Comparison of XH_2O Retrieved from GOSAT Short-Wavelength Infrared Spectra with Observations from the TCCON Network
Understanding the atmospheric distribution of water (H_2O) is crucial for global warming studies and climate change mitigation. In this context, reliable satellite data are extremely valuable for their global and continuous coverage, once their quality has been assessed. Short-wavelength infrared spectra are acquired by the Thermal And Near-infrared Sensor for carbon Observation-Fourier Transform Spectrometer (TANSO-FTS) aboard the Greenhouse gases Observing Satellite (GOSAT). From these, column-averaged dry-air mole fractions of carbon dioxide, methane and water vapor (XH_2O) have been retrieved at the National Institute for Environmental Studies (NIES, Japan) and are available as a Level 2 research product. We compare the NIES XH_2O data, Version 02.21, with retrievals from the ground-based Total Carbon Column Observing Network (TCCON, Version GGG2014). The datasets are in good overall agreement, with GOSAT data showing a slight global low bias of −3.1% ± 24.0%, good consistency over different locations (station bias of −1.53% ± 10.35%) and reasonable correlation with TCCON (R = 0.89). We identified two potential sources of discrepancy between the NIES and TCCON retrievals over land. While the TCCON XH_2O amounts can reach 6000–7000 ppm when the atmospheric water content is high, the correlated NIES values do not exceed 5500 ppm. This could be due to a dry bias of TANSO-FTS in situations of high humidity and aerosol content. We also determined that the GOSAT-TCCON differences directly depend on the altitude difference between the TANSO-FTS footprint and the TCCON site. Further analysis will account for these biases, but the NIES V02.21 XH_2O product, after public release, can already be useful for water cycle studies.
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). Academic Editors: Alfredo R. Huete and Prasad S. Thenkabail. Received: 10 January 2016; Accepted: 4 May 2016; Published: 17 May 2016. This paper is essential for the NIES GOSAT Project Office to ensure the qualification of the GOSAT TANSO-FTS SWIR XH_2O data as a standard product. Nicholas Deutscher is supported by an Australian Research Council fellowship, DE140100178. TCCON data were obtained from the TCCON Data Archive, hosted by the Carbon Dioxide Information Analysis Center (CDIAC): http://tccon.ornl.gov. The TCCON Network is supported by NASA through a grant to the California Institute of Technology: NNX14AI60G. Park Falls, Lamont and JPL are funded by NASA grants NNX14AI60G, NNX11AG01G, NAG5-12247, NNG05-GD07G and the NASA Orbiting Carbon Observatory Program. We are grateful to the United States of America's Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program for technical support in Lamont and to Jeff Ayers for technical support in Park Falls. Darwin and Wollongong are funded by NASA grants NAG5-12247 and NNG05-GD07G and the Australian Research Council grants DP140101552, DP110103118, DP0879468, LE0668470 and LP0562346. We are grateful to the DOE ARM program for technical support in Darwin. Bremen, Bialystok and Orléans are funded by the European Union's projects InGOS (Integrated non-CO_2 Greenhouse gas Observing System) and ICOS-INWIRE (Integrated Carbon Observing System-Improved sensors, Network and Interoperability for GMES (Global Monitoring for Environment and Security)) and by the Senate of Bremen. From 2004–2011, the Lauder TCCON program was funded by the New Zealand Foundation of Research, Science and Technology contracts CO1X0204, CO1X0703 and CO1X0406. Since 2011, the program has been funded by New Zealand's National Institute of Water and Atmospheric Research (NIWA)'s Atmosphere Research Programme 3 (2011/2013 Statement of Corporate Intent). Garmisch water vapor retrievals have been supported by the Bavarian Ministry of the Environment and Consumer Protection. Indianapolis and Edwards measurements are supported by the NASA Earth Science Division. Author Contributions: Eric Dupuy performed all of the NIES/TCCON comparisons for XH_2O presented in this paper within the NIES GOSAT Project, produced the figures and tables and wrote the entire paper. Nicholas Deutscher conducted the counterpart XH_2O comparisons among the TCCON partners; he collaborated closely throughout this analysis and the conception of the paper. Isamu Morino leads the GOSAT/GOSAT-2 Validation Team and took extensive part in the data analysis and the writing and review of this paper. Yukio Yoshida is responsible for the Level 2 processing of the GOSAT SWIR data at NIES; he was instrumental in the interpretation of the validation results from the point of view of retrieval biases. Osamu Uchino is the GOSAT Validation Manager and oversees the validation for all GOSAT data products at NIES, including XH2_O Version V02.21. Tatsuya Yokota and Tsuneo Matsunaga are the NIES Project Leaders for GOSAT and GOSAT-2, respectively, and coordinate all aspects of the GOSAT and GOSAT-2 Projects within NIES. Other authors are members of the TCCON Steering Committee and principal investigators or co-investigators of TCCON stations; they contributed to the acquisition, processing and delivery of the TCCON data products for the sites included in this study. All authors provided feedback on the data analysis and the writing of this manuscript. The authors declare no conflict of interest.
Supplemental Material - remotesensing-08-00414-s001.zip
Published - remotesensing-08-00414-v2.pdf
Erratum - remotesensing-08-00982.pdf