Sub-Hourly Precipitation Extremes in Mainland Portugal and Their Driving Mechanisms

Abstract

Sub-hourly heavy precipitation events (SHHPs) frequently underlie major meteorological hazards, but their comprehensive analysis is still lacking in Portugal. A 71-weather-station dataset for 2000–2020 is used in this article to (1) diagnose SHHPs corresponding to a 10-min precipitation event of at least 5.0 mm, (2) characterize their spatial-temporal distribution, and (3) identify their associated synoptic-scale conditions. Two synoptic types are associated with SHHPs: remote (RemL) and regional (RegL) low-pressure systems. RegL SHHPs display two marked maxima in spring and autumn, while RemL SHHPs show a single maximum in autumn. Most RegL events occur in the afternoon/evening, while RemL events show a slight bias toward midday occurrences. In the case of RemL, the wind is stronger for 2 to 3 h before and during SHHPs, veers from 180° to 210° near the event, the pressure decreases until 20 min before the event, and the wet-bulb temperature decreases around the time of the event and remains low, thus reflecting cold-front passages. For RegL, maximum winds coincide with precipitation peaks, and the wet-bulb temperature briefly decreases in association with downdrafts. A preliminary relationship between the SHHPs and mesoscale convective systems is established by detecting sudden surface-pressure surges, which are indicative of mesohighs caused by evaporatively cooled downdrafts. A calendar of mesohigh episodes linked to SHHPs is provided herein and their signatures are illustrated for the “Pedrógão-Grande” fires. Indicators of several downbursts, cold pools, and mesohighs were identified by the AROME forecast. This first, systematized analysis paves the way to identifying dynamic precursors, enabling their integration into early warning systems and climate projections.