Future projections of wet snow frequency and wet snow load on overhead high voltage conductors over Italy

Heavy snowfalls represent some of the most serious hazard for transmission and distribution grids because they trigger the formation of sleeves on overhead power lines whose loads may cause outages and, consequently, prolonged disruptions of the energy supply. In order to provide useful information in planning and decision making to cope with wet snow impacts and strength the resilience of the Italian transmission network, some future climate projections about the occurrences of wet snow events and the related loads on overhead power lines were elaborated for different 20-year periods until 2100 by using 12 high-resolution Euro-CORDEX models (spatial resolution of similar to 12 km) under the two different emission pathways RCP8.5 (without mitigation) and RCP4.5 (with moderate reductions of greenhouse gases emission). In addition, the reanalysis dataset MERIDA OI was used to bias-correct the climate models as well as to implement a methodology aimed to estimate climatic wet snow scenarios, by the use of a wet snow accretion model on power lines. The results were validated comparing them with wet snow loads observed at the Vinadio WILD station (Italy) and with the recorded failures in energy supply during some serious wet snow events occurred in Italy. Referring to Extreme Value Analysis, probability maps of wet snow loads exceeding some infrastructural hazard thresholds were estimated by means of the "Generalized Extreme Values" distributions, assuming a stationary climate in each 20-year period. In order not to have to assume climate stationarity, two new frequency indices were calculated directly from the outputs of the models, and the Wilcoxon test was applied to investigate the statistical significant of the trends, as well the agreement among the models to test the robustness of the results. In both methods the results point out relatively complex patterns of climate change effects: wet snow events will generally decrease in frequency and intensity at medium-low altitude, particularly under RCP8.5 pathway where projections highlight a sharply reduction of the events, as rainfalls will prevail over snowfalls due to global warming. But, by the end of the Century, wet snow events are likely to intensify over high Apennine and Alpine regions so far spared because of their historical cold temperatures. Finally, the evolution of wet snow events was investigated and tested more specifically for some mountain sites.