Sensitivity study of CH4 retrievals in the from satellite observations in the TIR spectral domain

Charles Robert, Justin Erwin, Bastien Vispoel, ANN CARINE VANDAELE, Séverine Robert, Martine De Mazière, Claude Camy-Peyret, Pascal Prunet, Yannick Kangah, Anne Grete Straume

Research output: Contribution to conferenceOtherpeer-review

Abstract

Atmospheric methane is measured continuously from space, providing valuable information at global/regional scales for atmospheric monitoring as well as for surface flux estimates. However, CH4 atmospheric concentration retrievals from TIR nadir sensors exhibit significant biases compared to independent observations, or when intercompared between different TIR and SWIR/TIR sensors products. It is necessary to analyse the possible causes of biases and to investigate potential retrieval improvements and/or bias-correction for proper and consistent CH4 measurements in the TIR.

In the framework of the ESA CH4TIR project, possible sources of CH4 biases in retrievals from TIR sensors have been investigated. In this work, we present a comprehensive sensitivity analysis carried out for the CH4 retrievals performed with IASI and TANSO-FTS data in the TIR region using ASIMUT. We assess the impact of the retrieval spectral range, the measurement uncertainty, uncertainties in the spectroscopic data, and the inclusion of different species in the retrieval. An exhaustive analysis of the retrieval residuals will be presented, along with the sensitivity of the CH4 retrieved profiles and vertical columns, in order to quantify the relative impact of these different retrieval parameters.

An analysis of the IASI spectral residuals shows that residuals are largest in the strongest part of the Q branch (1300-1310 cm-1), where line-mixing effects are most significant. To further improve the agreement with the measurements, we include line-mixing for methane in the forward radiative transfer model (RTM) in the TIR. We will present the work done to model the methane line-mixing effects (1st order approximation) and will compare the effect of the model improvements by using spectroscopic parameters from three sources: a) Tran et al. (2006) published values of line-mixing, b) HITRAN 2016 without line-mixing, and c) our effort to merge these two sources. We will show that implementing the line-mixing effects in the RTM leads to a better agreement with the observations near the strongest part of the Q branch.
Original languageEnglish
Publication statusPublished - 2021
EventThe 17th international workshop on greenhouse gas measurements from space - Virtual meeting
Duration: 14 Jun 202117 Jun 2021

Workshop

WorkshopThe 17th international workshop on greenhouse gas measurements from space
Period14/06/2117/06/21

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