The authors thank Dr. Peter Bechtold of ECMWF for the discussion related to the treatment of the top levels in the ECMWF model and to anonymous reviewers for their insightful comments, which led to improvements in the paper. The visit of N. Žagar to the University of Tsukuba was funded by the Center for Computational Sciences, by the Japan Society for the Promotion of Science, and by the Slovenian Research Agency. The Centre of Excellence for Space Sciences and Technologies SPACE-SI is an operation part financed by the European Union, European Regional Development Fund, and the Republic of Slovenia, Ministry of Higher Education, Science and Technology.
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