A decade of GOSAT Proxy satellite CH₄ observations

• Main Authors: R. J. Parker, A. Webb, H. Boesch, P. Somkuti, R. Barrio Guillo, A. Di Noia, N. Kalaitzi, J. S. Anand, P. Bergamaschi, F. Chevallier, P. I. Palmer, L. Feng, N. M. Deutscher, D. G. Feist, D. W. T. Griffith, F. Hase, R. Kivi, I. Morino, J. Notholt, Y.-S. Oh, H. Ohyama, C. Petri, D. F. Pollard, C. Roehl, M. K. Sha, K. Shiomi, K. Strong, R. Sussmann, Y. Té, V. A. Velazco, T. Warneke, P. O. Wennberg, D. Wunch
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-12-01
• Series: Earth System Science Data
• Online Access: https://essd.copernicus.org/articles/12/3383/2020/essd-12-3383-2020.pdf
• ISSN: 1866-3508; 1866-3516

<p>This work presents the latest release (v9.0) of the University of Leicester GOSAT Proxy <span class="inline-formula">XCH₄</span> dataset. Since the launch of the GOSAT satellite in 2009, these data have been produced by the UK National Centre for Earth Observation (NCEO) as part of the ESA Greenhouse Gas Climate Change Initiative (GHG-CCI) and Copernicus Climate Change Services (C3S) projects. With now over a decade of observations, we outline<span id="page3384"/> the many scientific studies achieved using past versions of these data in order to highlight how this latest version may be used in the future.</p> <p>We describe in detail how the data are generated, providing information and statistics for the entire processing chain from the L1B spectral data through to the final quality-filtered column-averaged dry-air mole fraction (<span class="inline-formula">XCH₄</span>) data. We show that out of the 19.5 million observations made between April 2009 and December 2019, we determine that 7.3 million of these are sufficiently cloud-free (37.6 %) to process further and ultimately obtain 4.6 million (23.5 %) high-quality <span class="inline-formula">XCH₄</span> observations. We separate these totals by observation mode (land and ocean sun glint) and by month, to provide data users with the expected data coverage, including highlighting periods with reduced observations due to instrumental issues.</p> <p>We perform extensive validation of the data against the Total Carbon Column Observing Network (TCCON), comparing to ground-based observations at 22 locations worldwide. We find excellent agreement with TCCON, with an overall correlation coefficient of 0.92 for the 88 345 co-located measurements. The single-measurement precision is found to be 13.72 ppb, and an overall global bias of 9.06 ppb is determined and removed from the Proxy <span class="inline-formula">XCH₄</span> data. Additionally, we validate the separate components of the Proxy (namely the modelled <span class="inline-formula">XCO₂</span> and the <span class="inline-formula">XCH₄∕XCO₂</span> ratio) and find these to be in excellent agreement with TCCON.</p> <p>In order to show the utility of the data for future studies, we compare against simulated <span class="inline-formula">XCH₄</span> from the TM5 model. We find a high degree of consistency between the model and observations throughout both space and time. When focusing on specific regions, we find average differences ranging from just 3.9 to 15.4 ppb. We find the phase and magnitude of the seasonal cycle to be in excellent agreement, with an average correlation coefficient of 0.93 and a mean seasonal cycle amplitude difference across all regions of <span class="inline-formula">−0.84</span> ppb.</p> <p>These data are available at <a href="https://doi.org/10.5285/18ef8247f52a4cb6a14013f8235cc1eb">https://doi.org/10.5285/18ef8247f52a4cb6a14013f8235cc1eb</a> <span class="cit" id="xref_paren.1">(<a href="#bib1.bibx68">Parker and Boesch</a>, <a href="#bib1.bibx68">2020</a>)</span>.</p>