Spatial–Temporal Variability of Hourly Precipitation Extremes in Portugal: Two Case Studies in Major Wine-Growing Regions

Abstract

Heavy precipitation events are a natural hazard that have significant socioeconomic implications, especially in sectors like agriculture, namely viticulture. This study is the first to present a detailed climatology of extreme precipitation across mainland Portugal, leveraging hourly data gathered from 71 meteorological stations over 23 years, spanning from 2000 to 2022. To understand spatial variability, we focus on Douro and Alentejo, two major winemaking regions where extreme weather events may have significant adverse consequences. High spatial variability in the β-index patterns (mean ratio between over-threshold precipitation and total precipitation) shows that hourly precipitation between 10 and 20 mm h⁻¹ contributes the most to winter and autumn precipitation, whereas events exceeding 20 mm h⁻¹ play a more significant role in summer and autumn. Extreme events occur most frequently between September and December, with a secondary peak observed in April and May, particularly pronounced in Alentejo. The diurnal cycle, which displays a peak in the afternoon, exhibits a close relationship with the occurrence of thunderstorms. A more in-depth analysis was conducted to examine the dynamic and thermodynamic mechanisms that contributed to two extreme precipitation events caused by thunderstorms in the Douro and Alentejo winemaking regions. Differing synoptic environments led to unstable conditions at the origin of these events. The Douro event was driven by a cut-off low, whereas the Alentejo event was related to an extratropical cyclone, both in their final stages of development. The regional indices and seasonal patterns identified offer insights into spatial distribution and seasonality, essential for future research and applied climate adaptation strategies.