Top-of-Atmosphere Albedo Estimation from Angular Distribution Models Using Scene Identification from Satellite Cloud Property Retrievals

International audienceThe next generation of earth radiation budget satellite instruments will routinely merge estimates of global top-of-atmosphere radiative fluxes with cloud properties. This information will offer many new opportunities for validating radiative transfer models and cloud parameterizations in climate models. In this study, five months of Polarization and Directionality of the Earth's Reflectances 670-nm radiance measurements are considered in order to examine how satellite cloud property retrievals can be used to define empirical angular distribution models (ADMs) for estimating top-of-atmosphere albedo. ADMs are defined for 19 scene types defined by satellite retrievals of cloud fraction and cloud optical depth. Two approaches are used to define the ADM scene types. The first assumes there are no biases in the retrieved cloud properties and defines ADMs for fixed discrete intervals of cloud fraction and cloud optical depth (fixed-τ approach). The second approach involves the same cloud fraction intervals, but uses percentile intervals of cloud optical depth instead (percentile-τ approach). Albedos generated using these methods are compared with albedos inferred directly from the mean observed reflectance field

Polarized light scattering by inhomogeneous hexagonal monocrystals. Validation with ADEOS-POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements

ANALYSE DES FLUCTUATIONS DU RAYONNEMENT ATMOSPHERIQUE AUTOUR DE 4,3 MICRONS POUR DES VISEES SATELLITALES PROCHES DU NADIR

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Thermal impact of Saharan dust over land. Part II : application to satellite IR remote sensing

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An improved derivation of the top-of-atmosphere albedo from POLDER/ADEOS-2: 2. Broadband albedo

International audienceThe narrowband albedos derived from Polarization and Directionality of Earth Reflectances (POLDER) measurements have been described in a companion paper (Buriez et al., 2005). Here, they are used to estimate the broadband shortwave albedo. Except for the gaseous absorption, the albedos at 443 and 670 nm are considered as representative of the UV-visible range, and the albedo at 865 nm is considered as representative of the near infrared. The gaseous absorption is estimated from TOMS data and from the POLDER 910- to 865-nm reflectance ratio. In a previous approach, the respective weights of the three narrowband albedos were based on radiative transfer simulations. Now, we take advantage of spatiotemporal coincidences between the second Advanced Earth Observing Satellite (ADEOS-2) and Terra to adjust these weights from the comparison between POLDER narrowband and CERES broadband reflectances. With no adjustment, the POLDER albedos are underestimated by 2% (in relative value) whereas the associated reflectances are underestimated by 4%. With an adjustment to the CERES reflectances, the POLDER shortwave albedos are overestimated by 2%. With or without adjustment the RMS difference between the POLDER and the CERES 1°-regional instantaneous shortwave albedos is 6%; this is quite satisfactory since it is comparable to the CERES SW albedo consistency between nadir and oblique viewing zenith angles. When considering only homogeneous areas, the agreement between POLDER and CERES estimates was rather less good in the previous approach; it is now more than twice better. The difference between POLDER and CERES shortwave albedo estimates appears, for a large part, due to our plane-parallel assumption that is crucial for the heterogeneous liquid water clouds. It results in a bias in monthly mean shortwave flux around 2 W m−2 that is found to present only small variations in latitude

An improved derivation of the top-of-atmosphere albedo from POLDER/ADEOS-2: Narrowband albedos

International audienceThe Polarization and Directionality of Earth Reflectances (POLDER) instrument was routinely functioning aboard the second Advanced Earth Observing Satellite (ADEOS-2) from April to October 2003. A series of algorithms dedicated to Earth radiation budget, water vapor, and clouds are applied to the POLDER data. This paper presents the derivation scheme of the narrowband albedos at the top of the atmosphere from POLDER measurements at 443, 670, and 865 nm. These narrowband albedos are used to estimate the broadband shortwave albedo at the top of atmosphere in a companion paper. Here we focus on the 670 nm (over land)/865 nm (over ocean) albedo. Although our derivation is based on the plane-parallel hypothesis, and thanks to the multidirectional capability of POLDER, results appear reliable for both cloudy and clear-sky scenes. A quality index is introduced from the comparison between the angular variability of the retrieved "directional" albedo values and that of the measured reflectances. This quality index is "good" in 80% of the cases. Moreover, the retrieved albedo values are found to be statistically little dependent on the viewing direction

Cloud cover observed simultaneously from POLDER and METEOSAT

International audienceThe POLDER instrument that was aboard the Japanese ADEOS platform between August 1996 and June 1997. is designed to the global observations of the polarisation and directionality of the sun-light reflected by the Earth-atmosphere system. The cloud detection from POLDER takes advantage of the original capabilities of the instrument (spectral polarisation and directionality). This cloud detection scheme uses 5 threshold tests based on pressure, reflectance, polarised reflectance and spectral variability. The results of the POLDER cloud detection scheme are compared to those of the LMD dynamical clustering method applied to visible and infrared METEOSAT data and local spatial variability of these two parameters. Special focus is given to the detection capabilities of the two kind of measurements for cloud situations such as small cumulus, thin cirrus and multilayered cloud cover. Results of this comparison would give some insight on the behaviour of the International Satellite Cloud Climatology Project (ISCCP) cloud detection scheme built mainly from visible and infrared measurements

Case study of inhomogeneous cloud parameter retrieval from MODIS data

International audienceCloud parameter retrieval of inhomogeneous and fractional clouds is performed for a stratocumulus scene observed by MODIS at a solar zenith angle near 60°. The method is based on the use of neural network technique with multispectral and multiscale information. It allows to retrieve six cloud parameters, i.e. pixel means and standard deviations of optical thickness and effective radius, fractional cloud cover, and cloud top temperature. Retrieved cloud optical thickness and effective radius are compared to those retrieved with a classical method based on the homogeneous cloud assumption. Subpixel fractional cloud cover and optical thickness inhomogeneity are compared with their estimates obtained from 250m pixel observations; this comparison shows a fairly good agreement. The cloud top temperature appears also retrieved quite suitably

Modeling of light scattering in cirrus clouds with inhomogeneous hexagonal monocrystals. Comparison with in-situ and ADEOS-POLDER measurements

International audienceAn Inhhomogeneous Hexagonal Monocrystal (IHM) model is used to simulate light scattering by randomly oriented hexagonal ice crystals containing air bubbles. This model based on a combination of ray-tracing, Mie theory and Monte-Carlo techniques, allows to retrieve the scattering phase function. In-situ measurements of the light scattering diagram in natural cirrus clouds with an airborne nephelometer have been performed. The results given by the IHM model have been favorably adjusted with these measurements. This agreement provides an opportunity to use this model in order to analyze ADEOS-POLDER reflectance measurements over cirrus clouds. POLDER uses an original concept to measure, for a given scene, total and polarized reflectances under several viewing directions. A first analysis of cirrus cloud spherical albedoes for the 10th November 1996 shows a rather good agreement between measurements and calculations

Modeling of light scattering in cirrus clouds with inhomogeneous hexagonal monocrystals. Comparison with in-situ and ADEOS-POLDER measurements

International audienceAn Inhhomogeneous Hexagonal Monocrystal (IHM) model is used to simulate light scattering by randomly oriented hexagonal ice crystals containing air bubbles. This model based on a combination of ray-tracing, Mie theory and Monte-Carlo techniques, allows to retrieve the scattering phase function. In-situ measurements of the light scattering diagram in natural cirrus clouds with an airborne nephelometer have been performed. The results given by the IHM model have been favorably adjusted with these measurements. This agreement provides an opportunity to use this model in order to analyze ADEOS-POLDER reflectance measurements over cirrus clouds. POLDER uses an original concept to measure, for a given scene, total and polarized reflectances under several viewing directions. A first analysis of cirrus cloud spherical albedoes for the 10th November 1996 shows a rather good agreement between measurements and calculations