Validation of GOSAT/TANSO-FTS TIR UTLS CO₂ data (Version 1.0) using CONTRAIL measurements

Abstract

Abstract. The thermal infrared (TIR) band of the Thermal and Near Infrared Sensor for Carbon Observation (TANSO)–Fourier Transform Spectrometer (FTS) on board the Greenhouse Gases Observing Satellite (GOSAT) has been observing carbon dioxide (CO2) concentrations in several atmospheric layers since its launch. This study compared TANSO-FTS TIR V1.0 CO2 data and CO2 data obtained in the Comprehensive Observation Network for TRace gases by AIrLiner (CONTRAIL) project in the upper troposphere and lower stratosphere (UTLS), where the TIR band of TANSO-FTS is most sensitive to CO2 concentrations, to validate the quality of the TIR V1.0 UTLS CO2 data from 287 to 162 hPa. From a comparison made during flights between Tokyo and Sydney, the averages of the TIR upper atmospheric CO2 data agreed well with the averages of the data obtained by the CONTRAIL Continuous CO2 Measuring Experiment (CME) within 0.1 % for all of the seasons in the Southern Hemisphere. The results of a comparison for all of the eight airline routes showed that the agreement between the TIR and CONTRAIL CO2 data was within 0.5 % on average in the Northern Hemisphere, which was better than the agreement between a priori and CONTRAIL CO2 data. The quality of TIR lower stratospheric CO2 data depends largely on the information content, and therefore has a seasonal dependence. In high latitudes, TIR V1.0 lower stratospheric CO2 data are only valid in the summer. The magnitude of bias in the TIR upper atmospheric CO2 data did not have a clear longitudinal dependence. The comparison results for flights in northern low and middle latitudes showed that the agreement between TIR and CONTRAIL CO2 data in the upper troposphere was worse in the spring and summer than in the fall and winter. This could be attributed to a larger negative bias in the upper atmospheric a priori CO2 data in the spring and summer and a seasonal dependence of spectral bias in TANSO-FTS TIR Level 1B (L1B) radiance data. The negative bias in northern middle latitudes made the maximum of TIR CO2 concentrations lower than that of CONTRAIL CO2 concentrations, which leads to underestimate the amplitude of CO2 seasonal variation.