Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean basin or land stations in the region from multi-year ground-based AERONET observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angström Exponent (AAE) dataset is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This dataset covers the 17-yr period 1996–2012 with most data being from 2003–2011 (~89% of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm > 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent < 1.0 in order to study absorption by carbonaceous aerosols. The SSA dataset includes AERONET level-2 products. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 (resp. 0.040 ± 0.01) and 0.050 ± 0.01 (0.055 ± 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear "moderately" absorbing with values of SSA close to ~0.94–0.95 ± 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA ~0.89–0.90 ± 0.04). The aerosol Absorption Angström Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate a possible East-West gradient, with higher values over the eastern basin (AAE_East = 1.39/AAE_West = 1.33). The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols

Abstract. Aerosols affect the Earth's energy budget ''directly'' by scattering and absorbing radiation and ''indirectly'' by acting as cloud condensation nuclei and, thereby, affecting cloud properties. However, large uncertainties exist in current estimates of aerosol forcing because of incomplete knowledge concerning the distribution and the physical and chemical properties of aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing from largely model-based to increasingly measurement-based. Here we assess the aerosol optical depth, direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements supplemented by global chemical transport model (CTM) simulations. The multi-spectral MODIS measures global distributions of aerosol optical thickness (τ) on a daily scale, with a high accuracy of ±0.03±0.05τ over ocean. The annual average τ is about 0.14 over global ocean, of which about 21% is contributed by human activities, as determined by MODIS fine-mode fraction. The multi-angle MISR derives an annual average AOT of 0.23 over global land with an uncertainty of ~20% or ± 0.05. These high-accuracy aerosol products and broadband flux measurements from CERES make it feasible to obtain observational constraints for the aerosol direct effect, especially over global ocean. A number of measurement-based approaches estimate the clear-sky DRE (on solar radiation) at the top-of-atmosphere (TOA) to be about −5.5±0.2 Wm−2 (median ± standard error) over global ocean. Accounting for thin cirrus contamination of the satellite derived aerosol field will reduce the TOA DRE to −5.0 Wm−2. Because of a lack of measurements of aerosol absorption and difficulty in characterizing land surface reflection, estimates of DRE over land and at the ocean surface are currently realized through a combination of satellite retrievals, surface measurements, and model simulations, and are less constrained. Over the ocean surface, the DRE is estimated to be −8.8±0.4 Wm-2. Over land, an integration of satellite retrievals and model simulations derives a DRE of −4.9±0.7 Wm−2 and −11.8±1.9 Wm−2 at the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that on average are smaller than the measurement-based DRE by about 30–40%, even after accounting for thin cirrus and cloud contamination. Despite these achievements, a number of issues remain open and more efforts are required to address them. Current estimates of the aerosol direct effect over land are poorly constrained. Uncertainties of DRE estimates are also larger on regional scales than on a global scale and large discrepancies exist between different approaches. The characterization of aerosol absorption and vertical distribution remains challenging. The aerosol direct effect in the thermal infrared range and under cloudy condition remains relatively unexplored and quite uncertain, because of a lack of global systematic aerosol vertical profile measurements. A coordinated research strategy needs to be developed for integration and assimilation of satellite measurements into models to constrain model simulations. Hopefully, enhanced measurement capabilities in the next few years and high-level scientific cooperation, will further advance our knowledge.

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Abstract. Land surface albedo constitutes a critical climatic variable, since it largely controls the actual amount of solar energy available to the Earth system. The purpose of this paper is to establish a theory for the exploitation of space observations to solve the atmosphere/surface radiation transfer problem on an operational basis and to generate surface albedo, aerosol load, and possibly land cover change products. Surface albedo is rather variable in space and time and depends both on the structure and on the radiative characteristics of the surface, as well as on the angular and spectral distribution of radiation at the bottom of the atmosphere. Weather and climate models often use preset distributions or simple parameterizations of this environment variable, even though such approaches do not accurately account for the actual effect of the underlying surface. From a mathematical point of view, the determination of the surface albedo corresponds to the estimation of a boundary condition for the radiation transfer problem in the coupled surface-atmosphere system. A relatively large database of 10 years or more of Meteosat data has been accumulated by EUMETSAT. These data, collected at half-hour intervals over the entire Earth disk visible from longitude 0 ø, constitute a unique resource to describe the anisotropy of the coupled surface-atmosphere system and provide the opportunity to document changes in surface albedo which may have occurred in these regions over that period. In addition, since the coupled surface-atmosphere radiation transfer problem must be solved, the proposed procedure also yields an estimate of the spatial and temporal distribution of aerosols. The proposed inversion procedure yields a characterization of surface radiative properties that may also be used to document and monitor land surface dynamics over the portion of the globe observed by Meteosat. Results from preliminary applications and an error budget analysis are discussed in a companion paper [Pinty et al., this issue]. IntroductionThe bulk of the solar radiation available to the Earth system is absorbed at or near the oceanic and continental surface and then ultimately released to the atmosphere through the fluxes of infrared radiation, as well as sensible and latent heat. The fraction of solar energy absorbed at the surface of the planet is controlled by its surface albedo, which is highly variable in space and time over terrestrial surfaces. Satellite-borne instruments constitute a priori a unique tool for monitoring surface albedo values at the global scale and at spatial and temporal resolutions adequate for meteorological and climate studies. However, the above assertion implies that the problems hindering the accurate estimation of surface albedo values from space measurements are correctly addressed.The contributions to the measured radiances due to the atmospheric layers and the variations due to the anisotropic reflectance of all terrestrial surfaces are of primary concern in this 18,099

Abstract. An advanced algorithm to retrieve the radiative properties of terrestrial surfaces sampled by the Meteosat visible instrument was derived in a companion paper IntroductionIn a companion paper, Pinty et al. [this issue] (hereinafter referred to as part 1) proposed a new approach to estimate land surface albedo from data acquired by the Meteosat instrument at a daily and pixel resolution. The cornerstone of this approach is the exploitation of the temporal sampling of Meteosat (data acquired every 30 min from sunrise to sunset) as if it were an instantaneous angular sampling. With the assumption that the geophysical system under investigation does not change significantly during the day, an analytical expression has been derived (part 1, equation (43)) to express the bidirectional reflectance field measured by Meteosat for "clear-sky" pixels. This expression accounts for the major radiation transfer processes occurring among the Sun, the Earth system, and the satellite and follows from mathematical developments aimed at deriving a formulation of the bidirectional reflectance field that can be operationally inverted against a set of daily Meteosat data. Application of this procedure permits estimating (1) the surface bidirectional reflectance factors (BRF) required to derive albedo-related quantities and (2) an "effective" aerosol load which, together with the surface BRF, provides a coherent interpretation of the daily sequence of This paper deals with a number of additional issues to be addressed when applying this approach to actual Meteosat-5 data. As such, it discusses and proposes methods to handle (1) the selection, for each pixel (location), of those time observations during the day which are not contaminated by cloud radiative effects, (2) the identification of the optimal solution for each pixel and each day through the set of potential solutions, and finally (3) the assessment of the limits inherent to the Meteosat-5 sensor itself. The application, performed on the basis of two sequences of Meteosat data sets available in June and November/December 1996, allows the production of an ensemble of geophysical maps and preliminary documentation of the seasonal changes in land surface over the regions studied. The impact of biomass burning on the surface albedo values of savannas and woodlands is examined and compared to the natural vegetation cycle related to the African monsoon events. 18,113