1Earth Observation Science, Department of Physics and Astronomy, University of Leicester, Leicester, UK
2National Centre for Earth Observation, Department of Physics and Astronomy, University of Leicester, Leicester, UK
3School of GeoSciences, University of Edinburgh, Edinburgh, UK
4National Centre for Earth Observation, School of GeoSciences, University of Edinburgh, Edinburgh, UK
5European Commission Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy
6Lab. des Sciences du Climat et de l'Environnement, CNRS, Gif-sur-Yvette, France
7Institute of Environmental Physics, University of Bremen, Bremen, Germany
8Karlsruhe Institut für Technologie, Karlsruhe, Germany
9Japan Aerospace Exploration Agency (JAXA), Tsukuba, Japan
10Finnish Meteorological Institute, Arctic Research, Sodankylä, Finland
11School of Chemistry, University of Wollongong, Wollongong, Australia
Received: 05 May 2015 – Discussion started: 17 Jun 2015
Abstract. We present 5 years of GOSAT XCH4 retrieved using the "proxy" approach. The Proxy XCH4 data are validated against ground-based TCCON observations and are found to be of high quality with a small bias of 4.8 ppb (∼ 0.27 %) and a single-sounding precision of 13.4 ppb (∼ 0.74 %). The station-to-station bias (ameasure of the relative accuracy) is found to be 4.2 ppb. For the first time the XCH4 / XCO2 ratio component of the Proxy retrieval is validated (bias of 0.014 ppb ppm−1 (∼ 0.30 %), single-sounding precision of 0.033 ppb ppm−1 (∼ 0.72 %)). The uncertainty relating to the model XCO2 component of the Proxy XCH4 is assessed through the use of an ensemble of XCO2 models. While each individual XCO2 model is found to agree well with the TCCON validation data (r = 0.94–0.97), it is not possible to select one model as the best from our comparisons. The median XCO2 value of the ensemble has a smaller scatter against TCCON (a standard deviation of 0.92 ppm) than any of the individual models whilst maintaining a small bias (0.15 ppm). This model median XCO2 is used to calculate the Proxy XCH4 with the maximum deviation of the ensemble from the median used as an estimate of the uncertainty. We compare this uncertainty to the a posteriori retrieval error (which is assumed to reduce with sqrt(N)) and find typically that the model XCO2 uncertainty becomes significant during summer months when the a posteriori error is at its lowest due to the increase in signal related to increased summertime reflected sunlight. We assess the significance of these model and retrieval uncertainties on flux inversion by comparing the GOSAT XCH4 against modelled XCH4 from TM5-4DVAR constrained by NOAA surface observations (MACC reanalysis scenario S1-NOAA). We find that for the majority of regions the differences are much larger than the estimated uncertainties. Our findings show that useful information will be provided to the inversions for the majority of regions in addition to that already provided by the assimilated surface measurements.
Revised: 20 Oct 2015 – Accepted: 29 Oct 2015 – Published: 17 Nov 2015
Parker, R. J., Boesch, H., Byckling, K., Webb, A. J., Palmer, P. I., Feng, L., Bergamaschi, P., Chevallier, F., Notholt, J., Deutscher, N., Warneke, T., Hase, F., Sussmann, R., Kawakami, S., Kivi, R., Griffith, D. W. T., and Velazco, V.: Assessing 5 years of GOSAT Proxy XCH4 data and associated uncertainties, Atmos. Meas. Tech., 8, 4785-4801, doi:10.5194/amt-8-4785-2015, 2015.