5:15 PM - 6:30 PM
[AAS12-P03] The impact on CH4 retrieval of GOSAT/TANSO-FTS TIR band from the uncertainty of the continuum absorption
We used 713 spectra observed by GOSAT/TANSO-FTS TIR band from March 2010 to September 2011. These observations satisfy following criteria. The coincidence criteria of distance and time between GOSAT-TIR observations and HIPPO aircraft observations (Wofsy et al., 2012) are 300 km and 72 hours, respectively, and the degrees of freedom for signal are greater than 0.7. We calculated forward spectrum using retrieved profile by LBLRTM V12.2 (Clough et al., 2005) and convolution of the forward spectrum and sinc function as an instrumental line shape function to set to low wavenumber resolution.
The spectral residual (forward spectrum - GOSAT-TIR observation spectrum) had fine structure from -20 K to 20 K, which was made from line absorption. We calculated moving average with window width of 11 channels to separate continuum from line absorption. The average of the moving-averaged residual were -2.0 K. According to Saitoh et al. (2009), 1 K difference of surface temperature make 5% bias at the altitude of 400 mbar in CO2 profile, there is a potential for bias in CH4 profile. There exist H2O self continuum, H2O foreign continuum, CO2 continuum, and O2 continuum in this band. The wavenumber dependencies of H2O self continuum and CO2 continuum are small in this band, while that of H2O foreign continuum and O2 continuum are large.
We calculated forward spectra using 10% larger continuum absorption coefficient than MT CKD model to know the impact on brightness temperature spectrum from continuum absorption. The brightness temperature changed about -0.00001 K and 0.01 K when we used 10% larger CO2 continuum absorption and 10% larger H2O self continuum absorption, respectively. The differences on brightness temperature were much smaller than the averaged residual. We are trying comparing GOSAT-TIR CH4 profile retrieved using 10% bias continuum absorption and other continuum model with HIPPO profile.
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Paynter, D. and Ramaswamy, V. (2011) JGR 116, D20302.
Saitoh, N., et al. (2009) JGR 114, D17305.
Wofsy, S. C., et al. (2012) http://dx.doi.org/10.3334/CDIAC/hippo_010.