-2009 data.called Surface Solar Radiation Data Set-Heliosat, Meteosat-East (SARAH-E), Edition 1 [4,5], is a satellite-based climate data record of the solar radiative quantities over India and surrounding regions. The dataset is derived from observations of the reflected solar radiation from the geostationary Meteosat Indian Ocean Data Coverage (IODC) satellites. The dataset spans 15 years at a spatial resolution of 0.05 • .In addition to SARAH-E, there exists a growing number of satellite-derived datasets of SSR, calculated with a variety of algorithms for both regional [6,7] and global coverage [8][9][10]. A multitude of studies also exists on the validation of said datasets (e.g., [11][12][13]), but to our knowledge, satellite-based SSR estimates, including SARAH-E, have not been validated over India at the sub-continental scale. Therefore, to facilitate the use of this satellite-based dataset, we must be able to ascertain its accuracy in determining SSR, its seasonal cycle and any possible trends. The goal of this paper is to provide this information, specifically answering these research questions:
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