Preprints
https://doi.org/10.5194/amt-2021-232
https://doi.org/10.5194/amt-2021-232

  07 Sep 2021

07 Sep 2021

Review status: this preprint is currently under review for the journal AMT.

Emissivity Retrievals with FORUM's End-to-end Simulator: Challenges and Recommendations

Maya Ben-Yami1, Hilke Oetjen1, Helen Brindley4, William Cossich3, Dulce Lajas1, Tiziano Maestri3, Davide Magurno3, Piera Raspollini5, Luca Sgheri2, and Laura Warwick4 Maya Ben-Yami et al.
  • 1ESA – ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
  • 2IAC – CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy
  • 3Università di Bologna – Dipartimento di Fisica e Astronomia, Viale Berti Pichat 6/2, 40126 Bologna, Italy
  • 4Space and Atmospheric Physics Group, Department of Physics, Imperial College London, SW7 2AZ, United Kingdom
  • 5IFAC – CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy

Abstract. Spectral emissivity is a key property of the Earth surface of which only very few measurements exist so far in the far-infrared (FIR) spectral region, even though recent work has shown its FIR contribution is important for accurate modelling of global climate. The European Space Agency's 9th Earth Explorer, FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) will provide the first global spectrally resolved measurements of the Earth's top-of-the-atmosphere (TOA) spectrum in the FIR. In clear-sky conditions with low water vapour content, these measurements will provide a unique opportunity to retrieve spectrally resolved FIR surface emissivity. In preparation for the FORUM mission with an expected launch in 2026, this study takes the first steps towards the development of an operational emissivity retrieval for FORUM by investigating the sensitivity of the emissivity product of a full spectrum optimal estimation retrieval method to different physical and operational parameters. The tool used for the sensitivity tests is the FORUM mission's end-to-end simulator. These tests show that spectral emissivity of most surface types can be retrieved for dry scenes in the 350–600 cm−1 region with an uncertainty ranging from 0.005 to 0.01. In addition, the quality of retrieval is quantified with respect to the precipitable water vapour content of the scene, and the uncertainty caused by the correlation of emissivity with surface temperature is investigated. Two main recommendations are made based on these investigations: (1) As the extent of TOA sensitivity to the surface in the FIR depends on the atmospheric state, the spectral region of the emissivity product should be decided using a so-called information quantifier, calculated from the ratio of the retrieval uncertainty to the a-priori uncertainty. (2) Depending on retrieval input parameters, the correlation of emissivity with surface temperature allows for retrieved emissivities within a small range around the true emissivity. Thus the impact of this correlation on the uncertainty estimates of the product should be quantified in detail during further development of the operational retrieval.

Maya Ben-Yami et al.

Status: open (until 13 Oct 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-232', Anonymous Referee #1, 08 Sep 2021 reply
  • RC2: 'Comment on amt-2021-232', Anonymous Referee #2, 13 Sep 2021 reply

Maya Ben-Yami et al.

Maya Ben-Yami et al.

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Short summary
Spectral emissivity is a key property of the Earth surface. Few measurements exist in the far-infrared, despite recent work showing its contribution is important for accurate modelling of global climate. In preparation for ESA’s EE9 FORUM mission (launch 2026), this study takes first steps towards the development of an operational emissivity retrieval for FORUM by investigating the sensitivity of the emissivity product to different physical and operational parameters.