Statistically Optimized Inversion Algorithm for Enhanced Retrieval of Aerosol Properties from Spectral Multi-Angle Polarimetric Satellite Observations

The proposed development is an attempt to enhance aerosol retrieval by emphasizing statistical optimization in inversion of advanced satellite observations. This optimization concept improves retrieval accuracy relying on the knowledge of measurement error distribution. Efficient application of such optimization requires pronounced data redundancy (excess of the measurements number over number of unknowns) that is not common in satellite observations. The POLDER imager on board the PARASOL microsatellite registers spectral polarimetric characteristics of the reflected atmospheric radiation at up to 16 viewing directions over each observed pixel. The completeness of such observations is notably higher than for most currently operating passive satellite aerosol sensors. This provides an opportunity for profound utilization of statistical optimization principles in satellite data inversion. The proposed retrieval scheme is designed as statistically optimized multi-variable fitting of all available angular observations obtained by the POLDER sensor in the window spectral channels where absorption by gas is minimal. The total number of such observations by PARASOL always exceeds a hundred over each pixel and the statistical optimization concept promises to be efficient even if the algorithm retrieves several tens of aerosol parameters. Based on this idea, the proposed algorithm uses a large number of unknowns and is aimed at retrieval of extended set of parameters affecting measured radiation

Remote sensing of aerosols by using polarized, directional and spectral measurements within the A-Train: the PARASOL mission

Instruments dedicated to aerosol monitoring are recently available and the POLDER (POLarization and Directionality of the Earth's Reflectances) instrument on board the PARASOL (Polarization & Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) mission is one of them. By measuring the spectral, angular and polarization properties of the radiance at the top of the atmosphere, in coordination with the other A-Train instruments, PARASOL provides the aerosol optical depths (AOD) as well as several optical and microphysical aerosol properties. The instrument, the inversion schemes and the list of aerosol parameters are described. Examples of retrieved aerosol parameters are provided as well as innovative approaches and further inversion techniques

Aerosol retrieval experiments in the ESA Aerosol_cci project

Within the ESA Climate Change Initiative (CCI) project Aerosol_cci (2010–2013), algorithms for the production of long-term total column aerosol optical depth (AOD) datasets from European Earth Observation sensors are developed. Starting with eight existing pre-cursor algorithms three analysis steps are conducted to improve and qualify the algorithms: (1) a series of experiments applied to one month of global data to understand several major sensitivities to assumptions needed due to the ill-posed nature of the underlying inversion problem, (2) a round robin exercise of "best" versions of each of these algorithms (defined using the step 1 outcome) applied to four months of global data to identify mature algorithms, and (3) a comprehensive validation exercise applied to one complete year of global data produced by the algorithms selected as mature based on the round robin exercise. The algorithms tested included four using AATSR, three using MERIS and one using PARASOL. This paper summarizes the first step. Three experiments were conducted to assess the potential impact of major assumptions in the various aerosol retrieval algorithms. In the first experiment a common set of four aerosol components was used to provide all algorithms with the same assumptions. The second experiment introduced an aerosol property climatology, derived from a combination of model and sun photometer observations, as a priori information in the retrievals on the occurrence of the common aerosol components. The third experiment assessed the impact of using a common nadir cloud mask for AATSR and MERIS algorithms in order to characterize the sensitivity to remaining cloud contamination in the retrievals against the baseline dataset versions. The impact of the algorithm changes was assessed for one month (September 2008) of data: qualitatively by inspection of monthly mean AOD maps and quantitatively by comparing daily gridded satellite data against daily averaged AERONET sun photometer observations for the different versions of each algorithm globally (land and coastal) and for three regions with different aerosol regimes. The analysis allowed for an assessment of sensitivities of all algorithms, which helped define the best algorithm versions for the subsequent round robin exercise; all algorithms (except for MERIS) showed some, in parts significant, improvement. In particular, using common aerosol components and partly also a priori aerosol-type climatology is beneficial. On the other hand the use of an AATSR-based common cloud mask meant a clear improvement (though with significant reduction of coverage) for the MERIS standard product, but not for the algorithms using AATSR. It is noted that all these observations are mostly consistent for all five analyses (global land, global coastal, three regional), which can be understood well, since the set of aerosol components defined in Sect. 3.1 was explicitly designed to cover different global aerosol regimes (with low and high absorption fine mode, sea salt and dust)

Ice crystal shapes in cirrus clouds derived from POLDER-1/ADEOS-1.

International audienceThis paper discusses the retrieval of ice crystal shapes of cirrus clouds on a global scale using observations collected with POLDER-1 (POLarization and Directionality of the Earth Reflectance) onboard the ADEOS-1 platform. The retrieval is based on polarized bidirectional observations made by POLDER. First, normalized polarized radiances are simulated for cirrus clouds composed of ice crystals that differ in shape and are randomly oriented in space. Different values of cloud optical depths, viewing geometries and solar zenith angles are used in the simulations. This sensitivity study shows that the normalized polarized radiance is highly sensitive to the shape of the scatterers for specific viewing geometries, and that it saturates after a few scattering events, which makes it rapidly independent of the optical depth of the cirrus clouds. Next, normalized polarized radiance observations obtained by POLDER have been selected, based on suitable viewing geometries and on the occurrence of thick cirrus clouds composed of particles randomly oriented in space. For various ice crystal shapes these observations are compared with calculated values pertaining to the same geometry, in order to determine the shape that best reproduces the measurements. The method is tested fully for the POLDER data collected on January 12, 1997. Thereafter, it is applied to six periods of 6 days of observations obtained in January, February, March, April, May, and June 1997. This study shows that the particle shape is highly variable with location and season, and that polycrystals and hexagonal columns are dominant at low latitudes, whereas hexagonal plates occur more frequently at high latitudes

Intercomparison of desert dust optical depth from satellite measurements

This work provides a comparison of satellite retrievalsof Saharan desert dust aerosol optical depth (AOD)during a strong dust event through March 2006. In this event,a large dust plume was transported over desert, vegetated,and ocean surfaces. The aim is to identify the differencesbetween current datasets. The satellite instruments consideredare AATSR, AIRS, MERIS, MISR, MODIS, OMI,POLDER, and SEVIRI. An interesting aspect is that the differentalgorithms make use of different instrument characteristicsto obtain retrievals over bright surfaces. These includemulti-angle approaches (MISR, AATSR), polarisationmeasurements (POLDER), single-view approaches using solarwavelengths (OMI, MODIS), and the thermal infraredspectral region (SEVIRI, AIRS). Differences between instruments,together with the comparison of different retrievalalgorithms applied to measurements from the same instrument,provide a unique insight into the performance andcharacteristics of the various techniques employed. As wellas the intercomparison between different satellite products,the AODs have also been compared to co-located AERONETdata. Despite the fact that the agreement between satellite andAERONET AODs is reasonably good for all of the datasets,there are significant differences between them when comparedto each other, especially over land. These differencesare partially due to differences in the algorithms, such as assumptionsabout aerosol model and surface properties. However,in this comparison of spatially and temporally averageddata, it is important to note that differences in sampling, relatedto the actual footprint of each instrument on the heterogeneousaerosol field, cloud identification and the qualitycontrol flags of each dataset can be an important issue

Validation of spaceborne estimates of aerosol loading from Sun photometer measurements with emphasis on polarization

International audienceOver land, aerosol products from the Polarization and Directionality of the Earth's Reflectances (POLDER) are derived from the multispectral analysis of multiangular measurements of polarized radiance. A retrieval algorithm, developed on the basis of radiative transfer simulations and airborne observations, will process all measurements during the instrument mission. An evaluation of the algorithm performance and possible improvements is needed before the release of the aerosol products to the scientific user communities. A validation effort has been planned to test the derived products against independent measurements. The validation is achieved against ground-based Sun photometer measurements from a worldwide station network. Although both measurements depend on the optical properties of the aerosols, they differ from the surface contribution and from the different viewing geometry. In this paper, we describe the planned procedure for POLDER aerosol product validation, and we discuss possible sources of uncertainties in comparing ground-based measurements with satellite based ones. The study shows that ground-based aerosol products are less affected by uncertainties than satellite-based ones and therefore constitute a reliable ground truth and a valid source of information in the process of the POLDER algorithm improvement. However, because a Sun photometer cannot observe the larger scattering angles, it may not be adapted to validate the refractive index estimate

Maritime and dust aerosol retrieval from polarized and multispectral active and passive sensors

International audienceThe two-dimensional structure and the optical properties of Saharan dust and maritime aerosols have been investigated over the Mediterranean Sea during October 2001 using airborne and satellite observations. A new airborne downward looking multispectral (490–2200 nm) micropolarimeter (MICROPOL) is used to derive the aerosol optical thickness and effective radius. We present two case studies corresponding to the observations performed during a mineral dust transport from the Sahara to Europe and to an undisturbed marine boundary layer. The dust plume is associated with aerosol optical thickness above 0.2 (at 865 nm), whereas relatively low loading conditions are observed in the maritime case (down to 0.1). The MICROPOL-derived aerosol optical thickness is in an excellent agreement with coincident Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals for both days. The effective radius retrieved by MICROPOL is also in a good agreement with the one from MODIS in the dust case. In the pure maritime case, this parameter is significantly underestimated by MICROPOL compared to MODIS retrieval. The vertical distribution of aerosol optical parameters is derived from combined two-wavelength backscattering airborne lidar observations and MICROPOL passive measurements. As expected in the case of a long-range transport, the aerosol effective radius within the dust layer is rather constant as a function of the altitude. A surprising low lidar depolarization factor of about 4% is retrieved within the dust plume, suggesting a major contribution of spherical particles. No significant depolarization has been observed in the marine boundary layer. For the given geometry of observation the retrieved aerosol models, which are based on the Mie theory, reproduce the MICROPOL polarized measurements within 5–8% in the dust and maritime case. The use of a nonspherical model increases by a factor of 2 the residual fitting error in polarization in the case of the dust observation. This result is confirmed by the lidar depolarization ratio and indicates that a large part of particles in the dust plume are spherical

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