We thank Huw Davies for invaluable advice, Jeffrey Annis for his help with a pilot study to this project, Daniela Domeisen for proofreading, George Kiladis and an anonymous reviewer for their valuable feedback and suggestions, and Meteo Swiss for granting access to the ERA-Interim data set. The satellite images were downloaded from the National Climatic Data Center. Publication costs have been covered by the German Research Foundation (DFG) as part of the research unit PANDOWAE (FOR 896).
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The following groups were chosen: PV > 4 PVU, 3 < PV < 4 PVU, 2 < PV < 3 PVU, 1 < PV < 2 PVU, and PV < 1 PVU. The choice of PV categories is to a certain degree arbitrary.