GHG Session 2

The GHG-CCI project (http://www.esa-ghg-cci.org/) is one of several projects of the European Space Agency’s (ESA) Climate Change Initiative (CCI). The goal of the CCI is to generate and deliver data sets of various satellite-derived Essential Climate Variables (ECVs) in line with GCOS (Global Climate Observing System) requirements. The “ECV Greenhouse Gases” (ECV GHG) is the global distribution of important climate relevant gases – specifically atmospheric CO₂ and CH₄ - with a quality sufficient to obtain information on regional CO₂ and CH₄ sources and sinks. The main goal of GHG-CCI is to generate long-term highly accurate and precise time series of global near-surface sensitive satellite observations of CO₂ and CH₄.  SCIAMACHY on ENVISAT and TANSO-FTS/GOSAT are currently the two main GHG-CCI satellite instruments as their spectral radiance observations in the near-infrared range of the spectrum permit retrievals of CO₂ and CH₄ columns that are sensitive down to the Earth's surface. This is an important property of the observations to be useful for constraining the sources and sinks of CO₂ and CH₄ by inverse modelling. In addition other satellite instruments such as IASI/METOP and MIPAS/ENVISAT are also used. Phase 1 of the GHG-CCI project finished end of 2013 and entered into phase 2 in March 2014, which will continue until end of February 2017. During this ongoing phase the focus will be on improving the quality of the satellite-derived GHG data sets, on extending the time series and on performing a comprehensive validation of the products. Furthermore, a dedicated "Climate Research User Group" will assess the products with respect to their utility for model evaluation, surface flux inverse modelling, and cross ECV activities. An overview about the current status and an outlook to future activities will be presented.

The Fourier Transform Spectrometer (TANSO-FTS) measures Earth radiances in the Short-Wave InfraRed (SWIR) and Thermal InfraRed (TIR) wavelength ranges. In this study, we discuss the capacity to retrieve methane solely from TIR measurements in the spectral range 1210-1310 cm^-1. Simultaneously, we infer atmospheric abundances of H₂O and N₂O, a spectral shift, and the skin temperature. The temperature profile is taken from ECMWF data and the emissivity is calculated at high spectral resolution with the HSR code developed at the university of Wisconsin and is based on MODIS measurements. To verify the retrieval performance of CH₄ total columns, we consider clear-sky spectra co-located in space and time with ground-based measurements at seven sites of the TCCON network (Bialystok, Bremen, Darwin, Lamont, Orleans, Park Falls, and Wollongong) in the period June 2009 to December 2010. In addition, a comparison is made with the HIPPO aircraft campaigns, and the retrieved vertical profiles are also compared against TM5 model results on a global scale. The validation indicates a high bias of the TIR methane product of 46 ppbv (2.5%) with respect to co-located TCCON measurements with a small inter-stational variation (6 ppbv, or 0.3%). We investigate forward model errors and radiometric errors as a potential reason of the retrieval bias. Finally, we discuss the synergistic use of GOSAT shortwave infrared and thermal infrared measurements. The two spectral ranges differ in their sensitivity to the vertical distribution of methane. SWIR measurements are sensitive to the vertically integrated amount of methane whereas the TIR spectral range is mainly sensitive to methane in the upper troposphere. When both spectral regions are combined, height information on methane can be inferred from the measurements. This retrieval approach is applied to GOSAT measurements and first results will be presented.

Quantifying the global distribution and emission fluxes of anthropogenic and naturally occurring atmospheric methane is a key scientific objective for the Greenhouse gases Observing SATellite (GOSAT) mission (launched in January 2009). GOSAT’s payload consists of the Thermal And Near-infrared Sensor for carbon Observation (TANSO) that comprises the Fourier Transform Spectrometer (FTS) and the Cloud Aerosol Imager (CAI) enabling coincident observations of atmospheric constituents, clouds and aerosols. The FTS observes through 3 short-wave infrared (SWIR) bands (1 to 3) and band 4 which covers the thermal Infrared (TIR) spectral range of 700 cm^-1  to 1800 cm^-1 (14.3 to 5.5 µm). GOSAT’s complementary SWIR-TIR measurements can be used to better quantify the vertical distribution of methane; space-borne SWIR observations of total column dry-air mole fraction methane are well- suited to improve our knowledge of the underlying surface fluxes whereas the TIR observations characterise concentrations in the free troposphere. 

We present GOSAT methane profile retrievals from the newly-developed University of Leicester GOSAT TIR retrieval scheme and assess the quality of the retrieval by comparing with HIAPER Pole-to-Pole Observation (HIPPO) in situ aircraft profiles and the in situ TCCON measurement network. We demonstrate the ability of the TIR retrieval to observe regional and large-scale variability in methane through model inter-comparisons and we demonstrate the potential for synergistic SWIR/TIR retrievals to obtain better estimates of atmospheric CH₄ concentration from the surface to the upper troposphere.