Retrieval performance of GOSAT thermal infrared FTS sensor for measuring CO 2 concentrations

Abstract

The Greenhouse gases Observing Satellite (GOSAT) is a Japanese satellite that is intended to observe CO₂ concentrations from space and to contribute to advancement of research related to CO₂ source/sink estimation. The GOSAT main sensor is a Fourier Transform Spectrometer (FTS) named "TANSO-FTS", which covers a wide terrestrial radiation spectrum including CO₂ absorption bands at 1.6 μm (Short Wavelength InfraRed, SWIR), and 15 μm (Thermal InfraRed, TIR). The former band is used to estimate columnar concentration of CO₂; the latter is used to retrieve the vertical profile of CO₂ in the upper atmosphere above the ca. 700 hPa pressure level. We adopt the maximum a posteriori method (MAP) to retrieve the vertical profile of CO₂ concentrations using the meteorological analysis data for temperature profiles. Key techniques for retrieving CO₂ concentrations are 1) reduction of temperature estimation error through channel selection, 2) optimization of the a priori CO₂ profile based on the output from a CO₂ transport model, and 3) usage of SWIR data as an additional constraint in retrieval of vertical profiles of CO₂. Simulation studies using the output from a CO₂ transport model show that, although thermal infrared spectrum has poor sensitivity to the CO₂ concentration change in the lower atmosphere, particularly in the boundary layer, we expect that CO₂ concentration profiles in the lower atmosphere can be reproduced statistically by combining CO₂ columnar data derived from SWIR as an additional constraint in retrieving a CO₂ concentration profile from TIR data.