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Pinty et al. (2000)<\/a> were the first to propose an algorithm for the joint retrieval of surface reflectance and aerosol properties to demonstrate the possibility of generating CDR from data acquired by operational weather geostationary satellites. Due to limited operational computational resources available in 1998 in the EUMETSAT reprocessing ground segment, where the data were processed, the development of this algorithm was subject to strong constraints. The Radiative Transfer Equation (RTE) solutions were precomputed and stored in lookup tables with a very coarse resolution, limiting the maximum retrieved AOT to 1, which represents a severe limitation over the Sahara region where AOT values can easily exceed such a limit. Furthermore, the radiative coupling between aerosol scattering and gaseous absorption was not taken into account. This algorithm, referred to as Geostationary Surface Albedo (GSA), has been substantially improved and generalised for the processing of any satellite data (Govaerts and Luffarelli, 2018<\/a>). This new algorithm, referred to as Combined Inversion of Surface and AeRosol (CISAR) includes the following features:<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n The CISAR algorithm has been successfully applied on data acquired by radiometers on board polar and geostationary orbiting platforms (Luffarelli and Govaerts, 2019<\/a>).<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n The spectral width of the MVIRI\/VIS band represents however a serious challenge for the generation of a physically-based CDR. Radiative transfer theory is formally valid only for monochromatic cases, i.e<\/em>., no important spectral variations of the optical properties of the observed media are expected to take place in the processed spectral intervals.<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n This assumption is clearly violated for the MVIRI\/VIS band as it can be seen on Figure 2. This spectral interval contains some strong gas absorption bands and it also subject to aerosol scattering processes whose magnitude varies with the wavelength. Vegetated surface reflectance exhibits also strong and fast spectral variations over this spectral region as a consequence of the differences in the radiation transfer regimes occurring on both sides of 0.7 \u03bcm, i.e<\/em>., mainly governed by absorption (scattering) at wavelengths shorter (larger) than 0.7 \u03bcm. Conversely, atmospheric radiative processes are dominated by scattering (absorption) at wavelengths shorter (larger) than 0.7 \u03bcm. \u00a0<\/p>\r\n \r\n\r\n<\/p>\r\n It is therefore not possible to perform a retrieval in that spectral interval based on the radiative transfer theory assuming a monochromatic spectral interval, i.e<\/em>., with invariant optical properties. As an effect, the calculation of the Top Of Atmopshere (TOA) Bidirectional Reflectance Factor<\/a> (BRF) in the MVIRI\/VIS band \\tilde r_p =\\int_\\lambda r(\\lambda) <\/span> expressed as<\/p>\r\n \r\n\r\n \\tilde r_p = \\int_\\lambda r(\\Omega; \\omega_0(\\lambda), \\tau(\\lambda), \\rho_0(\\lambda), k(\\lambda), \\Theta(\\lambda), h(\\lambda))<\/span> \r\n\r\n<\/p>\r\n differs from the one expressed as<\/p>\r\n \r\n\r\n \\tilde r_m = r(\\Omega;\\int_\\lambda \\omega_0(\\lambda),\\int_\\tau (\\lambda),\\int_\\lambda\\rho_0 (\\lambda),\\int_\\lambda k(\\lambda),\\int_\\lambda\\Theta a(\\lambda),\\int_\\lambda h(\\lambda)<\/span> \r\n\r\n<\/p>\r\n where<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n This difference is illustrated in Figure 3 where the blue curve represents the TOA BRF \\tilde{r}_p <\/span> calculated for one day of MVIRI\/VIS band observation over a tropical forest and the red curve shows the corresponding BRF \\tilde{r}_m <\/span> value.<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n A workaround has been implemented in the CISAR algorithm to address this issue that relies on two successive corrections. A black surface is first assumed and the VIS band single scattering albedo<\/p>\r\n \r\n\r\n \\tilde{\\omega}_0 = \\int_\\lambda \\omega_0(\\lambda) <\/span> \r\n\r\n<\/p>\r\n of the FASTRE scattering layer is adjusted to minimize the difference between \\tilde{r}_m <\/span> and \\tilde{r}_p <\/span>.<\/p>\r\n \r\n\r\n<\/p>\r\n The correction factor \\gamma <\/span> of the \\tilde{\\omega}_0 <\/span> value adjustment is estimated for different values of the water vapour total column concentration and aerosol optical thickness \\tilde{\\tau} <\/span>.<\/p>\r\n \r\n\r\n<\/p>\r\n The second correction step concerns the magnitude of the surface BRF \\tilde{\\rho}_0 <\/span> . This correction is also estimated so that \\tilde{r}_m \\rightarrow \\tilde{r}_p <\/span>. As it is not possible to know a priori<\/em> the surface spectral variations within the MVIRI\/VIS band, the correction was determined for a large number of pre-calculated surface reflectance spectral variations.<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n MVIRI\/VIS FCDR data are accumulated during the course of one day to form a multi-angular observation vector. The CISAR algorithm is applied on this observation vector to derive:<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n These values are derived on a daily basis at the native MVIRI\/VIS band pixel resolution.<\/p>\r\n \r\n\r\n<\/p>\r\n A level-3 CDR is generated from these values aggregating them on a monthly basis at one-degree resolution (Figure 4). Over ocean surface, the MVIRI AOT compare favourably with the one derived from MODIS observations to the exception of Antartic and Groenland where very high optical thickness are derived. Over land surfaces, the AOT derived in the MVIRI\/VIS band exceeds the MODIS one.<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n The MVIRI\/VIS band FCDR<\/a>\u00a0presents decisive advantages for the generation of a CDR with respect to the original Meteosat First Generation level 1.5 data in the so-called RECT2LP format. This original format contains only the digital count values. The quantitative use of these data required significant efforts to generate the corresponding TOA BRF from the digital count values. Viewing geometry is also missing is that format. The new MVIRI\/VIS band FCDR provides to the users directly TOA BRF values accounting for the reconstructed sensor spectral response<\/a>, together with the illumination and viewing geometries and acquisition time. The availability of detailed pixel-based information on the various types of radiometric uncertainties represents a significant breakthrough of this new FCDR. Such information is indeed of critical importance for retrievals performed in an Optimal Estimation framework as is the case with the CISAR algorithm.<\/p>\r\n \r\n\r\n<\/p>\r\n \r\n\r\n<\/p>\r\n The use of reconstructed VIS band spectral responses accounting for its spectral ageing is undoubtedly the most important benefit of this FCDR. Figure 5 and 6 illustrate the difference in AOT retrieval based on original RECT2LP and FCDR data. The CISAR algorithm has been applied on two time series of MVIRI observations acquired over AERONET<\/a> stations corresponding to water and vegetated surface types. The first time series is directly based on the original RECT2LP format where data have been calibrated with the original pre-launch sensor spectral function<\/a>. The second time series is based on FCDR relying on the reconstructed sensor spectral function<\/a> accounting\u00a0 for its spectral ageing. Over water surface the correlation coefficient between the MVIRI AOT values and the AERONET ones increases from 0.38 with the original sensor spectral function up to 0.72 with the reconstructed ones. There is also a benefit over vegetated \u00a0surfaced thought it is less pronounced as over water surfaces.<\/p>\r\n\r\n
![\"\"](\"http:\/\/35.193.178.118\/wp-content\/uploads\/2019\/07\/aerosol-and-albedo-fig1.png\")
\r\nThe scientific issues<\/h2>\r\n
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\r\nResults<\/h2>\r\n
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![\"\"](\"http:\/\/35.193.178.118\/wp-content\/uploads\/2019\/07\/aerosol-and-albedo-fig4-1024x444.png\")
\r\nFCDR benefits for CDR generation<\/h2>\r\n
![\"\"](\"http:\/\/35.193.178.118\/wp-content\/uploads\/2019\/07\/aerosol-and-albedo-fig5.png\")
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![\"\"](\"http:\/\/35.193.178.118\/wp-content\/uploads\/2019\/07\/aerosol-and-albedo-fig6.png\")
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