201905_341006-en

Evaluation of bias correction methods for GOSAT SWIR XH2O using TCCON data

  • Thi Ngoc Trieu Tran
    Research Associate, Satellite Remote Sensing Section, Center for Global Environmental Research
  • Isamu Morino, Osamu Uchino, Hirofumi Oyama, et al.
    Center for Global Environmental Research

The study evaluated three bias correction methods of systematic biases in column-averaged dry-air mole fraction of water vapor (XH2O) data retrieved from Greenhouse gases Observing SATellite (GOSAT) Short-Wavelength InfraRed (SWIR) observations compared with ground-based data from the Total Carbon Column Observing Network (TCCON). They included an empirically multi-linear regression method, altitude bias correction method, and combination of altitude and empirical correction for three cases defined by the temporal and spatial collocation around TCCON site. The large altitude differences between GOSAT observation points and TCCON instruments are the main cause of bias, and the altitude bias correction method is the most effective bias correction method. The lowest biases result from GOSAT SWIR XH2O data within a 0.5° × 0.5° latitude × longitude box centered at each TCCON site matched with TCCON XH2O data averaged over ±15 min of the GOSAT overpass time. Considering land data, the global bias changed from −1.3 ± 9.3% to −2.2 ± 8.5% and station bias from −2.3 ± 9.0% to −1.7 ± 8.4%. In mixed land and ocean data, global bias and station bias changed from −0.3 ± 7.6% and −1.9 ± 7.1% to −0.8 ± 7.2% and −2.3 ± 6.8%, respectively after bias correction. The results also confirmed that the fine spatial and temporal collocation criteria is necessary in bias correction methods. In addition, GOSAT SWIR XH2O data are correlated mainly with the retrieved albedo and air mass (a function of the solar zenith angle and the satellite-viewing angle). However, only the mixed data are correlated with the retrieved AOD at 1.6 µm, whereas land data are correlated strongly with surface pressure and altitude of GOSAT observation point. The correlation coefficient R, which ranged from 0.91 to 0.99 after bias correction, demonstrated that corrected GOSAT SWIR XH2O data are very strongly correlated with TCCON data. Besides that, the imperfect retrieval algorithms of GOSAT data as well potential bias from TCCON might also contribute to GOSAT SWIR XH2O total bias.

FigureCorrelations between GOSAT SWIR XH2O data within a 0.5° × 0.5° latitude × longitude box centered at each TCCON site matched with TCCON XH2O data averaged over ±15 min of the GOSAT overpass time. (a) Original GOSAT SWIR XH2O and TCCON XH2O; (b) GOSAT SWIR XH2O corrected by bias correction method (A) and TCCON XH2O.

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