Precipitation extremes in Romania

What is it about?

A new index quantifying the maximum rain intensity (IMAX) within a rainfall event is proposed. The paper goal is twofold. First, the characteristics of variability (trends, change points) for maximum values of this index calculated for spring (April-May), summer (June- August) and autumn (September – October) are analysed, in comparison with other two precipitation indices (daily maximum and total amount). Secondly, the scaling of the IMAX high percentiles with temperature is investigated and the results are compared to hourly precipitation extremes. The analysis is carried out at six Romanian stations over the period 1966-2007, extended over 1902-2007 for one station. Our results reveal a statistically significant increase of IMAX over the 20th century (intensified over the second half), in contrast with no significant trends of the other two precipitation indices. On the other hand, an opposite phase between the IMAX variability and its corresponding duration (e.g. shorter durations correspond to higher intensities) is noted. Regarding the scaling behaviour of the IMAX percentiles, there is a difference between summer and spring/autumn. In summer, the 90th and 99th percentiles show a Clausius-Clapeyron (CC) scaling for temperatures between 140C and 260C and then a decrease, while the 99.9th percentile shows a super-CC scaling for temperatures lower than 180C and then a decrease. The spring and autumn the 90th and 99th percentiles show an approximate 2CC scaling for temperatures range of 80C-200C. The 99.9th percentile exhibits a scaling close to the 2CC relationship for temperatures range of 120C-180C (spring) and then a decrease, while the autumn 99.9th percentile exhibits a dependence close to 2CC scaling for temperatures < 180C and then a scaling close to CC. Comparing these results with those referring to hourly and daily precipitation extremes we could conclude that the magnitude of the CC scaling in Romania is mainly dependent on the temporal scale of the extreme precipitation event, storm intensity and season. The proposed index (IMAX) is more sensitive to temperature increase than the hourly and daily extremes and therefore it is more appropriate to quantify the climate signal related to intense precipitation events in a warmer climate.

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Why is it important?

Our results reveal a robust climate signal over the entire country referring to an opposite phase between the variability of the maximum rainfall intensity (IMAX) and its corresponding duration (DIMAX) (e.g. shorter durations correspond to higher intensities).A new index (IMAX) quantifying the maximum rain intensity within a rainfall event is proposed in this study. IMAX values and their corresponding durations (DIMAX) at the six stations in Romania, seasonally stratified (spring, summer and autumn), are analysed in two ways corresponding to the main two objectives of this paper. The IMAX scaling behaviour depends on the season, storm intensity and varies over the temperature range.

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