Ensemble homogenization of Slovenian monthly air temperature series

This paper presents an attempt to obtain high‐quality data series of monthly air temperature for Slovenian stations network in the period from 1961 to 2011. Intensive quality control procedure was applied to mean, maximum and minimum air temperature datasets from the Slovenian Environment Agency. Recently developed semi‐automatic homogenization tool HOMER (HOMogenisation softwarE in R) was used to homogenize the selected high‐quality datasets. To estimate the reliability of homogenized datasets, three to six experts independently homogenized the same datasets or their subsets. Different homogenization parameter settings were used by each of the experts, thus comprising ensemble homogenization experiment. Resulting datasets were compared by break statistics, root‐mean‐squared‐difference (RMSD) of monthly and annual values, and RMSD of the long‐term trend. This semi‐automatic homogenization approach based on metadata gave more reliable homogenization results than a fully automatic approach without metadata. While the network‐wide linear trend of the dataset did not change after semi‐automatic homogenization was applied, the distribution of the trends of individual stations became spatially more uniform. The arithmetic mean of the homogenized datasets of three experts was assigned as a reference homogenized dataset and it was compared with some publicly available homogenized datasets. The calculated linear trend on an annual level for Slovenia is strongly positive in all datasets, though the trend values are significantly different between the datasets. We conclude that the warming trend of near‐surface air temperature in Slovenia in 1961–2011 is significant and unequivocal in all seasons, except for autumn. Mean, maximum and minimum temperature series indicate linear trend of around 0.3–0.4 °C decade –1 on an annual level.