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Title: Trends in the extremes of environments associated with severe US thunderstorms Authors:  Jonathan Koh - EPFL (Switzerland) [presenting]
Erwan Koch - EPFL (Chair of Statistics STAT) (Switzerland)
Anthony Davison - EPFL (Switzerland)
Abstract: Concurrently high values of convective available potential energy (CAPE) and storm relative helicity (SRH) are conducive to hazardous convective weather associated with severe thunderstorms. Hence, it is highly relevant to have probabilistic models for both variables' extremes that use relevant covariate information to account appropriately for their spatial and temporal dependence. We consider a large area of the contiguous United States over the period 1979--2015 and use statistical extreme value models and appropriate multiple testing procedures to identify trends in the extremes. In the first step, we show that there is a significant time trend in the extremes for CAPE and SRH maxima in the spring and summer months. These increases in CAPE are also relevant for rainfall extremes and are expected in a warmer climate but have not previously been reported. In the second step, we focus on the local spatial extremal dependence structure and thus model the pointwise maxima using max-stable random fields. We focus especially on the Brown--Resnick field with a power variogram and investigate the effect of the El Ni\~no-Southern Oscillation (ENSO) on its parameters with the use of tensor product splines. Our results show that the range parameters for CAPE and SRH are lower in the spring and summer, so the corresponding extremes are more localized during these seasons than in the winter. Lastly, we find that these seasonal differences are more pronounced during La Ni\~na events.