Articles | Volume 15, issue 22
Atmos. Chem. Phys., 15, 13023–13040, 2015
Atmos. Chem. Phys., 15, 13023–13040, 2015

Research article 24 Nov 2015

Research article | 24 Nov 2015

Does GOSAT capture the true seasonal cycle of carbon dioxide?

H. Lindqvist1,2, C. W. O'Dell1, S. Basu3,4, H. Boesch5, F. Chevallier6, N. Deutscher7,8, L. Feng9, B. Fisher10, F. Hase11, M. Inoue12,13, R. Kivi14, I. Morino12, P. I. Palmer9, R. Parker5, M. Schneider11, R. Sussmann15, and Y. Yoshida12 H. Lindqvist et al.
  • 1Department of Atmospheric Science, Colorado State University, CO, USA
  • 2Finnish Meteorological Institute, Helsinki, Finland
  • 3NOAA Earth System Research Laboratory, Boulder, CO, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 5National Centre for Earth Observation, Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 6Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, Gif-sur-Yvette, France
  • 7School of Chemistry, University of Wollongong, Wollongong, Australia
  • 8Institute of Environmental Physics, University of Bremen, Bremen, Germany
  • 9National Centre for Earth Observation, School of GeoSciences, University of Edinburgh, Edinburgh, UK
  • 10Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 11Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany
  • 12National Institute for Environmental Studies (NIES), Tsukuba, Japan
  • 13Department of Biological Environment, Akita Prefectural University, Akita, Japan
  • 14Finnish Meteorological Institute, Arctic Research Centre, Sodankylä, Finland
  • 15Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany

Abstract. The seasonal cycle accounts for a dominant mode of total column CO2 (XCO2) annual variability and is connected to CO2 uptake and release; it thus represents an important quantity to test the accuracy of the measurements from space. We quantitatively evaluate the XCO2 seasonal cycle of the Greenhouse Gases Observing Satellite (GOSAT) observations from the Atmospheric CO2 Observations from Space (ACOS) retrieval system and compare average regional seasonal cycle features to those directly measured by the Total Carbon Column Observing Network (TCCON). We analyse the mean seasonal cycle amplitude, dates of maximum and minimum XCO2, as well as the regional growth rates in XCO2 through the fitted trend over several years. We find that GOSAT/ACOS captures the seasonal cycle amplitude within 1.0 ppm accuracy compared to TCCON, except in Europe, where the difference exceeds 1.0 ppm at two sites, and the amplitude captured by GOSAT/ACOS is generally shallower compared to TCCON. This bias over Europe is not as large for the other GOSAT retrieval algorithms (NIES v02.21, RemoTeC v2.35, UoL v5.1, and NIES PPDF-S v.02.11), although they have significant biases at other sites. We find that the ACOS bias correction partially explains the shallow amplitude over Europe. The impact of the co-location method and aerosol changes in the ACOS algorithm were also tested and found to be few tenths of a ppm and mostly non-systematic. We find generally good agreement in the date of minimum XCO2 between ACOS and TCCON, but ACOS generally infers a date of maximum XCO2 2–3 weeks later than TCCON. We further analyse the latitudinal dependence of the seasonal cycle amplitude throughout the Northern Hemisphere and compare the dependence to that predicted by current optimized models that assimilate in situ measurements of CO2. In the zonal averages, models are consistent with the GOSAT amplitude to within 1.4 ppm, depending on the model and latitude. We also show that the seasonal cycle of XCO2 depends on longitude especially at the mid-latitudes: the amplitude of GOSAT XCO2 doubles from western USA to East Asia at 45–50° N, which is only partially shown by the models. In general, we find that model-to-model differences can be larger than GOSAT-to-model differences. These results suggest that GOSAT/ACOS retrievals of the XCO2 seasonal cycle may be sufficiently accurate to evaluate land surface models in regions with significant discrepancies between the models.

Short summary
Atmospheric carbon dioxide concentration varies seasonally mainly due to plant photosynthesis in the Northern Hemisphere. We found that the satellite GOSAT can capture this variability from space to within 1ppm. We also found that models can differ by more than 1ppm. This implies that the satellite measurements could be useful in evaluating models and their prior estimates of carbon dioxide sources and sinks.
Final-revised paper