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Assessment of extreme rainfall thresholds for an early warning system in Croatia

The changes in rainfall extremes due to climate change may have negative consequences in the engineering designs, particularly for urban infrastructure that relies on the short-term rainfall amounts. In Croatia, the early warning system aside on the official web page is also integrated into the Meteoalarm severe weather alert system for Europe. The current alerts for heavy rain are issued for eight regions in Croatia with the thresholds based on 6 and 24 hours return values associated with 1, 2 and 5 years return periods for yellow, orange and red alarming categories, respectively. The corresponding return values were estimated by fitting the Generalized Extreme Value distribution (GEV) to the series of the annual maxima of 6 and 24 hours precipitation amounts for the 1959-2005 period. Eight regions were covered by 17 meteorological stations from DHMZ station network. The main goal of this study was to upgrade the alarm system by means of improving thresholds due to a longer period of measurements and moving from the regions defined by the climatological characteristics to the county level (21 counties). To that end, new rainfall thresholds are estimated by employing ombrographic data from 58 stations covering the period 1961-2020. Different scales of sub-daily rainfall amounts, from 10 minutes to 24 hours, are analysed by means of extreme value theory. Both stationary and non-stationary GEV models are tested and the results revealed the adequacy of the stationary one for estimation of the return values. The spatial distribution of the corresponding 1, 2, 5, 10, 25, 50 and 100 years return values showed a clear distinction between the continental and Adriatic regions with a pronounced maximum in the northern Adriatic (Kvarner) region. The new proposal for the warning rainfall thresholds in Croatia is discussed. It would be still based on the 6 and 24 hours duration, but considering higher return periods than in the current system by shifting to 1, 5 and 25 years for yellow, orange and red colours, respectively. Generally, the new threshold values are higher in each category than in the current system, especially for orange and red warnings along the coast and in the mountain region. Following the international standards and procedures for using the longer data sets as well as spatially smaller warning regions, the current warning system will be refined.

Extreme short-term precipitation, Meteoalarm severe weather alert system, Generalized Extreme Value distribution

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