The effect of infrared radiation reflected from the Earth's surface observed in the atmospheric emission spectra in remote sensing of the Earth from space

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Aerosol indirect effects in POLDER satellite data and the Laboratoire de Météorologie Dynamique–Zoom (LMDZ) general circulation model

International audienceThe POLDER-1 instrument was able to measure aerosol and cloud properties for eight months in 1996-1997. We use these observational data for aerosol concentration (the aerosol index), cloud optical thickness, and cloud droplet effective radius to establish statistical relationships among these parameters in order to analyze the first and second aerosol indirect effects. We also evaluate the representation of these effects as parameterized in the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) general circulation model. We find a decrease in cloud top droplet radius with increasing aerosol index in both the model and the observations. Our results are only slightly changed if the analysis is done at fixed cloud liquid water path (LWP) instead of considering all LWP conditions. We also find a positive correlation between aerosol index and cloud liquid water path, which is particularly pronounced over the Northern Hemisphere midlatitudes. This may be interpreted as observational evidence for the second aerosol indirect effect on a large scale. The model-simulated relationship agrees well with that derived from POLDER data. Model simulations show a rather small change in the two relationships if preindustrial rather than present-day aerosol distributions are used. However, when entirely switching off the second aerosol indirect effect in our model, we find a much steeper slope than we do when including it

Scattering layer statistics from space borne GLAS observations

International audienceCloud and aerosol layers detected by the space borne Geoscience Laser Altimeter System (GLAS) are used to derive statistics of clear and almost clear atmospheres, the latter defined to be those with some scattering material but total optical thickness less than 0.2. Such statistics are needed to evaluate the potential coverage of NASA's forthcoming Orbital Carbon Observatory. The global fraction of clear cases is approximately 15%, with large scale spatial structures similar to those found by passive sensors. The spatial distribution of almost clear cases is similar to that for clear, with global fraction approximately 20%. The mean altitude of optically thin scattering layers is generally below one kilometer, indicating that they are composed mostly of boundary layer aerosol rather than high altitude cloud. The spatial correlation function of clear cases is accurately reproduced by the analytical function F(d) = exp[−(d/d0)0.5], where d0 is a correlation scale length. Between 60N and 60S, d0 shows little zonal variation, and its average value is 320 km. Over the Arctic d0 falls to 250 km, but rises to 450 km over the Antarctic

Enhanced discrimination of boreal forest covers with directional reflectances from the airborne polarization and directionality of Earth reflectances (POLDER) instrument

International audienceDuring the Boreal Ecosystem-Atmosphere Study (BOREAS), directional and spectral reflectance measurements were acquired from May to July 1994 with the polarization and directionality of Earth reflectances (POLDER) instrument on board a NASA C-130 aircraft. The instrument has a wide field-of-view optics, a two-dimensional CCD array, and a rotating wheel carrying filters in the visible and near infrared. Measurements were obtained (1) over coniferous forests at the young and old jack pine and old black spruce sites, (2) over a deciduous forest at the old aspen site, and (3) over a fen at the fen site. A prominent hot spot feature was apparent at each site, with an additional strong peak in the specular direction for the fen site. Strong variations of the bidirectional reflectance distribution function (BRDF) with sun zenith angle were observed. For a constant sun zenith angle, the variation of the BRDF of conifer stands between May and July was relatively weak. A key objective of this paper is to quantify the improvement of discrimination of various forest covers when remotely sensed directional signatures are added to the more conventional spectral signatures. The experimental protocol consisted of the following steps. First, 150 pixels pertaining to five different classes of forest covers were selected on land cover maps available in the BOREAS Information System (BORIS) data base. Second, the BRDF measurements acquired by POLDER at each pixel were adjusted against a three-parameter semiempirical BRDF model and processed to retrieve the reflectance seen in three different viewing directions. Third, the results of supervised classifications were compared on all selected pixels, using as input, either the reflectances in only one direction (this simulates the case of conventional spectral signatures), or reflectances acquired in three directions (this simulates the case of spectral + directional signatures). The results showed that when only one spectral band was used, the proportion of correctly classified pixels increased from 36-59% with one viewing direction to 64-84% with three viewing directions. When three spectral bands were considered, this proportion improved from 72-87% to 83-97%. These results demonstrate that the account of directional information enhances the ability to discriminate forest covers by remote sensing

Atmospheric water vapor estimate by a differential absorption technique with the polarisation and directionality of the Earth reflectances (POLDER) instrument

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Mild winter and spring 2007 over western Europe led to a widespread early vegetation onset

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Remote sensing of airborne desert dust mass over oceans using Meteosat and CZCS imagery

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Assimilation of POLDER aerosol optical thickness into the LMDz-INCA model: Implications for the Arctic aerosol burden

The large spatial and temporal variability of atmospheric aerosol load makes it a challenge to quantify aerosol effect on climate. This study is one of the first attempts to apply data assimilation for the analysis of global aerosol distribution. Aerosol optical thickness (AOT) observed from the Polarization and Directionality of the Earth Reflectances (POLDER) spaceborne instrument are assimilated into a three-dimensional chemistry model. POLDER capabilities to distinguish between fine and coarse AOT are used to constrain them separately in the model. Observation and model errors are a key component of such a system and are carefully estimated on a regional basis using some of the high-quality surface observations from the Aerosol Robotic Network (AERONET). Other AERONET data provide an independent evaluation of the a posteriori fields. Results for the fine mode show improvements, in terms of reduction of root-mean-square errors, in most regions with the largest improvements found in the Mediterranean Sea and Eurasia. We emphasize the results for the Arctic, where there is growing evidence of a strong aerosol impact on climate, but a lack of regional and continuous aerosol monitoring. The a posteriori fields noticeably well reproduce the winter-spring “Arctic Haze” peak measured in Longyearbyen (15°E, 78°N) and typical seasonal variations in the Arctic region, where AOT increase by up to a factor of three between a posteriori and a priori. Enhanced AOT are found over a longer period in spring 2003 than in 1997, suggesting that the large Russian fires in 2003 have influenced the Arctic aerosol load

Remote sensing of aerosols over land surfaces including polarization measurements and application to POLDER measurements

International audienceGround-based measurements of the diffuse skylight and airborne measurements of the light reflected by land surfaces are examined, especially with regard to their polarization properties. The reported land surface reflections correspond to multidirectional polarized measurements performed by the Polarization and Directionality of Earth Reflectances (POLDER) airborne version on very clear days. These observations are analyzed for retrieving the polarization properties of scattering by terrestrial aemml.q and reflection by ground targets, respectively. The results suggest that the polarized light is much more sensitive to atmospheric scattering than to reflection by natural surfaces, especially by vegetative cover. Theoretical modeling supports this hypothesis. Finally, application of these results to aerosol remote sensing over land surfaces from POLDER measurements is discussed