CO_2 retrieval performance of TANSO-FTS (TIR) sensor aboard greenhouse gases observing satellite (GOSAT)

摘要

The Greenhouse gases Observing Satellite (GOSAT) is a Japanese satellite that is intended to observe CO_2 concentration from space and to contribute to advancement of research of the source/sink estimation of CO_2. The GOSAT main sensor is a Fourier Transform Spectrometer (FTS) named "TANSO-FTS", which covers a wide terrestrial radiation spectrum including CO_2 absorption bands at 1.6 μm, 2.0 μm, and 15 μm. The former two bands are used to estimate columnar concentration of CO_2; the latter is used to retrieve the vertical profile of CO_2 in the upper atmosphere above about 700 hPa pressure level. In addition, another installed on the satellite is an imaging sensor that will be used to detect clouds and aerosols: Cloud and Aerosol Imager (CAI). The Center for Climate System Research (CCSR) has contracted with the Japan Aerospace Exploration Agency (JAXA) to develop an algorithm to retrieve CO_2 concentration profiles from data measured by the thermal infrared (TIR) band of the TANSO-FTS sensor. We adopt the maximum a posteriori method (MAP) to retrieve the vertical profile of atmospheric parameters from thermal infrared spectra. Key techniques for retrieving CO_2 concentrations are 1) reduction of temperature estimation error through channel selection, 2) optimization of the initial guess for CO_2 profile based on the output from a chemical transport model (CTM), and 3) usage of data from the 1.6 μm band of TANSO-FTS as an additional constraint in retrieval of vertical profiles of CO_2. Although thermal infrared spectrum data have poor vertical resolving power for CO_2 concentration in the lower atmosphere, particularly in the boundary layer, we expect that CO_2 amount in the lower atmosphere can be deduced by substituting the upper level concentration from the columnar concentration estimated from the 1.6 μm band data.