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Temporal changes in precipitation extremes since the beginning of the 20th century in Croatia

This analysis deals with time-series of precipitation amounts (annual and seasonal) and six indices of precipitation extremes indicating intensity and frequency of extreme rainfall events. They are proposed by World Meteorological Organization-Commission for Climatology (WMO-CCl) and the Research Programme on Climate Variability and Predictability (CLIVAR), and calculated after daily precipitation amounts. The data are from the period 1901-2008 for the five meteorological stations in Croatia covering different climate regions: continental, mountainous and maritime. Trends are calculated by a simple least squares fit of the linear model, and tested for statistical significance at the 95% confidence level using a non-parametric Mann-Kendall rank statistics t. For the series showing the significant trend identified by the Mann-Kendall coefficient t, a Sneyers progressive analysis of the time series was performed by means of the statistic u(t) in order to determine the beginning of this phenomenon. In order to eliminate short-term fluctuations and show the longer time scale changes more clearly, the noise was taken out of the data series by means of the weighted 11-year binomial moving average filter, which is often used for the analysis of climate variability. A search for change in variability has been performed by time series analysis of coefficients of variation (cv) in consecutive 30-year periods (1901-1930, 1902-31, ...., 1979-2008). During the 20th century annual amounts of precipitation showed a downward trend in all parts of Croatia, thus joining the trend of drying across the Mediterranean. Decadal trends in annual and seasonal precipitation amounts have not been significantly changed according to data series prolonged by 2008. Precipitation amounts have large interannual variability, both on annual and seasonal scales. Therefore, in order to find out position of 10 driest years in the observed 108-year period, it could be seen that they did not occur grouped in some period. Variability of annual precipitation amounts in the period 1901-2008, indicates a decrease in NW Croatia, mountainous region and northern littoral. Such a decrease was present in the eastern lowland by the end of the 20th century as well, but the changes since the beginning of the 21st century contribute to an increase of variability. Dalmatian islands experienced an increase of variability in a period from the middle of the 20th century. In the area of drying such as Croatia there is no signal of major secular changes in extremes related to the high amounts of precipitation (fraction of annual total precipitation due to very wet days (R95%T) and annual 1-day and 5-day maxima) and frequency of wet (Rd≥R75%) and very wet (R95%T) days over the larger part of Croatia. The reduction in the annual amounts of precipitation can be attributed to changes in the frequency of low-intensity rain days and significant increase in incidence of dry days (Rd<1.0 mm) all over Croatia.

trend analysis; precipitation variability; indices of precipitation extremes; Croatia

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