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B. Pinty and D. Tanre

Abstract

The inference of surface reflectance from satellite observations requires the knowledge of the double-way transmittance through the atmosphere. Since the existing pyranometer networks routinely provide measurements of the incident transmittance over sensitive climatic regions, it would be useful for subsequent applications to relate this ground-based measurement to the corresponding double-way transmittance. A variety of satellite radiance simulations corresponding to clear sky conditions has been made in order to derive a suitable parameterized expression between the two quantities. The accuracy of this expression when making use of additional meteorological observations is shown and discussed. Finally, the derived expression is used to improve a method recently proposed by Pinty et al. for retrieving surface albedo over the African Sahel from METEOSAT radiances.

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I. Jankowiak and D. Tanré

Abstract

An operational algorithm for detecting dust outbreaks over ocean at global scales is presented. It is shown to be efficient for identifying dusty areas over the eastern Atlantic Ocean using the European Meteorological Satellite (Meteosat). The retrieved values of the dust optical thickness, which is related to the importance of the event, are shown to be in good agreement with simultaneous ground measurements. First results concerning the frequencies and the trajectories of the dust outbreaks that occurred over five years (from 1984 to 1988) are also provided.

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J. Lenoble, D. Tanre, P. Y. Deschamps, and M. Herman

Abstract

A simple three-layer model of the earth-atmosphere system, including the ground, troposphere and stratosphere, with their interactions, is developed. The model permits the radiative characteristics of both the troposphere and stratosphere to be separately adjusted to describe any atmospheric state. The accuracy is tested against the more precise computations of Herman et al. (1976), which take into account the aerosol profile and the angular variation of reflectance at the top of the troposphere. Analytical expressions are obtained for the albedo variation due to a thin stratospheric aerosol layer. The physical procedures are outlined, as well as the influence of the main parameters: aerosol optical thickness, single scattering albedo and asymmetry factor, and sublayer albedo.

The method is applied to compute the variation of the zonal albedo and the planetary radiation balance due to a stratospheric aerosol layer of background H2SO4 droplets and of volcanic ash. The resulting ground temperature perturbation is evaluated, using a Budyko type climate model.

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Michael D. King, Yoram J. Kaufman, Didier Tanré, and Teruyuki Nakajima

Tropospheric aerosol particles originate from man-made sources such as urban/industrial activities, biomass burning associated with land use processes, wind-blown dust, and natural sources. Their interaction with sunlight and their effect on cloud microphysics form a major uncertainty in predicting climate change. Furthermore, the lifetime of only a few days causes high spatial variability in aerosol optical and radiative properties that requires global observations from space.

Remote sensing of tropospheric aerosol properties from space is reviewed both for present and planned national and international satellite sensors. Techniques that are being used to enhance our ability to characterize the global distribution of aerosol properties include well-calibrated multispectral radiometers, multispectral polarimeters, and multiangle spectroradiometers. Though most of these sensor systems rely primarily on visible to near-infrared spectral channels, the availability of thermal channels to aid in cloud screening is an important additional piece of information that is not always incorporated into the sensor design. In this paper, the various satellite sensor systems being developed by Europe, Japan, and the United States are described, and the advantages and disadvantages of each of these systems for aerosol applications are highlighted. An important underlying theme is that the remote sensing of aerosol properties, especially aerosol size distribution and single scattering albedo, is exceedingly difficult. As a consequence, no one sensor system is capable of providing totally unambiguous information, and hence a careful intercomparison of derived products from different sensors, together with a comprehensive network of ground-based sunphotometer and sky radiometer systems, is required to advance our quantitative understanding of global aerosol characteristics.

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J. L. Deuze, C. Devaux, M. Herman, R. Santer, and D. Tanre

Abstract

In July 1983, the summer transport of Saharian aerosols across the Mediterranean Sea was observed. The dust cloud was particularly dense and was clearly detected in A.V.H.R.R. and METEOSAT imageries. Optical thickness and Angström coeffcients have been derived from these pictures. During the same period, ground based observations—transmission, aureole and polarization measurements—were performed at the Observatoire de Haute Provence (southeast of France). Measured aerosol optical thickness at 550 nm were as large as about 1.5.

The optical thicknesses and Angström coefficients derived from the two experiments are compared and are in good agreement.

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F. Waquet, J. Riedi, L. C. Labonnote, P. Goloub, B. Cairns, J-L. Deuzé, and D. Tanré

Abstract

The detection of aerosol above clouds is critical for the estimate of both the aerosol and cloud radiative impacts. In this study, the authors present a new method to retrieve the aerosol properties over clouds that uses the multiangle polarization measurements of the Polarization and Directionality of Earth Reflectances (POLDER)–Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL) instrument. The method is illustrated and applied to a case study exploiting the coincident observations from other passive and active sensors of the NASA A-Train satellite constellation. The case study is relative to an elevated biomass burning aerosol layer that originates from southern Africa and is then transported over low-level clouds extending over the Atlantic Ocean. It is shown that the comparison between the cloud-top heights retrieved with the different passive techniques developed for the A-Train sensors can be used to detect the presence of aerosols above clouds. The analysis of the PARASOL observations showed that the aerosols significantly affect the polarized light reflected by the clouds over the 80°–120° scattering angle range and in the rainbow region. A single scattering model permitted the reproduction of the polarization observations and the retrieval of an estimate of the aerosol layer optical thickness of 0.225 at 0.865 μm. The retrieved aerosol optical thicknesses over clouds agree quantitatively with the closest ones retrieved over clear-sky ocean (±0.04 as a maximum departure), demonstrating the value of the method. This innovative technique based solely on passive measurements is expected to provide a better understanding of aerosol properties in regions where significant cloud cover usually prevents the retrieval of aerosol optical thickness. As such, this new retrieval method can provide significant and valuable information about the radiative impact of clouds and aerosols, especially where they can potentially interact strongly with each other.

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Y. J. Kaufman, D. Tanré, B. N. Holben, S. Mattoo, L. A. Remer, T. F. Eck, J. Vaughan, and Bernadette Chatenet

Abstract

Accurate measurements of the spectral solar flux reaching the surface in cloud-free conditions are required to determine the aerosol radiative impact and to test aerosol models that are used to calculate radiative forcing of climate. Spectral flux measurements are hampered in many locations by persistent broken cloud fields. Here a new technique is developed to derive the diffuse solar spectral flux reaching the surface from principal-plane measurements conducted in the last six years by the Aerosol Robotic Network (AERONET). This 50–100 instrument global network measures the principal-plane radiances in four spectral bands (0.44–1.02 μm) approximately every hour every day. These instruments also measure the spectral optical thickness and derive the aerosol size distribution and other properties from sky measurements. The advantage of the AERONET measurements is that collimated sky radiance is measured for each 1° × 1° field of view. Clouds and cloud shadows are rejected before the total sky brightness is reconstructed and the flux is derived. The results compare favorably with shadow band measurements and with aerosol models. Studied are smoke aerosol in Brazil; Saharan dust in Cape Verde; and urban–industrial pollution in Créteil, near Paris, France, and near Washington, D.C. The spectral attenuation of total (diffuse+direct) solar flux reaching the surface is given by f λ = exp(−a λb λ τ λ), where a λ is attenuation by an atmosphere with no aerosol and b λ is the aerosol attenuation coefficient. Remarkably, it is found that for these sites except for the Washington, D.C., site, the spectrally averaged value of b λ does not vary significantly from one aerosol type to another: {b λ} = 0.35 ± 0.03 (for solar zenith angle of 50°). The measured 24-h average aerosol impact on the solar flux at the surface per unit optical thickness is ΔFτ = −80 W m−2 in these sites, almost independent of the aerosol type: smoke, dust, or urban–industrial pollution. In Washington, D.C., it is suspected, and demonstrated in a back of the envelope calculation, that the high amount of broken cloudiness and its correlation with the aerosol optical thickness are responsible for the apparent small aerosol forcing at the surface of ΔFτ = −50 W m−2.

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Oleg Dubovik, Brent Holben, Thomas F. Eck, Alexander Smirnov, Yoram J. Kaufman, Michael D. King, Didier Tanré, and Ilya Slutsker

Abstract

Aerosol radiative forcing is a critical, though variable and uncertain, component of the global climate. Yet climate models rely on sparse information of the aerosol optical properties. In situ measurements, though important in many respects, seldom provide measurements of the undisturbed aerosol in the entire atmospheric column. Here, 8 yr of worldwide distributed data from the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations. Established procedures for maintaining and calibrating the global network of radiometers, cloud screening, and inversion techniques allow for a consistent retrieval of the optical properties of aerosol in locations with varying emission sources and conditions. The multiyear, multi-instrument observations show robust differentiation in both the magnitude and spectral dependence of the absorption—a property driving aerosol climate forcing, for desert dust, biomass burning, urban–industrial, and marine aerosols. Moreover, significant variability of the absorption for the same aerosol type appearing due to different meteorological and source characteristics as well as different emission characteristics are observed. It is expected that this aerosol characterization will help refine aerosol optical models and reduce uncertainties in satellite observations of the global aerosol and in modeling aerosol impacts on climate.

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L. A. Remer, Y. J. Kaufman, D. Tanré, S. Mattoo, D. A. Chu, J. V. Martins, R.-R. Li, C. Ichoku, R. C. Levy, R. G. Kleidman, T. F. Eck, E. Vermote, and B. N. Holben

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard both NASA’s Terra and Aqua satellites is making near-global daily observations of the earth in a wide spectral range (0.41–15 μm). These measurements are used to derive spectral aerosol optical thickness and aerosol size parameters over both land and ocean. The aerosol products available over land include aerosol optical thickness at three visible wavelengths, a measure of the fraction of aerosol optical thickness attributed to the fine mode, and several derived parameters including reflected spectral solar flux at the top of the atmosphere. Over the ocean, the aerosol optical thickness is provided in seven wavelengths from 0.47 to 2.13 μm. In addition, quantitative aerosol size information includes effective radius of the aerosol and quantitative fraction of optical thickness attributed to the fine mode. Spectral irradiance contributed by the aerosol, mass concentration, and number of cloud condensation nuclei round out the list of available aerosol products over the ocean. The spectral optical thickness and effective radius of the aerosol over the ocean are validated by comparison with two years of Aerosol Robotic Network (AERONET) data gleaned from 132 AERONET stations. Eight thousand MODIS aerosol retrievals collocated with AERONET measurements confirm that one standard deviation of MODIS optical thickness retrievals fall within the predicted uncertainty of Δτ = ±0.03 ±0.05τ over ocean and Δτ = ±0.05 ± 0.15τ over land. Two hundred and seventy-one MODIS aerosol retrievals collocated with AERONET inversions at island and coastal sites suggest that one standard deviation of MODIS effective radius retrievals falls within Δr eff = ±0.11 μm. The accuracy of the MODIS retrievals suggests that the product can be used to help narrow the uncertainties associated with aerosol radiative forcing of global climate.

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