A Channel Selection Method for CO2 Retrieval Using Information Content AnalysisLe Kuai1, Vijay Natraj1, Run-Lie Shia1, Charles Miller2, Yuk Yung11. Division of Geological and Planetary Sciences, California Institute of Technology2. Jet Propulsion Laboratory, California Institute of Technology
A major challenge in retrieving the CO2 concentrations from thermal infrared remote sensing comes from the fact that measurements in the 4.3 and 15 μm absorption bands (AIRS or TES) are sensitive to both temperature and CO2 variations. This complicates the selection of absorption channels with maximum CO2 concentration information content. In contrast, retrievals using near infrared (NIR) CO2 absorption bands are relatively insensitive to temperature and are most sensitive to changes of CO2 near the surface, where the sources and sinks are located. The Orbiting Carbon Observatory (OCO-2) was built to measure reflected sunlight in three NIR spectral regions (the 0.76 μm O2 A-band and two CO2 bands at 1.61 and 2.06 μm). In an effort to significantly increase the speed of accurate CO2 retrieval algorithms for OCO-2, we performed an information content analysis to identify the 20 best channels from each CO2 spectral region to use in OCO-2 retrievals. Retrievals using these 40 channels provide as much as 75% of the total CO2 information content compared to retrievals using all 1016 channels in each CO2 spectral region. The CO2 retrievals using our selected channels have a precision better than 0.1 ppm. This technique is general and equally applicable to the retrieval of other geophysical variables (e.g. temperature or CH4), or modified for other instruments, such as AIRS or TES.
Below are the selected 40 channels that are sensitive to CO2, but insensitive to temperature, water vapor and surface pressure.
Fig. 1 Weighting functions for CO2 have peak near surface. a) 1.61 μm CO2 band; b) 2.06 μm CO2 band
CO2 retrieval comparison. Case 2: All channel retrieval; Case 3: Average of 100 all-channel retrievals; Case 5: 40-channel retrieval; Case 6: Average of 100 40-channel retrievals. The a priori is 375 ppm (constant) for all cases. Random noise is included in the pseudo-measurements.
Definition of information content (H) and degree of freedom (ds)
Channels with highest information content (CO2) are from the channels with intermediate radiance in both bands. For very weak channels, the absorption is too low and no signal is received. On the other hand, for the saturated channels, the absorption is too high so that there is no sensitivity to the signal.
Sa: the a priori covariance matrix;
Sξ: the measurement error covariance matrix;
K: the Jacobian; A: the averaging Kernel;
λi: the eigenvalues of
1) The intermediate absorption channels provide most information for CO2. 2) No influence of scenarios on the channels selection. 3) The reduced-channel retrieval performs good enough compared to the full-channel retrieval.
Crevoisier, C., Chedin, A. and Scott, N., (2003), AIRS channel selection for CO2 and other trace-gas retrievals. Q. J. R. Meteorol. Soc., 129, pp. 2719-2740.
Crisp, D., et al., (2004), The Orbiting Carbon Observatory (OCO-2) mission. Advances in Space Research,34, pp. 700-709.
Rodgers, C. D. (2000), Inverse Methods of Atmospheric Sounding: Theory and Practice. Singapore: World Scientific Publishing Company. pp. 240.
Sato, M., Tahara, S., and Usami, M., (2009), FIP’s Environmentally Conscious Solutions and GOSAT. FUJITSU Sci. Tech. J., Vol. 45, No. 1, pp. 134-140.
Yokomizo, M. (2008), Greenhouse gases Observing SATellite (GOSAT) Ground Systems. FUJITSU Sci. Tech. J.,Vol. 44, No. 4, pp. 410-417.
The simultaneous retrieval using all 2032 channels in both 1.61 μm CO2 band and 2.06 μm CO2 bands provide 1.67 DOF and 5.9 bits of IC. This figure shows that the retrieval using the first 200 channels in each band would have 1.55 DOF and 5.35 bits of information content (IC). It is about 90% of the information provided by the retrieval using all channels. The retrieval using the first 20 channels has 75% of the information from the full channel retrieval. The channels were selected by ranking the IC from highest to lowest in each band.
First 20 channels with highest IC for CO2 (cross), temperature (diamond), H2O (star) and surface pressure (square). The left column is clear sky scenario; right column is cloudy sky scenario. The O2 A-band channels are only sensitive to temperature and pressure. This figure also shows that the channels with high IC for CO2 are mostly different from those sensitive for temperature, water vapor and surface pressure. However, the channels most sensitive to the one variable are the same under both scenarios.