[A-AS21] Atmospheric Chemistry

[AAS21-P22] Comparisons between NICAM-TM and GOSAT/TANSO-FTS TIR CO2 data

*Ryo SUGIMURA¹, Naoko SAITOH¹, Ryoichi IMASU², Shuji KAWAKAMI³, Kei SHIOMI³, Yosuke NIWA⁴, Toshinobu MACHIDA⁵, Yousuke SAWA⁴, Hidekazu MATSUEDA⁴ (1.Center for Environmental Remote Sensing, Chiba University, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Japan Aerospace Exploration Agency, 4.Meteorological Research Institute, 5.National Institute for Environmental Studies)

Keywords:CO2, satellite remote sensing, GOSAT

Greenhouse gases Observing SATellite (GOSAT), which was the first satellite for global observations of greenhouse gases, was successfully launched on 23 January 2009. Our recent analysis suggested that CO₂ vertical profiles retrieved from Thermal and Near Infrared Sensor for Carbon Observation (TANSO) - Fourier Transform Spectrometer (FTS) thermal infrared (TIR) band had a negative bias in the middle troposphere. In this study, we globally evaluated the magnitude of the bias through the comparisons between the TIR CO₂ data and Nonhydrostatic Icosahedral Atmospheric Model - based Transport Model (NICAM-TM) CO₂ data [Niwa et al., 2011]. Furthermore, we calculated a correction factor to modify the bias for each latitude band and applied the latitude-dependent correction factors to the TIR CO₂ data on 500 hPa; here, we estimated the correction factors on the basis of comparisons between CO₂ profiles observed over airports by Continuous CO₂ Measuring Equipment (CME) in Comprehensive Observation Network for Trace gases by Airliner (CONTRAIL) project [Machida et al., 2008] and the coincident TIR CO₂ profiles. Then, we analyzed seasonal variations of the modified TIR CO₂ data to check the validity of the correction factors estimated here.

Comparisons of the differences of CO₂ concentrations on 500 hPa and 200 hPa (500 hPa minus 200 hPa) between TIR CO₂ data and NICAM-TM CO₂ data showed that the differences of the TIR CO₂ data were larger than those of the NICAM-TM CO₂ data because of the negative bias of the mid-tropospheric TIR data. The CO₂ differences between the two pressure levels of the TIR data were particularly large (~8 ppmv) in low latitudes; this characteristic was not seen both in the NICAM-TM CO₂ data and the a priori CO₂ data (NIES-TM05). Next, we applied the latitude-dependent correction factors to the TIR CO₂ data on 500 hPa, and then compared the CO₂ differences on 500 hPa and 200 hPa. In low latitudes (25⁰S-25⁰N), the CO₂ differences between the two pressure levels of the TIR data became closer to the CO₂ differences of the NICAM-TM CO₂ data and the a priori CO₂ data when we applied the correction factor estimated over Bangkok. On the other hand, in northern high latitudes (northern latitudes of ~40⁰N), most of the CO₂ differences between the two pressure levels of the TIR data were positive unlike the NICAM-TM CO₂ and the a priori CO₂ data when we applied the correction factor estimated over Amsterdam. In boreal summer, surface CO₂ concentrations are lower than middle and upper tropospheric CO₂ concentrations; in that sense, the correction factor applied here was not appropriate in northern high latitudes in summer. These results suggest that the magnitude of the negative bias seen in TIR CO₂ data would vary depending on seasons as well as regions, and therefore, we should estimate a latitude-dependent correction factor for each season.

Furthermore, we compared time series of TIR CO₂ data with those of NICAM-TM CO₂ data and the a priori CO₂ data for several different regions that were categorized in terms of climatic divisions and latitude bands [Niwa et al., 2011]. Our preliminary results showed that the seasonal variations of the TIR CO₂ data in some regions were closer to those of the NICAM CO₂ data than of the a priori CO₂ data. For future work, we should review how to compare the three CO₂ data sets, and then closely analyze the differences in seasonal variations among the three data sets globally.

Acknowledgements
We thank the staff and engineers of Japan Airlines, the JAL Foundation, and JAMCO Tokyo for supporting the CONTRAIL project.