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Name: Helga Kristin Olafsdottir

Talk Title: Frequency increase in extreme rainfall events in the Northeastern USA with stable intensity distribution

Abstract: For infrastructure planning, information of both intensities and frequencies of individual extreme rainfall events are of importance. Climate change brings an additional complexity, since extreme daily rainfalls can be increased by the individual extreme rainfalls becoming more frequent, more intense, or both. We develop a new statistical extreme value model, the PGEV model, to predict to what extent these three scenarios will occur. The model combines the Generalized Extreme Value (GEV) distribution for annual maxima series and General Pareto (GP) distribution for exceedances over threshold for the partial duration series. This allows the use of high quality annual maxima series data instead of less well checked daily data to estimate trends in intensity and frequency separately.

The method is applied to annual maxima data from the NOAA Atlas 14, Volume 10. For the majority of stations in the Northeastern USA, the frequency of extreme rainfall events increases with increased mean temperature, without much evidence of trends in the distribution of the intensities of individual extreme daily rainfall events. Naturally, increasing trends in frequency also increase the yearly or 10-yearly risks of very extreme rainfall events.

The median of the frequency trends corresponds to extreme rainfalls becoming 83% more frequent for each centigrade degree of temperature increase, but there is an important local variation in the trends. For many stations, the frequency increase exceeds 150% per centigrade temperature increase. The analysis is extended to three other large areas in the contiguous USA, showing a similar but weaker effect.

This talk is a contributed talk at EVA 2021.