TIR retrievals of CO2, N2O and CH4 over the Arctic ocean from TANSO-FTS and comparison with IASI
Résumé
The TANSO-FTS thermal infrared (TIR) spectra of GOSAT in band B4 are quite interesting to provide additional atmospheric information on what is available from the SWIR (B3 and B2) and NIR bands (B1). In this presentation the retrieval scheme based on the LARA radiative transfer and inversion algorithm will be presented for the determination of XCO2, XN2O and XCH4 from spectra recorded over the Arctic ocean during summer. This choice was made to characterize the TANSO-FTS performances in rather difficult conditions (large solar zenith angle (SZA) and cold surface), pushing the performances of TIR sounders to the limit. It also allows us to compare the performances of TANSO-FTS (high spectral resolution for the nominal L1B product of ~ 0.25 cm-1) with those of the lesser resolved Fourier transform space instrument IASI (L1C product of ~ 0.5 cm-1 resolution), which has a better noise figure. Comparisons of the two sounders are possible at high latitude where simultaneous off-nadir observations (SONO) are possible. Comparisons of the expected theoretical performances will be presented together with the actual performances on nearly coinciding SONO footprints (or IFOVs) of the two TIR sounders over water. The IFOVs where carefully filtered for clouds and the choice of Arctic ocean observations is reducing the uncertainties related to IFOV homogeneity. The retrieved surface temperatures Tsurf is used as a common diagnostic and is also compared to the L2 Eumetsat IASI product. A progress report on the issues related to the knowledge of the TANSO-FTS noise and instrument line shape (ILS) as well as spectroscopic issues with line-by-line parameters or heavy molecule cross-sections will be presented. Results will be presented for 2010 (the period of the SCORE-MIP exercises) and possibly for 2013 (to demonstrate that trends are indeed detectable with TANSO-FTS on GOSAT).