Impact of differences in line parameter databases on methane retrieval through radiative transfer calculation

Abstract:

The thermal infrared (TIR) band of Thermal and Near-infrared Sensor for Carbon Observation Fourier Transform Spectrometer (TANSO-FTS) onboard Greenhouse Gases Observing Satellite (GOSAT) observes CH₄ profile at wavenumber range from 1210 cm^-1 to 1360 cm^-1 including CH₄ ν₄ band. The current retrieval algorithm (V1.0) uses LBLRTM v12.1 with AER v3.1 line database to calculate optical depth. The purpose of this study is to compare several line parameter databases to assess their impact on CH₄ retrieval: HITRAN2004 database, HITRAN2008 database, AER v3.2 database, and HITRAN2012 database (Rothman et al. 2005, 2009, and 2013. Clough et al., 2005). The CH₄ line parameters of AER v3.1 and v3.2 databases are based on HITRAN2008 and it updates until May 2009 with line mixing parameters. The line strengths are almost the same between HITRAN2008 and HITRAN2012 in the CH₄ ν₄ band, although their line centers are shifted about 0.002 cm^-1 (Brown et al., 2013). We used LBLRTM v12.2 to calculate optical depth of each layer with wavenumber resolution of 0.0034 cm^-1 (Clough et al., 2005). Then we calculated forward radiance spectra and convoluted them with sinc function. At 1247.8 cm^-1 where there exist three relatively strong CH₄ absorption lines (1247.705 cm^-1, 1247.823 cm^-1, and 1247.842 cm^-1), no H₂O interference, and small N₂O interference, the differences in calculated brightness temperatures between HITRAN2008 and HITRAN2004, between HITRAN2008 and AER v3.2, and between HITRAN2012 and HITRAN2008 were 0.002 K, 0.4 K, and 0.06 K, respectively. These differences may affect CH₄ retrieval. In wavenumber range from 1210 cm^-1 to 1360 cm^-1, several H₂O line parameters were updated from HITRAN2008 to HITRAN2012. These updates could also affect on CH₄ retrieval through simultaneous retrieval with H₂O at CH₄ ν₄ band.