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The long-term comparison between simulated and observed spectrally resolved outgoing longwave radiation (OLR) can represent a stringent test for the direct verification and improvement of general circulation models (GCMs), which are regularly tuned by adjusting parameters related to subgrid processes not explicitly represented in the model to constrain the integrated OLR energy fluxes to observed values. However, a good agreement between simulated and observed integrated OLR fluxes may be obtained from the cancellation of opposite-in-sign systematic errors localized in specific spectral ranges. Since the mid-2000s, stable hyperspectral observations of the mid-infrared region (667 to 2750 cm−1) of the Earth emission spectrum have been provided by different sensors (e.g. AIRS, IASI and CrIS). Furthermore, the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission, selected to be the ninth ESA Earth Explorer, will measure, starting from 2027, the terrestrial radiation emitted to space at the top of the atmosphere (TOA) from 100 to 1600 cm−1, filling the observational gap in the far-infrared (FIR) region, from 100 to 667 cm−1. In this work, in anticipation of FORUM measurements, we compare Infrared Atmospheric Sounding Interferometer (IASI) Metop-A observations to radiances simulated on the basis of the atmospheric fields predicted by the EC-Earth Global Climate Model (version 3.3.3) in clear-sky conditions. To simulate spectra based on the atmospheric and surface state provided by the climate model, the radiative transfer model σ-IASI has been integrated in the Cloud Feedback Model Intercomparison Project (COSP) package. Therefore, online simulations, provided by the EC-Earth model equipped with the new COSP–σ-IASI module, have been performed in clear-sky conditions with prescribed sea surface temperature and sea ice concentration, every 6 h, over a time frame consistent with the availability of IASI data. Systematic comparisons between observed and simulated brightness temperature (BT) have been performed in 10 cm−1 spectral intervals, on a global scale over the ocean, with a specific focus on the latitudinal belt between 30∘ S and 30∘ N. The analysis has shown a warm BT bias of about 3.5 K in the core of the CO2 absorption band and a cold BT bias of approximately 1 K in the wing of the CO2 band, due to a positive temperature bias in the stratosphere and a negative temperature bias in the middle troposphere of the climate model, respectively. Finally, considering a warm BT bias in the rotational–vibrational water vapour band, we have highlighted a dry bias of the water vapour concentration in the upper troposphere of the model.

The long-term comparison between simulated and observed spectrally resolved outgoing longwave radiation (OLR) can represent a stringent test for the direct verification and improvement of general circulation models (GCMs), which are regularly tuned by adjusting parameters related to subgrid processes not explicitly represented in the model to constrain the integrated OLR energy fluxes to observed values. However, a good agreement between simulated and observed integrated OLR fluxes may be obtained from the cancellation of opposite-in-sign systematic errors localized in specific spectral ranges. Since the mid-2000s, stable hyperspectral observations of the mid-infrared region (667 to 2750 cm−1) of the Earth emission spectrum have been provided by different sensors (e.g. AIRS, IASI and CrIS). Furthermore, the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission, selected to be the ninth ESA Earth Explorer, will measure, starting from 2027, the terrestrial radiation emitted to space at the top of the atmosphere (TOA) from 100 to 1600 cm−1, filling the observational gap in the far-infrared (FIR) region, from 100 to 667 cm−1. In this work, in anticipation of FORUM measurements, we compare Infrared Atmospheric Sounding Interferometer (IASI) Metop-A observations to radiances simulated on the basis of the atmospheric fields predicted by the EC-Earth Global Climate Model (version 3.3.3) in clear-sky conditions. To simulate spectra based on the atmospheric and surface state provided by the climate model, the radiative transfer model σ-IASI has been integrated in the Cloud Feedback Model Intercomparison Project (COSP) package. Therefore, online simulations, provided by the EC-Earth model equipped with the new COSP–σ-IASI module, have been performed in clear-sky conditions with prescribed sea surface temperature and sea ice concentration, every 6 h, over a time frame consistent with the availability of IASI data. Systematic comparisons between observed and simulated brightness temperature (BT) have been performed in 10 cm−1 spectral intervals, on a global scale over the ocean, with a specific focus on the latitudinal belt between 30∘ S and 30∘ N. The analysis has shown a warm BT bias of about 3.5 K in the core of the CO2 absorption band and a cold BT bias of approximately 1 K in the wing of the CO2 band, due to a positive temperature bias in the stratosphere and a negative temperature bias in the middle troposphere of the climate model, respectively. Finally, considering a warm BT bias in the rotational–vibrational water vapour band, we have highlighted a dry bias of the water vapour concentration in the upper troposphere of the model.

In the frame of Earth observation remote-sensing data analysis, synergistic retrieval (SR) and complete data fusion (CDF) are techniques used to exploit the complementarity of the information carried by different measurements sounding the same air mass and/or ground pixel. While more difficult to implement due to the required simultaneous access to measurements originating from different instruments, the SR method is sometimes preferred over the CDF method as the latter relies on a linear approximation of the retrieved states as functions of the true atmospheric and/or surface state. In this work, we study the performance of the SR and CDF techniques when applied to simulated measurements of the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) and the Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG) missions that will be operational in a few years, from two polar-orbiting satellites. The study is based on synthetic measurements generated for the two missions in clear-sky atmospheres. The target parameters of the inversion are the vertical profiles of temperature, water vapor and ozone mixing ratios, surface temperature, and spectral emissivity. We find that for exact matching of the measurements, the results of the SR and CDF techniques differ by less than 1/10 of their errors estimated through the propagation of measurement noise. For measurements with a realistic mismatch in space and time, the two methods provide more different results. Still in this case, however, the differences between the results are within the error bars due to measurement noise. We conclude that, when applied to FORUM and IASI-NG missions, the two methods are equivalent from an accuracy point of view.

The new platforms for Earth observation from space are characterized by measurements made at great spatial and temporal resolutions. While this abundance of information makes it possible to detect and study localized phenomena, it may be difficult to manage this large amount of data for the study of global and large-scale phenomena. A particularly significant example is the use by assimilation systems of Level 2 products that represent gas profiles in the atmosphere. The models on which assimilation systems are based are discretized on spatial grids with horizontal dimensions of the order of tens of kilometres in which tens or hundreds of measurements may fall in the future. A simple procedure to overcome this problem is to extract a subset of the original measurements, but this involves a loss of information. Another option is the use of simple averages of the profiles, but this approach also has some limitations that we will discuss in the paper. A more advanced solution is to resort to the so-called fusion algorithms, capable of compressing the size of the dataset while limiting the information loss. A novel data fusion method, the Complete Data Fusion algorithm, was recently developed to merge a set of retrieved products in a single product a posteriori. In the present paper, we apply the Complete Data Fusion method to ozone profile measurements simulated in the thermal infrared and ultraviolet bands in a realistic scenario. Following this, the fused products are compared with the input profiles; comparisons show that the output products of data fusion have smaller total errors and higher information contents in general. The comparisons of the fused products with the fusing products are presented both at single fusion grid box scale and with a statistical analysis of the results obtained on large sets of fusion grid boxes of the same size. We also evaluate the grid box size impact, showing that the Complete Data Fusion method can be used with different grid box sizes even if this possibility is connected to the natural variability of the considered atmospheric molecule.

In the frame of Earth observation remote sensing data analysis, Synergistic Retrieval (SR) and Complete Data Fusion (CDF) are techniques used to exploit the complementarity of the information carried by different measurements sounding the same air mass and / or ground pixel. While more difficult to implement due to the required simultaneous access to measurements originating from different instruments / missions, the SR method is sometimes preferred over the CDF method as the latter relies on a linear approximation of the retrieved states as functions of the true atmospheric and / or surface state. In this work, we study the performance of the SR and CDF techniques when applied to simulated measurements of the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) and the IASI-NG (Infrared Atmospheric Sounding Interferometer - New Generation) missions that will be operational in a few years, from two polar orbiting satellites. The study is based on synthetic measurements generated for the two missions, in clear-sky Antarctic atmospheres. The target parameters of the inversion are the vertical profiles of temperature, water vapour and ozone mixing ratios, surface temperature and spectral emissivity. We find that for exact matching of the measurements, the results of the SR and CDF techniques differ by less than 1/10 of their errors estimated trough the propagation of measurement noise. For measurements with a realistic mismatch in space and time, the two methods provide more different results. Still in this case, however, the differences between the results are within the error bars due to measurement noise. We conclude that, when applied to FORUM and IASI-NG missions, the two methods are equivalent from the accuracy point of view.

In recent years, the availability of a large amount of data has stimulated the use of synergistic approaches to gain the largest amount of information from measurements provided by a great number of satellite missions. Two classes of strategies are widely used to combine multiple sets of independent retrievals of the same atmospheric target: the synergistic retrieval (SR) and the a posteriori combination of the retrieved products. The Complete Data Fusion (CDF) algorithm was presented as a new a posteriori method to combine independent measurements of the same vertical profile of an atmospheric parameter into a single estimate for a concise and complete characterization of the atmospheric state. The CDF is able to reduce the complexity of managing high volumes of data and to improve their quality with respect to the operational outcome of individual instruments. FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment has been selected to be the ninth Earth Explorer mission of the European Space Agency (ESA) and it is scheduled for launch on a polar orbiting satellite in 2027. The core instrument will be a Far-InfraRed (FIR) spectrometer measuring, at nadir, the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum, from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a spectral resolution of 0.5 cm-1. FORUM will fly in loose formation with the MetOp-SG satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). Since IASI-NG will measure the Middle-InfraRed (MIR) part of the upwelling atmospheric spectrum (from 645 to 2760 cm-1), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements. The Level 2 products of FORUM include the water vapour profile, surface spectral emissivity in dry regions, cloud parameters and spectral fluxes. In this work, we present the results obtained from the application of two synergistic approaches, the SR and the CDF, to IASI-NG and FORUM synthetic measurements. We carried out two sets of test retrievals, based on synthetic clear-sky measurements, emulating an idealized situation in which both FORUM and IASI-NG measure, either with perfect matching or with a realistic mismatch, for 900 times, the same area over the Antarctic Plateau surface covered by coarse snow. The retrieval products considered in this study include the profiles of water vapour and temperature, surface temperature and spectral emissivity from 100 to 2300 cm-1. To characterize the quality of synergistic and fused products we evaluate their number of degrees of freedom as well as their total errors from the statistics of the differences between retrieved and true state parameters. Finally we compare the results obtained by the two methods by considering the statistics of the differences between synergistic and fused products. We find that in case of perfectly matching measurements, SR and CDF provide results that differ by less than 1/10 of their associated noise retrieval error. If a realistic mismatch (both in time and space) between the measurements is considered, the two methods provide more different results but with differences that are still within the retrieval error. The results obtained from the two methods are compared and, for both of them, the effect of the mismatch on the synergistic products is quantified.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) will flight as the 9th ESA’s Earth Explorer mission, and an End-to-End Simulator (E2ES) has been developed as a support tool for the mission selection process and the subsequent development phases. The current status of the FORUM E2ES project is presented, together with the characterization of the capabilities of a full physics retrieval code applied to FORUM data. We show how the instrument characteristics and5the observed scene conditions impact on the spectrum measured by the instrument, accounting for the main sources of error related to the entire acquisition process, and the consequences on the retrieval algorithm. Both homogeneous and heterogeneous case studies are simulated in clear and cloudy conditions, validating the E2ES against two independent codes: KLIMA (clear sky) and SACR (cloudy sky). The performed tests show that the performance of the retrieval algorithm is compliant with the project requirements both in clear and cloudy conditions. The far infrared (FIR) part of the FORUM spectrum is shown to be10sensitive to surface emissivity, in dry atmospheric conditions, and to cirrus clouds, resulting in improved performance of the retrieval algorithm in these conditions. The retrieval errors increase with increasing the scene heterogeneity, both in terms of surface characteristics and in terms of fractional cloud cover of the scene.

The knowledge of the water vapor (WV) distribu- tion in the Earth's atmosphere is of great importance for weather prediction. Meteorological models, in particular, the so-called limited area models, can assimilate humidity measurements, increasing the reliability of the simulated atmospheric dynamics. An important improvement can be achieved, for instance, if we are able to provide the total column with a sufficient precision and accuracy. In this paper, the novel normalized differential spectral attenuation (NDSA) approach is applied to retrieve the vertical profile of WV--and thus the total column--from measurements of differential attenuation signals at microwaves. A forward model (FM) has been used to simulate the ray- tracing of a microwave signal from a transmitter to a receiver in the atmosphere by using the 3-D atmospheric parameters as provided by a numerical weather prediction (NWP) model. From the NDSA measurement, the integrated WV (IWV) content can be directly derived. A further retrieval code is able to invert the measurements of IWV along the path length, providing the vertical humidity profile, which is directly related to the total vertical column assimilated by weather prediction models. In this paper, we show that the values of the total column can be retrieved with a precision and accuracy up to about 0.6% and 2.1%, respectively, which could have a positive impact on NWP models at short time scale.

The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, selected for the ESA's 9th Earth Explorer (EE9) mission, will be launched in 2026. For the first time, spectrally resolved radiance observations covering the Far InfraRed (FIR) band from 100 to 667 cm ?¹ with global coverage and with high spectral resolution and radiometric accuracy will be available. This spectral region is highly sensitive to upper-tropospheric-lower-stratospheric (UTLS) water vapor concentration. One of the tools used to support the FORUM proposal during the EE9 competition, and previously, for the data analysis of the REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) balloon observations, is KLIMA (Kyoto protocoL Informed Management of the Adaptation) retrieval code. KLIMA analysed the synthetic FORUM observations in clear sky providing simultaneously the vertical profile of the minor atmospheric gases (such as the water vapour), atmospheric temperature profile, surface temperature, and surface emissivity. During the EE9 phase A selection, KLIMA was employed in various ESA studies such as: FORUM-req for the assessment of the FORUM mission requirements definition; FORUM-E2E for the validation of the End-to-End Simulator (E2ES); FIRMOS for the analysis of the ground-based observations of the FORUM prototype FIRMOS (Far-Infrared Radiation Mobile Observation System). Recently, the KLIMA code was used in the context of the ASI FORUM-science project for the generation of gases optical thicknesses used by the fast code ?-FORUM. In this framework, KLIMA has been upgraded to retrieve also the concentration of the isotopologues of the atmospheric gases. In this work, we will show the assessment of the performance of the retrieval of water vapour isotopologues using the spectral coverage and radiometric accuracy of the FORUM observations, and the comparison with the results obtained when only the middle-infrared information is used. Results both on single measurement and averaged measurements will be shown to evaluate the effect of the radiometric accuracy of the FORUM observations.

FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment will be the ninth Earth Explorer mission of the European Space Agency (ESA), it is scheduled for launch on a Polar satellite in 2026. The FORUM core instrument will be a Fourier Transform Spectrometer covering both the Far-Infrared (FIR) and the Middle-InfraRed (MIR) spectral regions. It will measure, at nadir, the upwelling spectral radiance emitted by the Earth across the most relevant infrared part of the electromagnetic spectrum, from 100 to 1600 cm-1 (from 100 to 6.25 microns in wavelength), with a full-width-half-maximum spectral resolution of 0.5 cm-1. FORUM will fly in loose formation with the MetOp-SG-A satellite, hosting the Infrared Atmospheric Sounding Interferometer - New Generation (IASI-NG). The MIR (645 to 2760 cm-1) upwelling atmospheric spectrum measured by IASI-NG will nicely complement the FORUM FIR spectrum, providing an unprecedented spectral coverage, from 100 to 2760 cm-1. While the FIR part of the spectrum (100-667 cm-1) measured by FORUM is the most sensitive to the water vapour content in the UTLS, the atmospheric windows in the MIR are measured by IASI-NG with a very high signal-to-noise ratio, thus supplying very precise information on the surface temperature and on the temperature profile, that are essential to constrain atmospheric and surface parameter retrievals. In this study, we compare two alternative approaches to determine atmospheric and surface state parameters by exploiting simultaneously both FORUM and IASI-NG measurements. We examine the synergistic retrieval of state parameters from the simultaneous inversion of both measurements, and the a posteriori combination, via the Complete Data Fusion method, of the state parameters retrieved from the individual measurements. We perform test retrievals based on synthetic clear-sky measurements and characterize the quality of synergistic and fused products by means of their error evaluated both from the covariance matrix of the retrieval and from the statistics of the differences between retrieved and true state parameters. The retrieval products considered in this study include the profiles of water vapour and temperature, surface temperature and spectral emissivity from 100 to 2300 cm-1. As expected, we find that the exploitation of IASI-NG measurements permits to improve significantly the quality of FORUM retrieval products. These improvements, however, are progressively lost as soon as the mismatch between the two measurements increases. While synergistic retrieval and Complete Data Fusion provide almost identical results for perfectly matching measurements, differences comparable to the noise error are observed in the presence of time- and / or space- mismatches. We discuss the advantages and disadvantages of each of the two methods and analyze the limits of their applicability.

In a climate change era, measuring atmospheric parameters is fundamental for monitoring the status and predicting the patterns of the atmosphere. For this reason, several space missions, already launched or to be launched by different space agencies are devoted to these measurements. While most key parameters are or will be available, some others are still missing. Among them, the water vapor (WV) in the bottom part of the troposphere, which can contribute to improve numerical weather prediction models (NWP) on short time scales. In order to fill these gaps, a method for obtaining the integrated water vapor (IWV) along a microwave link and based on a pair of attenuation measurements, called NDSA (Normalized Differential Spectral Attenuation), was proposed years ago. The NDSA method is based on measuring a parameter called "spectral sensitivity", which is the normalized incremental ratio of the spectral attenuation and was found to be linearly related to the IWV along the radio-link path. Some studies, supported by ESA, have shown the NDSA capability to effectively estimate the IWV along the path between two Low Earth Orbit (LEO) satellites - one carrying a transmitter, the other a receiver - in a limb measurement geometry, using transmission frequencies in the Ku and K bands for paths crossing the low troposphere (below 10 km), or much higher in case higher tropospheric layers are involved. The ESA studies focused on the retrieval of vertical profiles of IWV and WV in the case of LEO satellites orbiting in the same plane, but along opposite directions (counter-rotating), and only marginally the very different case of the application of the NDSA technique to satellites orbiting in the same plane and along the same direction (co-rotating). The latter is the concept of a recent project, SATCROSS, supported by the Italian Space Agency: the objective is providing a pre-feasibility study for a space remote sensing system based on a train of co-rotating LEO satellites. In such a configuration, in which one or more LEO satellites with an on-board transmitter follow(s) one or more LEO satellites with receiving apparatuses, the NDSA measurements refer to links that cross the troposphere at certain altitudes and "brush" an entire annular region in the orbital plane. It is thus possible to estimate the two-dimensional field of WV in the aforementioned annular region starting from the entire set of IWV measurements based on an inverse problem formulation. Specific SATCROSS activities are: 1) analysis and application of appropriate two-dimensional inversion algorithms; 2) development of an end-to-end simulator; 3) analysis of the impact of the WV products on NWP; 4) definition of the mission characteristics and of the satellites payload, based on Cubesat technology. As a support to these activities, a measurement campaign is being carried out in a ground-to-ground configuration with an instrument able to provide NDSA measurements at 19 GHz. The purpose of this presentation is to illustrate the overall framework of the SATCROSS project, while the details of the related activities are given by companion presentations in ATMOS 2021.

The measure of water vapor (WV) in the lower troposphere is a critical issue, which still leaves important margins for improvements of observational data quality, as necessary for instance to enhance the forecast performance of Numerical Weather Prediction systems.A novel technique developed in recent theoretical ESA studies, called NDSA (Normalized Differential Spectral Attenuation), is based on the measure of differential attenuation of two continuous tones transmitted at 18.8 and 19.2 GHz on a line of sight radio link.These studies proved the good capabilities of the technique in the estimation of the integrated water vapor (IWV), thus fostering further research efforts, including those aimed at developing and testing a demonstrator capable to acquire the first NDSA measurements. We recently designed and implemented a first demonstrator of such technique operating in a ground-to-ground link configuration. The first prototype was upgraded with the realization of an improved transmitter and receiver units to ensure more accurate measurements of IWV. We will report and discuss the results of this activity and the results of a new measurements campaign conducted with the last prototype.The poster presentation will also describe the activities envisaged in the near and mid-term for further advancement of the instrument prototype, such as, in particular, a ground-to-air measurement configuration. The aim of this is a feasibility study for the realization of a system based on the NDSA technique that satisfies the basic requirements laying the foundations for the ultimate ambition of a NDSA space mission deployment.

Remote sensing observations of the composition of Earth's atmosphere are performed with instruments operating on space-borne and airborne platforms, as well as from ground-based stations. In this context, vertical profiles of atmospheric variables are often obtained with an inversion procedure (retrieval) from the observed radiances. When one or more instruments observe the same portion of atmosphere, the information obtained from the different measurements can be combined in order to obtain a unique vertical profile of improved quality with respect to that of the profiles retrieved from the single observations. The most comprehensive way to combine different measurements of the same quantity is considered to be the simultaneous retrieval, in which all the observations are used together as inputs of the retrieval that produces the common profile. However, recently a new method, referred to as Complete Data Fusion 1 , was proposed that, in linear approximation conditions, provides products equivalent to that of the simultaneous retrieval with simpler implementation requirements. Here, we present the first results of the application of the Complete Data Fusion method to measurements of ozone vertical profiles performed by IASI and GOME-2. The inputs of the method are the profiles retrieved from IASI and from GOME-2 measurements characterized by their a priori information, covariance matrices and averaging kernel matrices. The output consists of a single profile also characterized by an a priori information, a covariance matrix and an averaging kernel matrix, which collect all the information content present in the input profiles. The quality of the fused profile is compared with the quality of the original profiles retrieved from IASI and GOME-2 measurements, in terms of errors and number of degrees of freedom. The discussion also faces the strategies adopted to allow the fusion of non-coincident measurements that are characterized by different systematic errors and the implications of these choices in terms of information content of the fused product. Possible follow-ons of this research can be the validation of the results of Complete Data Fusion against collocated ozone-soundings and the comparison of the fused products with the ozone profiles retrieved by simultaneous inversion of IASI and GOME-2 measurements, in order to verify the equivalence between the two approaches.

The Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission will provide a significant opportunity to verify and potentially improve the ability of global circulation models (GCMs) in simulating the Outgoing Longwave Radiation (OLR). Accurate simulation of the OLR is in fact crucial to better constrain the different radiative feedbacks. Planned for launch in 2026, FORUM will measure nadir-looking, spectrally resolved radiances of the Earth's emission spectrum at the top of the atmosphere (TOA) from 100 to 1600 cm-1 filling the existing observational gap of the far-infrared (FIR) region, from 100 to 667 cm-1. In addition, FORUM will fly in loose formation with IASI-NG, which will continue to cover the middle infrared range of IASI from 675 to 2760 cm-1. In anticipation of FORUM measurements, we aim at comparing IASI existing observations to synthetic radiances extracted from the EC-Earth GCM (version 3.3.2), a recent European model based on ECMWF's Integrated Forecasting System (IFS) for the atmosphere-land component and the ocean model NEMO, including sea ice (LIM2) and land surface components (1). Climate model outputs only provide simulated energy fluxes integrated over the whole Earth emission spectrum making difficult the detection of biases and the identification of possible compensation errors in the estimation of the OLR. Conversely, comparing simulated to observed spectra, allows to point out the potential model criticalities in specific spectral bands which contain the signatures of particular climate variables. In order to extract simulated spectra from the climate model, EC-Earth has been equipped with COSP, a simulator package able to map the model state into synthetic observations from different satellite-borne active (CloudSat (radar) and CALIPSO (lidar)) and passive (ISCCP, MISR and MODIS) (2) sensors. We have further developed the package by implementing inside COSP the radiative transfer model ?-FORUM, a monochromatic code able to reproduce synthetic radiances in the Far-Infrared and Mid-Infrared regions compatible with future FORUM and existing IASI observations (3). Due to the high computation cost of the operation, the efficiency of the EC-Earth model equipped with the new COSP module has been improved by modifying the ?-FORUM original code structure and by reducing the original resolution of the model. Therefore, simulations provided by EC-Earth model equipped with the new COSP + ?-FORUM module have been performed with prescribed sea surface temperature and sea-ice cover every 6 hours, over a timeframe consistent with the availability of IASI METOP-A L1C data, from 2007 to 2016. The comparison between nadir radiances simulated by the EC-Earth climate model and the climatology built from ten years of IASI observations represents a very high confidence test for the direct verification and improvement of the GCM avoiding potential uncertainties related to application of retrieval methods. The same approach could be extended to other climate models and, in the near future, it will involve FORUM measurements for a comprehensive analysis of the climate model ability in reproducing the whole Earth emission spectrum. References 1. Hazeleger, Wilco, et al. "EC-Earth: a seamless earth-system prediction approach in action." Bulletin of the American Meteorological Society 91.10 (2010): 1357-1364. 2. Bodas-Salcedo, Alejandro, et al. "COSP: Satellite simulation software for model assessment." Bulletin of the American Meteorological Society 92.8 (2011): 1023-1043. 3. Amato, Umberto, et al. "Corrigendum to: "The sigma-IASI code for the calculation of infrared atmospheric radiance and its derivatives" [Environmental Modelling & Software (2002) 17 (7) 651-667]." Environmental Modelling and Software 18.1 (2003): 97

Remote sounding of atmospheric composition makes use of satellite measurements with very heterogeneous characteristics. In particular, the determination of vertical profiles of gases in the atmosphere can be performed using measurements acquired in different spectral bands and with different observation geometries. The most rigorous way to combine heterogeneous measurements of the same quantity in a single Level 2 (L2) product is simultaneous retrieval. The main drawback of simultaneous retrieval is its complexity, due to the necessity to embed the forward models of different instruments into the same retrieval application. To overcome this shortcoming, we developed a data fusion method, referred to as Complete Data Fusion (CDF), to provide an efficient and adaptable alternative to simultaneous retrieval. In general, the CDF input is any number of profiles retrieved with the optimal estimation technique, characterized by their a priori information, covariance matrix (CM), and averaging kernel (AK) matrix. The output of the CDF is a single product also characterized by an a priori, a CM and an AK matrix, which collect all the available information content. To account for the geo-temporal differences and different vertical grids of the fusing profiles, a coincidence and an interpolation error have to be included in the error budget.In the first part of the work, the CDF method is applied to ozone profiles simulated in the thermal infrared and ultraviolet bands, according to the specifications of the Sentinel 4 (geostationary) and Sentinel 5 (low Earth orbit) missions of the Copernicus program. The simulated data have been produced in the context of the Advanced Ultraviolet Radiation and Ozone Retrieval for Applications (AURORA) project funded by the European Commission in the framework of the Horizon 2020 program. The use of synthetic data and the assumption of negligible systematic error in the simulated measurements allow studying the behavior of the CDF in ideal conditions. The use of synthetic data allows evaluating the performance of the algorithm also in terms of differences between the products of interest and the reference truth, represented by the atmospheric scenario used in the procedure to simulate the L2 products. This analysis aims at demonstrating the potential benefits of the CDF for the synergy of products measured by different platforms in a close future realistic scenario, when the Sentinel 4, 5/5p ozone profiles will be available.In the second part of this work, the CDF is applied to a set of real measurements of ozone acquired by GOME-2 onboard the MetOp-B platform. The quality of the CDF products, obtained for the first time from operational products, is compared with that of the original GOME-2 products. This aims to demonstrate the concrete applicability of the CDF to real data and its possible use to generate Level-3 (or higher) gridded products.The results discussed in this presentation offer a first consolidated picture of the actual and potential value of an innovative technique for post-retrieval processing and generation of Level-3 (or higher) products from the atmospheric Sentinel data.

In the context of a growing need for innovatory techniques to take advantage of the largest amount of information from the great number of available remote sensing data, the Complete Data Fusion (CDF) algorithm was presented as a new method to combine independent measurements of the same vertical profile of an atmospheric parameter into a single estimate for a concise and complete characterization of the atmospheric state. The majority of the atmospheric composition measurements determine the altitude distribution of a great number of quantities: multi-target retrievals (MTRs) are increasingly applied to remote sensing observations to determine simultaneously atmospheric constituents with the purpose to reduce the systematic error caused by interfering species. In this work, we optimised the CDF for the application to MTR products. We applied the method to simulated retrievals in the thermal infrared and in the far infrared spectral ranges, considering the instrumental specifications and performances of IASI-NG (Infrared Atmospheric Sounding Interferometer New Generation) and FORUM (Far-Infrared Outgoing Radiation Understanding and Monitoring) instruments, respectively. The obtained results show that the CDF algorithm can cope with state vectors from MTRs, that must share at least one retrieved variable. In particular, the results show that the fused profile has the greatest number of degrees of freedom and the smallest error for all considered cases. The comparison between the CDF products and the synergistic retrieval ones shows the equivalence of the two methods when the linear approximation is adopted to simplify the treatment of the retrieval problem.

The outgoing longwave radiation (OLR) emitted to space is a fundamental component of the Earth’s energy budget. There are numerous, entangled physical processes that contribute to OLR and that are responsible for driving, and responding to, climate change. Spectrally resolved observations can disentangle these processes, but technical limitations have precluded accurate space-based spectral measurements covering the far infrared (FIR) from 100 to 667 cm−1 (wavelengths between 15 and 100 µm). The Earth’s FIR spectrum is thus essentially unmeasured even though at least half of the OLR arises from this spectral range. The region is strongly influenced by upper-tropospheric–lower-stratospheric water vapor, temperature lapse rate, ice cloud distribution, and microphysics, all critical parameters in the climate system that are highly variable and still poorly observed and understood. To cover this uncharted territory in Earth observations, the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has recently been selected as ESA’s ninth Earth Explorer mission for launch in 2026. The primary goal of FORUM is to measure, with high absolute accuracy, the FIR component of the spectrally resolved OLR for the first time with high spectral resolution and radiometric accuracy. The mission will provide a benchmark dataset of global observations which will significantly enhance our understanding of key forcing and feedback processes of the Earth’s atmosphere to enable more stringent evaluation of climate models. This paper describes the motivation for the mission, highlighting the scientific advances that are expected from the new measurements.

Earth's emitted energy in the wavelength range 4 ?m-100 ?m is commonly termed Outgoing Longwave radiation (OLR) and its accurate measurement is a first-priority requirement to assess the energy balance of the planet. Integrated broadband OLR fluxes provided by CERES energy balanced and filled (EBAF) dataset constitutes the standard reference to study the Earth's energy budget and consequently represents the target values of energy fluxes at top of atmosphere (TOA) for climate models tuning. Even if tuning operations guarantee a good agreement on a global basis between observed and simulated OLR at TOA, large regional biases are present all over the globe and a significant spread exists between different climate model simulations. The Far-Infrared Outgoing Radiation and Monitoring (FORUM) mission, planned for launch in 2026, will provide nadir-looking spectrally resolved radiance measurements of the Earth's emission spectrum at TOA from 100 to 1600 cm-1 (6.25 ?m-100 ?m) with a resolution better than 0.50 cm-1. For the first time FORUM measurements will fill the existing observational gap in the far-infrared (FIR) radiation range from 100 to 667 cm-1 (15 ?m-100 ?m) also providing spectrally resolved OLR fluxes (level 2 data product). In anticipation of FORUM measurements, we want to quantify the potential improvement of climate models in simulating OLR fluxes at TOA due to the future availability of spectrally resolved fluxes data from FORUM, extending to the Far InfraRed. At this stage, we focus on the analyses of simulations provided by EC-Earth (version 3.3.2), a new climate model developed by the joint effort of 27 research institutes of 10 European countries which has directly contributed to Coupled Model Intercomparison (CMIP) in its phases 5 and 6. The EC-Earth model is based on ECMWF's Integrated Forecasting System (IFS) for the atmosphere-land component and the ocean model NEMO, including sea ice (LIM2) and land surface components. Atmospheric profiles and information on the surface and on the clouds produced by EC-Earth simulations in realistic but simplified conditions are provided in input to SIGMA-IASI, a radiative transfer model able to calculate spectral radiances compatible with those that will be measured by FORUM. From these synthetic measurements, we estimate OLR fluxes by applying angular distribution models (ADM) of increasing complexity. The comparison between the OLR fluxes provided by the EC-Earth simulation and those calculated from synthetic FORUM measurements may allow to identify potential improvements for the parametric radiative module included in EC-Earth to estimate the OLR radiative fluxes.

FIRMOS (Far-Infrared Radiation Mobile Observation System) is a Fourier transform spectrometer covering the range from 100 to 1000 cm-1, developed to support the preparation of the satellite mission FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) planned in 2025/2026 (Earth Explorer 9 - ESA). FIRMOS was deployed at the Alpine observatory of Mt. Zugspitze, in the South of Germany, at about 3000 m a.s.l. for 2 months (December 2018 and January-February 2019). In January-February 2019, FIRMOS acquired about 600 measurements of the downwelling spectral radiance in clear sky condition. In this presentation, we present results of the retrieval of the water vapour and temperature profiles obtained from Level 2 analysis of the FIRMOS measurements in clear sky condition. Dedicated and coincident measurements allowed us to perform a comparison with integrated water vapour obtained from E-AERI (Extended-range Atmospheric Emitted Radiance Interferometer) at Zugspitze summit station and with radio-sounding launched at Garmisch-Partenkirchen. Comparisons with further independent information are also shown. Acknowledgments. The authors gratefully acknowledge the funding support by the European Space Agency with the FIRMOS project (ESA-ESTEC Contract No. 4000123691/18/NL/LF) and the Italian Space Agency with the research projects SCIEF (Italian acronym of Development of the National Competences for the FORUM experiment - ASI contract No. 2016-010-U.0)

The complete data fusion method, generalized to the case of fusing profiles of atmospheric variables retrieved on different vertical grids and referred to different true values, is applied to ozone profiles retrieved from simulated measurements in the ultraviolet, visible, and thermal infrared spectral ranges for the Sentinel-4 and Sentinel-5 missions of the Copernicus program. In this study, the production and characterization of combined low Earth orbit (Sentinel-5) and geostationary Earth orbit (Sentinel-4) fused ozone data is performed. Fused and standard products have been compared and a performance assessment of the generalized complete data fusion is presented. The analysis of the output products of the complete data fusion algorithm and of the standard processing using quality quantifiers demonstrates that the generalized complete data fusion algorithm provides products of better quality when compared with standard products.

The new platforms for Earth observation from space are characterized by measurements made with great spatial and temporal resolution. While this abundance of information makes it possible to detect and study localized phenomena, on the other hand it may be difficult to manage this large amount of data in the study of global and large scale phenomena. A particularly significant example is the use by assimilation systems of level 2 products that represent gas profiles in the atmosphere. The models on which assimilation systems are based are discretized on spatial grids with horizontal dimensions of the order of tens of kilometres in which tens or hundreds of measurements may fall. A simple procedure to overcome this problem is to extract a subset of the original measurements. However, this procedure involves a loss of information and is therefore justifiable only as a temporary solution. A more refined solution is to resort to the so-called fusion algorithms, capable of compressing the size of the dataset limiting the information loss. A novel data fusion method, the Complete Data Fusion, was recently developed to merge a-posteriori a set of retrieved products in a single one. In the present paper, the Complete Data Fusion method is applied to ozone profile measurements simulated in the thermal infrared and ultraviolet bands, in a realistic scenario, according to the specifications of the Sentinel 4 and 5 missions of the Copernicus programme. Then the fused products are compared with the input profiles; comparisons show that the output products of data fusion have in general smaller errors and higher information contents. The most significant improvement is an increased vertical resolution together with a reduction of the errors. The comparisons of the fused with the fusing products are presented both at single fusion grid-box scale and with a statistical analysis. The grid box size impact was also evaluated, showing that the Complete Data Fusion method can be used with a wide range of grid-box size, the quality of the products improving with larger grid boxes.

HAPS: fixed WINGS or LTA : limits and potentiality The presentation will discuss the limit and potentiality of the typology of two different HAPs. A fixed wings and a LTA platform. The two performances will be compared in the specific case of potential use of two proposed HAP in the framework of two different systems for on-going project and studies for deployment from space of innovative observation systems and concepts aimed at monitoring of the Earth's atmosphere. The first one is the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, candidate mission for the Earth Explorer 9 programme, currently under competitive feasibility study. The second is the NDSA (Normalized Differential Spectral Attenuation) measurement of water vapour in the troposphere, an innovative technique for active sounding of atmospheric water vapour between two co-rotating (or counter-rotating) LEO satellites. Possible scenarios will be described in some detail of perspectives to deploy onboard the HAP an airborne version of the FORUM instrument and/or a demonstrator of the NDSA technique applied to water vapour measurements from a flying platform.

In order to ensure global and continuous measurements of atmospheric species, an increasingly great number of space-borne missions, airborne and ground-based campaigns have been providing observations of the Earth's atmosphere from over the past two decades. These measurements are extremely important to improve the accuracy in the vertical profiling of atmospheric variables and are used as input to the physical and chemical models predicting the evolution of the atmospheric state. Measurements of two or more instruments that sound the same portion of atmosphere either in different spectral regions or with different observation geometries, can be combined in order to obtain a single vertical profile of improved quality with respect to that of the profiles retrieved from the single observations. The simultaneous retrieval is the most comprehensive way to combine different measurements of the same quantity but this method requires to integrate different forward models in the same retrieval process and this complexity could be a limit to its application. In recent years a great interest in developing innovative techniques to exploit all the available information from independent observations of the same portion of the atmosphere has been growing to face with the great amount of available satellite data. In this context, the Complete Data Fusion (CDF) was proposed as a new method to combine independent measurements of the same profile into a single estimate for a comprehensive and concise description of the atmospheric state. The CDF is an a posteriori algorithm that requires a very simple implementation and uses standard retrieval products characterized by their a priori information, covariance matrix and averaging kernel matrix. In the analysis of remote sensing observations, multi-target retrievals are frequently applied in order to determine simultaneously atmospheric constituents reducing the systematic error caused by interfering species. It was thus crucial to generalize the CDF algorithm to fuse profiles obtained from multi-target retrievals, extending its use to a greater number of remote sensing data. In this work, we present the results of the first application of the CDF method to multi-target retrieval products showing how to modify the inputs to take into account that the state vectors of the fusing measurements may contain only the same atmospheric variables or include different variables as well. We applied the method to simulated measurements in the thermal infrared and in the far infrared spectral ranges, considering the instrumental specifications and performances of IASI-NG (Infrared Atmospheric Sounding Interferometer - Next Generation) and FORUM (Far Infrared Outgoing Radiation Understanding and Monitoring) instruments, respectively. The results obtained demonstrate that the CDF can deal with state vectors from multi-target retrievals both when they contain the same variables and when they have only a subset of variables in common, providing outputs of improved quality with respect to the input data.

Working with Copernicus Services for our Future Ugo Cortesi National Research Council of Italy (CNR) Atmospheric monitoring (space and ground) Copernicus Space component for the atmosphere Copernicus Atmosphere Monitoring Service (CAMS) Copernicus Climate Change Service (C3S) Air Quality, UV Surface Radiation, Applications, Conclusions Discussed the various Sentinel technologies and their capacities for atmospheric monitoring, showed Sentinel 4 to be launched in 2022 and Sentinel 5 to be launched in 2023 Contributing to - Copernicus Atmosphere Service and Copernicus Climate Change Service Reasons why it's important - satellite technology revealing trends in topics like acid rain, regional smog, climate forcing, disasters, ozone layer Climate forecasting - offers climate change, mitigation and adaptation, weather forecast, pollution, environment, and health. Global and local analyses First Geostationary Satellite Constellation for Air Quality Monitoring in the Northern Hemisphere Air-Portal combines Earth observation data, meteo data, land use information, traffic, in-situ measurements Up to date data on chemicals like NO2 and Ozone - showed an example of the European map of NO2 component of air pollution created with 6 months of Sentinel-5p measurements Conclusions - CAMS and C3S services provide unprecedented quantity and quality of information Wealth of issues from global to local level can be tackled using this information (disasters, air pollution, weather forecasts, climate projections, mitigation and adaptation) The Philippines can largely benefit from Copernicus atmospheric data Free, full, and open data policy applies to the program - "DATA ARE FREE: USE IT"

Remote sensing observations of the atmosphere are performed with instruments operating on space-borne and airborne platforms, as well as from ground-based stations and vertical profiles of atmospheric variables are often obtained with an inversion procedure from the observed radiances. When an instrument or several instruments observe more times the same portion of atmosphere, the information obtained from the different measurements can be combined in order to obtain a single vertical profile of improved quality with respect to that of the profiles retrieved from the single observations. The most comprehensive way to combine different measurements of the same quantity is considered to be the simultaneous retrieval, in which all the observations are used as inputs of a single retrieval algorithm that produces a single profile. However, recently a new method, referred to as Complete Data Fusion, was proposed that, with simple implementation requirements, provides products of quality equivalent to that of the simultaneous retrieval products. The use of the Complete Data Fusion highlighted a problem that we believe to be common to simultaneous retrieval and data fusion. The measurements that we wish to fuse often present some inconsistencies due to three causes: (i) the profiles to be fused (in the following referred to as fusing profiles) are represented on different vertical grids, (ii) a variability is present in the observed species and the fusing profiles refer to different times and space locations and (iii) the fusing profiles are affected by different forward model errors. These inconsistencies may spoil the quality of the fused profile. In order to apply the Complete Data Fusion method to inconsistent measurements without a degradation of the product, it is necessary to add to the error covariance matrix of each fusing profile a covariance matrix that takes into account the inconsistencies. Therefore, the main problem in the fusion of inconsistent measurements is the realistic estimate of these inconsistency covariance matrices. The value of the cost function, which is minimized in the Complete Data Fusion, depends on the inconsistency covariance matrices and can be used to establish some constraints on their amplitude. To this purpose, we have analytically calculated the expected value and the variance of the cost function and used these quantities to define a procedure that estimates the inconsistency covariance matrices. Modelling the inconsistency covariance matrices with one parameter, we determine the value of this parameter that makes the reduced cost function equal to its expected value and use the variance to assign an error to this determination. The use of the Complete Data Fusion will be particularly relevant for the analysis of the future atmospheric Sentinel missions of the Copernicus program. The amount of data that will be available from these missions will pose technical challenges to most applications and the Complete Data Fusion can be used to reduce the number of products while maintaining the information content of the full datasets. For this reason, we test the proposed procedure on simulated measurements of ozone profiles as they could be obtained by the Infrared Sounder operating in the thermal infrared on board the Meteosat Third Generation satellite in the framework of the Sentinel 4 mission. In this context, the procedure is used to estimate the coincidence covariance matrices that take into account the variability of ozone when the fusing profiles refer to different times and space locations. The merits of this new procedure in the case of this specific dataset are presented and discussed.

Tropospheric ozone is a significant greenhouse gas (GHG) formed and destroyed by chemical reactions involving other atmospheric species and a key constituent for understanding the interactions between climate and chemistry. In contrast to most GHGs, tropospheric ozone is produced photo-chemically via its precursors, notably nitrogen oxides (NOx), carbon monoxide (CO), CH4 and non-methane hydrocarbons, all of which have large anthropogenic sources. Crucial uncertainties, however, remain in the assessment of the tropospheric ozone budget, its precursors, and the chemical and physical processes involved. Large spatial and temporal variability is observed in tropospheric ozone, resulting from important regional differences in the factors controlling its concentration. For all these reasons, accurate global measurements of ozone vertical profiles in the troposphere are essential. AURORA (Advanced Ultraviolet Radiation and Ozone Retrieval for Applications) is a three-year project (ending in July 2019) supported by the European Union in the frame of its H2020 Call EO-2-2015. Its overarching objective is to simulate the provision of synergistic data products, having unprecedented accuracy, for the vertical profiling of atmospheric ozone and to assess their quality with respect to the one expected for the operational products of the geostationary (GEO) mission Sentinel-4 and of the Low Earth Orbit (LEO) missions Sentinel-5p and Sentinel-5. In particular, the main scientific purpose of AURORA is to investigate the potential of synergistic exploitation of complementary ozone measurements acquired in different spectral regions - from the UV over the visible to the thermal infrared - through the assimilation of fused GEO and LEO ozone profile products resulting from application of the innovative method of complete data fusion. The impact of combining complementary capabilities of the atmospheric Sentinels observations, especially in terms of vertical sensitivity, might result in advanced performances that are of specific relevance for tropospheric ozone monitoring. In the frame of the project it has been demonstrated that the complete data fusion method is able to reduce the complexity of managing the high volume of Copernicus S-4 and S-5 data and to increase its quality with respect to the operational outcome of individual instruments. In particular, the fused products show higher degrees of freedom and an enhanced vertical sensitivity. In this project, the 3D ozone analyses and forecasts obtained from the data assimilation systems after assimilating the fused ozone observations are used to derive tropospheric ozone columns and profiles. An extensive ozone product validation system has been developed in the frame of the project to assure that the AURORA tropospheric ozone and data products will be warily verified and documented by means of in-depth QA/validation studies. In this presentation we will discuss the first results of the AURORA synergistic approach, based on data fusion and assimilation, applied to the tropospheric ozone profiling.

The primary goal of the FORUM mission is to measure the Earth's top-of-atmosphere emission spectrum in the 100 to 1600 cm-¹ spectral region covering the observational gap of the Far IR (100-667 cm-¹), which has never been observed from space in its entirety. This measurement will provide an improved understanding of the climate system by supplying, for the first time, most of the spectral features of the Far IR contribution to the Earth radiation budget, particularly focusing on the water vapour contribution related to the continuum absorption in the rotational band, cirrus cloud and ice/snow surface emissivity. This goal is achieved by measuring the spectrum with a nominal resolution of 0.3 cm-¹, and with an absolute calibration accuracy of 0.1 K, benchmarked against international standards. A precision of 0.4 mW/m² sr cm-¹ on the radiance will also allow the retrieval of the atmospheric state and cloud parameters with improved accuracy. A demonstration of the FORUM capabilities in improving the characterisation of the atmospheric radiative properties using the Far IR is shown by means of ground-based observations performed at high-altitude sites where the air is sufficiently transparent to sound the Far IR spectral features of the atmospheric emission. To this aim, two instruments are used: the REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) instrument, which has been deployed on the Antarctic Plateau, where it has been acquiring downwelling longwave radiation spectra continuously since 2012, and the FORUM prototype, which will be deployed at the Zugspitze summit at the end of 2018 - early 2019. Both instruments are Fourier transform spectrometers with an optical configuration of the interferometer similar to FORUM. They cover the Far IR spectral region down to 100cm-¹ and can be operated under all sky non-precipitating conditions. In this presentation, a review of the measurement requirements of the FORUM mission will be provided with a description of the measurement results obtained during the high-altitude Antarctic and Zugspitze campaigns.

Not available

Vertical profiles of atmospheric variables are often obtained with the inversion of remote sensing observations performed by instruments operating on space-borne and airborne platforms, as well as from ground-based stations. When the same portion (or nearby portions) of atmosphere is observed more times by the same instrument or by different instruments the measurements can be combined in order to obtain a single vertical profile of improved quality with respect to that of the profiles retrieved from the single observation. The simultaneous retrieval from several observations is considered the most comprehensive way to combine different measurements of the same quantity, however, recently a new method, referred to as Complete Data Fusion, was proposed that, with simple implementation requirements, provides products of quality equivalent to that of the simultaneous retrieval products. The adoption of the Complete Data Fusion has highlighted a problem that we believe to be common to simultaneous retrieval and data fusion. The measurements that we wish to fuse often present some inconsistencies, due to representations on different vertical grids, not perfect time and space coincidence and different forward model errors, which may spoil the quality of the fused profile. In order to apply the Complete Data Fusion method to inconsistent measurements without a degradation of the product, it is necessary to add to the measurement covariance matrices of each fusing profile a covariance matrix that takes into account these inconsistencies. Therefore, the main problem in the fusion of inconsistent measurements is the realistic estimate of these inconsistency covariance matrices. We propose to use the value of the cost function that is minimized in the Complete Data Fusion to constrain the values of the inconsistency covariance matrices. To this purpose, we have analytically calculated the expected value of the cost function and its variance: the result is presented and is used in the proposed procedure. Modelling the inconsistency covariance matrices with one parameter, we tune the value of this parameter using the constraint provided by the requirement of the value of the cost function being equal to the expected value and using the variance to assign an error to the obtained value of the parameter. The use of Complete Data Fusion will be particularly useful for the analysis of the future atmospheric Sentinel missions of the Copernicus program. Indeed, the amount of data that will be available from these missions will pose technical challenges to many applications and the Complete Data Fusion can be used to reduce the number of products while maintaining the information content of the original datasets. For this reason, we test the proposed method on simulated measurements of ozone profiles obtained in the thermal infrared in the framework of the Sentinel 4 mission. In this context, the method is used to estimate the coincidence covariance matrices that take into account the variability of ozone when the fusing profiles refer to different times and space locations. The merits of this new procedure in the case of this specific data set are presented and discussed.

With the launch of the Sentinel-5 Precursor (S-5P, lifted-off on 13 October 2017), Sentinel-4 (S-4) and Sentinel-5 (S-5)(from 2021 and 2023 onwards, respectively) operational missions of the ESA/EU Copernicus program, a massive amount of atmospheric composition data with unprecedented quality will become available from geostationary (GEO) and low Earth orbit (LEO) observations. Enhanced observational capabilities are expected to foster deeper insight than ever before on key issues relevant for air quality, stratospheric ozone, solar radiation, and climate. A major potential strength of the Sentinel observations lies in the exploitation of complementary information that originates from simultaneous and independent satellite measurements of the same air mass. The core purpose of the AURORA (Advanced Ultraviolet Radiation and Ozone Retrieval for Applications) project is to investigate this exploitation from a novel approach for merging data acquired in different spectral regions from on board the GEO and LEO platforms. A data processing chain is implemented and tested on synthetic observations. A new data algorithm combines the ultraviolet, visible and thermal infrared ozone products into S-4 and S-5(P) fused profiles. These fused products are then ingested into state-of-the-art data assimilation systems to obtain a unique ozone profile in analyses and forecasts mode. A comparative evaluation and validation of fused products assimilation versus the assimilation of the operational products will seek to demonstrate the improvements achieved by the proposed approach. This contribution provides a first general overview of the project, and discusses both the challenges of developing a technological infrastructure for implementing the AURORA concept, and the potential for applications of AURORA derived products, such as tropospheric ozone and UV surface radiation, in sectors such as air quality monitoring and health.

Many remote sensing observations of vertical profiles of atmospheric variables are obtained with instruments operating on space-borne and airborne platforms, as well as from ground-based stations. When the same portion (or nearby portions) of atmosphere is observed more times by the same instrument or by different instruments the measurements can be combined in order to obtain a single vertical profile of improved quality with respect to that of the profiles retrieved from the single measurements. Recently, a new method of data fusion, referred to as Complete Data Fusion (CDF), was proposed for use in the combination of independent measurements of the same profile. This is an a posteriori method that uses standard retrieval products and with simple implementation requirements provides products equivalent to those of the simultaneous retrieval, which is considered to be the most comprehensive way of exploiting different observations of the same quantity. As part of the AURORA project, we have applied the CDF method to ozone profiles obtained from simulated measurements in the ultraviolet and in the thermal infrared in the framework of the Sentinel 4 mission of the Copernicus programme. We observe that the quality of the fused products is very good when we fuse consistent profiles, instead the quality is degraded when we fuse profiles that are either retrieved on different vertical grids or referred to different true profiles. In order to address this shortcoming, a generalization of the CDF method, which takes into account interpolation and coincidence errors, was developed. We determine the expressions of these errors and show how they enter in the CDF formula. This upgrade overcomes the encountered problems and provides products of good quality also when the fusing profiles are both retrieved on different vertical grids and referred to different true profiles. The approach developed to account for the interpolation and coincidence errors can also be followed to include other error components, such as forward model errors.

AURORA is a project financed by the European Commission in the framework of the Horizon 2020 Framework Program that concerns the application of fusion and assimilation algorithms to simulated VMR ozone profiles in different spectral bands, according to the specifications of atmospheric sentinels 4 and 5(p). In the context of this project, several verification activities have been carried on to study the behavior of the Complete Data Fusion (CDF) algorithm. One of them consists in applying the CDF algorithm to sets of coincident pixels, simulated with different errors to study the behavior of the fused profile. A detailed analysis will be presented on the behavior of the fused profile, its averaging kernel and its variance covariance matrix, when the number of the input profiles increases.