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.
Synergistic Retrieval and Complete Data Fusion applied to FORUM and IASI-NG simulated measurements
Marco Ridolfi;Cecilia Tirelli;Simone Ceccherini;Ugo Cortesi;Luca Palchetti
2021
Abstract
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
