The Record-Breaking Precipitation Event of December 2022 in Portugal: Synoptic Background

Extreme precipitation over western Iberia is mostly concentrated in the winter half-year. While relatively rare, intense precipitation events can disrupt and are often associated with major human, social and economic damages. Most of the time these extreme precipitation events are triggered by intense extratropical cyclones and associated frontal systems. However, in the last decade a number of studies have shown the important role played by Atmospheric Rivers (ARs) in the occurrence of extreme precipitation events in western Europe, particularly in the Iberia Peninsula.

In this study we analyse the all-time 24h record-breaking precipitation values recorded in Lisbon, Portugal between the 12 and 13 December 2022 in terms of the synoptic background. We obtained a comprehensive synoptic characterization of the atmospheric circulation between the 1^st and 15^th of December, considering a wide range of meteorological fields, such as vertically integrated water vapor flux, sea level pressure, geopotential height and divergence at the 850 hPa isobar, divergence at the 200 hPa, vertical velocity at the 500 hPa and temperature and specific humidity at 900 and 600 hPa.

Results show that on the 8 December by 06 UTC an extratropical cyclone was present in the middle of the North Atlantic, with a high moisture content and that by 18 UTC on the following day a cut-off low was formed in the northwest Atlantic. This cut-off system was well characterized by relatively high vertical velocities and convergence at the low levels, combined with high rates of evaporation acquired over the Gulf Stream, intensifying the moisture content to its south side. Both systems converged on 10 December by 12 UTC and by the 18 UTC the algorithm detected an AR located southward of the extratropical cyclone. The combination between high IVT values, with maxima ranging between 947 kg m^-1 s^-1 and 1227 kg m^-1 s^-1, with a dynamical component characterised by winds above 20 m/s, as well as a suitable vertical motion, allowed the system to evolve and maintain the AR characteristics for 72 h. The AR progressed towards Iberia, affecting Portugal and central Spain as an extreme AR event, leading to the 24h precipitation record of 134.6 mm measured at the Geophysical Institute in Lisbon, the highest value since continuous measurements started in the 1860. The previous record was registered on the 18 February 2008, with a value of 118.4 mm.

This work was supported by the Portuguese Science Foundation (FCT) through the project AMOTHEC (DRI/India/0098/2020) with DOI 10.54499/DRI/India/0098/2020 and also through national funds (PIDDAC) – UIDB/50019/2020. Tiago Ferreira was supported by FCT through PhD grant UI/BD/154496/2022.