ひまわり8号/AHIによる海上のGOSAT/TANSO-FTS観測視野内雲判定の評価

抄録

Before retrieving greenhouse gas concentrations from spectra of the Thermal and Near Infrared Sensor for Carbon Observation (TANSO)-Fourier Transform Spectrometer (FTS) on board the Greenhouse Gases Observing Satellite (GOSAT), cloud contamination in the fields of view (FOVs) of TANSO-FTS, which could degrade the data quality of the retrieved gas concentrations, is assessed using the TANSO-Cloud and Aerosol Imager (CAI) and the short-wave infrared (SWIR) band of TANSO-FTS in the daytime and the thermal infrared (TIR) band of TANSO-FTS in the nighttime. This study has validated the daytime and nighttime cloud assessments in the FOVs of TANSO-FTS over the ocean in GOSAT data processing by comparing them with cloud assessments based on the reflectance and brightness temperature data of coincident measurements made by the Advanced Himawari Imager (AHI) on board the Himawari-8 satellite. About 90 % of the daytime cloud judgements made by TANSO-CAI and the TANSO-FTS SWIR band and by Himawari-8 in this study agreed with each other. In contrast, agreement between the TANSO-FTS TIR band and Himawari-8 in their nighttime cloud judgements was ～10 % lower than the daytime level of agreement. Due to the larger FOVs of TANSO-FTS (10.5 km), the TANSO-FTS TIR band could not detect subpixel-sized clouds that partly existed in the FOVs of TANSO-FTS. Although the cloud judgements made by Himawari-8 here were “clear conservative,” they suggest that some of the TANSO-FTS observations judged to be “clear” by the GOSAT cloud assessments were contaminated by clouds, especially in the nighttime. We should therefore evaluate the data quality of the GOSAT/TANSO-FTS GHG products from the view point of the accuracy of cloud assessments in the FOVs of TANSO-FTS.