Comment on amt-2022-129

Abstract

The Thermal and Near-Infrared Sensor for Carbon Observation Fourier-Transform Spectrometer-2 (TANSO-FTS-2) onboard the Japanese Greenhouse gases Observing SATellite-2 (GOSAT-2) observes a wide spectral region of the atmosphere, from the ShortWave-InfraRed (SWIR) to longwave Thermal InfraRed radiation (TIR) with 0.2 cm-1 spectral intervals. The TANSO-FTS-2 has operated nominally since Feb 2019, and the atmospheric radiance spectra it has acquired have been released to the public. This paper describes an updated model for spectral radiance calibration and its validation. The model applies to the version 210210 TIR products of the TANSO-FTS-2 and integrates polarization sensitivity correction for the internal optics and the scanner mirror thermal emission. These correction parameters are characterized by an optimization which depends on the difference between the spectral radiance of the TANSO-FTS-2 and coincident nadir observation data from the Infrared Atmospheric Sounding Interferometer (IASI) on METOP-B. To validate the updated spectral radiance product against other satellite products, temporally and spatially coincident observation points were considered for Simultaneous Nadir Overpass (SNO) from February 2019 to March 2021 from the Atmospheric Infrared Sounder (AIRS) on Aqua, IASI on METOP-B, and TANSO-FTS on GOSAT. The agreement of brightness temperatures between the TANSO-FTS-2 and AIRS and IASI was better than 0.3 K (1σ) from 180 K to 330 K for the 680 cm-1 CO2 channel. The brightness temperatures between the TANSO-FTS-2 and TANSO-FTS of version v230231, which implemented a new polarization reflectivity of the pointing mirror and was released in June 2021, generally agree from 220 K to 320 K. However, there is a discrepancy at lower brightness temperatures, pronounced for CO2 channels at high latitudes. To characterize the spectral radiance bias for along-track and cross-track angles, a 2-Orthogonal Simultaneous Off-Nadir Overpass (2O-SONO) is now done for the TANSO-FTS-2 and IASI, the TANSO-FTS-2 and AIRS, and the TANSO-FTS-2 and TANSO-FTS. The 2O-SONO comparison results indicate that the TIR product for the TANSO-FTS-2 has a bias that exceeds 0.5 K in the CO2 channel for scenes with forward and backward viewing angles greater than 20°. These multi-satellite sensor and multi-angle comparison results suggest that the calibration of spectral radiance for the TANSO-FTS-2 TIR, version v210210, is superior to that of the previous version in its consistency of multi-satellite sensor data. In addition, the paper identifies the remaining challenging issues in current TIR products.