The Accelerating Frequency of Extreme Weather Events in Lisbon, Portugal

This study addresses the nonstationary behavior of the frequency of extreme rainfalls, heatwaves, and droughts in the Lisbon area Portugal, based on daily data at the Lisboa-Geofísico meteorological station, from 1 October 1864 to 30 September 2021 157 hydrological years. To identify the extreme hydrological events, different techniques and descriptors were applied, namely: 1 the peaks-over-threshold POT sampling technique in what concerns extreme rainfalls; 2 the heatwave magnitude index to identify the prolonged periods of abnormally hot weather or heatwaves; and 3 the standardized precipitation evapotranspiration SPEI to select the periods under drought conditions. The analysis of the temporal frequency of the extreme events utilized a kernel occurrence rate estimator KORE applied to their dates of occurrence. Overall, the frequency of the extreme daily rainfall showed an increasing trend from the year 1920 to 1960; thereafter, a decade of lower occurrence rates, followed by a steeper increase from the late 1960s on, translated into more than 5 independent extreme rainfalls per year. The results for Tmax heatwave days showed that their occurrence rates have changed over time, with up to 35 heatwave days per year recently. Regarding the periods under drought conditions, a generalized increase was detected in their frequency, with more recurrent periods under drought conditions lately. As a concluding point, it should be noted that the results were achieved based on robust statistical models and reliable data that, in some circumstances, served as abstractions of the climate change phenomenon and its underlying causes.