Abstract
FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) has been approved to be the ninth Earth Explorer mission of the European Space Agency. The mission is scheduled for launch on a Polar satellite in the 2025–2026 time frame. The core FORUM instrument is a Fourier Transform Spectrometer measuring, with very high accuracy, the upwelling spectral radiance, from 100 to 1600 cm − 1 (from 100 to 6.25 microns in wavelength), thus covering the Far-Infrared (FIR), and a Mid-Infrared (MIR) portion of the spectrum emitted by the Earth. FORUM will fly in loose formation with the MetOp-SG-1A satellite, hosting the Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG). IASI-NG will measure only the MIR part of the upwelling atmospheric spectrum, from 645 to 2760 cm − 1 (from 15.5 to 3.62 microns in wavelength), thus, the matching FORUM measurements will supply the missing FIR complement. Together, the two missions will provide, for the first time, a spectrally resolved measurement of the full Earth emitted thermal spectrum. The calibrated spectral radiance will be, on its own, the main product of the FORUM mission, however, the radiances will also be processed up to Level 2, to determine the vertical profile of water vapour, surface spectral emissivity and cloud parameters in the case of cloudy atmospheres. In this paper we assess the performance of the FORUM Level 2 products based on clear-sky simulated retrievals and we study how the FORUM and IASI-NG matching measurements can be fused in a synergistic retrieval scheme, to provide improved Level 2 products. Considering only the measurement noise and the systematic calibration error components, we find the following figures for the synergistic FORUM and IASI-NG retrieval products. In the upper troposphere/lower stratosphere region, individual water vapour profiles can be retrieved with 1 km vertical sampling and an error ranging from 10% to 15%. In the range from 300 to 600 cm − 1 , surface spectral emissivity can be retrieved with an absolute error as small as 0.001 in dry Polar atmospheres. Ice cloud parameters such as ice water path and cloud top height can be retrieved with errors smaller than 10% and 1 km, respectively, for ice water path values ranging from 0.2 to 60 g/m 2 .
Highlights
Model simulations suggest that around 50% of the total clear-sky long-wave cooling of the Earth to space occurs within the Far InfraRed (FIR) spectral region [1,2], conventionally between 100 and 667 cm−1 or between 100 and 15 microns in wavelength
The Jacobians K1 and K2 are obtained by convolving with the respective FORUM and Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG) apodized Instrument Spectral Response Function (ISRF)’s, the high resolution Jacobians computed by the Linearized Discrete Ordinate Radiative Transfer (LIDORT) radiative transfer model [20], with gases optical depth computed by the Line-By-Line Radiative Transfer Model (LBLRTM) [26]
In the first part of this work we assess the performance of the Level 2 products expected from FORUM clear-sky measurements
Summary
Model simulations suggest that around 50% of the total clear-sky long-wave cooling of the Earth to space occurs within the Far InfraRed (FIR) spectral region [1,2], conventionally between 100 and 667 cm−1 or between 100 and 15 microns in wavelength. For technological reasons, to date the FIR upwelling radiance from the atmosphere and surface has never been measured spectrally resolved from space To cover this long-standing observational gap, in September 2019 the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment has been approved to be the ninth Earth Explorer mission of the European Space Agency (ESA, 2019 [10]). Since IASI-NG will measure the Middle-InfraRed (MIR) part of the upwelling atmospheric spectrum (from 645 to 2760 cm−1, or from 15.5 to 3.62 microns in wavelength), the matching FORUM FIR spectra can be effectively used to synergistically complement the IASI-NG measurements which, as compared to FIR measurements, are less sensitive to the water vapour content in the Upper Troposphere/Lower Stratosphere (UTLS), an important driver of the outgoing long-wave radiation flux [11,12].
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