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Recent variations in surface solar radiation (SSR) at the beginning of the 21st century (2000-2007) were determined at scales ranging from local/regional to hemispherical/global, on the basis of radiative transfer computations and information from satellites, reanalyses and surface measurements. Under all-sky conditions, in the Northern Hemisphere (NH) there is no clear dimming/brightening signal after 2000, whereas in the SH there is a more clear dimming arising from both increasing clouds and aerosols. Dimming is observed over land and ocean in the Southern Hemisphere (SH), and over oceans in the NH, whereas a slight brightening occurred over NH land. However, opposite tendencies are found even within the same continent, indicating the need to assess SSR changes at regional/local scales apart from hemispherical/global ones. Copyright (c) 2011 Royal Meteorological Society Atmospheric Science Letters

The spatial and temporal distribution of the downward solar radiation (DSR) at the Earth's surface, which is a key parameter for the Earth-atmosphere climate system, is studied on a global scale by applying the multivariate statistical method of factor analysis (FA) on mean monthly DSR values for the period 1984-2000 for winter and summer. DSR fluxes have been computed with a physical deterministic radiation transfer model, which uses satellite and reanalysis climatological input data. FA objectively groups grid points with common temporal variability of solar radiation, identifying areas with characteristic solar radiation variability and revealing teleconnection patterns. The globally distributed DSR exhibits strong variability and can be represented by about 30 factors (sub-areas) explaining approximately 85% of the total variance. The main factors are located in the tropical Pacific (El Nino Southern Oscillation), the northern Pacific (Aleutian low), desert areas (Africa, Middle East, and Australia), and oceanic areas (storm track zone around 60 S). Furthermore, some of these areas are teleconnected indicating common DSR variability in remote places of the Earth. The primary physical parameter for the co-variability of surface solar radiation is found to be cloud cover. The time series of the factor scores (solar radiation) exhibit features associated with natural climatic phenomena (e. g. NAO, ENSO), thus revealing that DSR might be considered as an indirect indicator for these phenomena. On the other hand, DSR determines these phenomena through its effects on major climate parameters such as surface temperature or evaporation, thus playing a key role in their formation and explanation. Finally, our analysis revealed some statistically significant trends of solar radiation, for example, a reduction in the northern Pacific and an increase in tropical and subtropical regions, which are important for global dimming/brightening and associated surface warming of the Earth. Copyright (C) 2009 Royal Meteorological Society International Journal of Climatology

The second generation Collection 005 (C005) MODIS operational algorithm for retrieval of aerosol properties was evaluated and validated for the greater Mediterranean basin (29.5 degrees N-46.5 degrees N and 10.5 degrees W-38.5 degrees E), a region with an atmosphere under siege by air pollution and diminishing water resources that are exacerbated by high aerosol loads and climatic change. The present study aims to quantify the differences between the C005 and the previous (C004) MODIS collections, and re-assess the results of previous studies that have been performed for the region using MODIS C004 aerosol optical depth (AOD) products. Daily data of AOD from EOS-Terra covering the 6-year period 2000-2006 were taken from both C005 and C004 Level-3 datasets, and were inter-compared and validated against ground-based measurements from 29 AERONET stations. The C005 data were found to significantly better agree with the AERONET data than those of C004. The correlation coefficient between MODIS and AERONET was found to increase from 0.66 to 0.76 and the slope of linear regression MODIS/AERONET from 0.79 to 0.85. The MODIS C005 data still overestimate/underestimate the AERONET AOD values smaller/larger than 0.25, but to a much smaller extent than C004 data. The better agreement of C005 with AERONET data arises from the generally lower C005 values, with regional mean AOD values equal to 0.27 and 0.22 for C004 and C005, respectively. This decrease, however, is not uniform over the region and involves a significant decrease over land and a small increase over the ocean for AOD values greater than 0.1 (opposite changes were found under aerosol-clean conditions). Both data sets indicate a decrease in the regional mean AOD over the period 2000-2006, equal to 20% based on C005 and 17% based on C004 datasets, though the intra-annual and inter-annual variation did not change significantly, thus indicating a systematic correction to C004 values. Atmospheric Chemistry and Physics

The temporal variability of aerosol optical properties is investigated over the broader Mediterranean basin, with emphasis on aerosol optical depth ( AOD) that is an effective measure of aerosol load. The study is performed using Collection 005 Level-3 mean daily spectral aerosol data from the Moderate Resolution Imaging Spectroradiometer ( MODIS) instrument on board the Terra and Aqua satellites, which cover the 6-year period from 2000 to 2006. The results of our analysis reveal a significant interannual variability of AOD in the study region. Specifically, the regional mean visible AOD over land and ocean has decreased over the period 2000-2006 by 20% in relative percentage terms ( or by 0.04 in absolute terms). This tendency is statistically significant according to the Man-Kendall test. However, the decreasing tendency of AOD is not uniform over the whole basin. It appears mainly in the western parts of Iberian, Italian, and Balkan peninsulas ( and coastal areas), as well as in the southern Anatolian peninsula. The analysis for summer ( June to September) and winter ( November to March) seasons revealed different tendencies in both AOD and precipitation. The summer-period AOD has decreased by 0.04 ( or by 14%) probably due to decreased emission rates of anthropogenic pollution. In contrast, the winter AOD has increased by 0.03 ( or 19%) mainly related to decreased precipitation ( associated with an increasing tendency in the NAO index). The decreasing tendency in MODIS AOD is in good agreement with corresponding AOD tendencies based on data from Aerobot Robotic Network ( AERONET) stations in the study region and ground based PM10 measurements at selected stations. Journal of Geophysical Research-Atmospheres