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Data is archived at ECMWF and available for download from ECMWF data services.
References
Poli, P., H. Hersbach, D.P. Dee, P. Berrisford, A.J. Simmons, F. Vitart, P. Laloyaux, D.G. Tan, C. Peubey, J. Thépaut, Y. Trémolet, E.V. Hólm, M. Bonavita, L. Isaksen, and M. Fisher. ERA-20C: An Atmospheric Reanalysis of the Twentieth Century, Journal of Climate, 2016, 29:11, 4083-4097, Doi: 10.1175/JCLI-D-15-0556.1
Some ERA-20C references available from the ECMWF E-Library.
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