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Extreme precipitation events (EPEs) present potentially an enormous societal risk and often lead to major human and economic impacts. In the mid-latitudes, such EPEs are often triggered by intense extratropical cyclones and their associated frontal systems. Over the last decade, several studies have shown the important and specific role played by Atmospheric Rivers (ARs) in the occurrence of EPEs in western Europe, particularly in the Iberia Peninsula. In this study we analyze the all-time 24 h record-breaking precipitation (120.3 mm) recorded in the historical Dom Luiz Observatory (since 1863) in Lisbon, Portugal, between 12 and 13 December 2022. A synoptic evaluation of surface and upper-level fields from 5 to 14 December is performed using ERA-5 reanalysis. The week before the EPE, there was a combined effect of a large-scale SLP gradient resembling the NAO negative phase, a southerly position of the jet stream and an above normal positive SST anomalies over the North Atlantic leading to the development of several low-pressure systems at relatively low latitudes, all travelling along the same mean path towards western Europe. The atmospheric river associated with this event was first detected on late 10 December associated with a deep extratropical cyclone. The combination of high Integrated Vapor Transport (and moisture inflow by the warm conveyor belt), with a dynamical component characterized by a suitable uplift motion, allowed the system to evolve and maintain its AR characteristics for 72 h. The extratropical cyclone and associated AR moved northeast towards Iberia, making landfall in Portugal, on 12 December, as an extreme AR event, leading to the 24 h precipitation record breaking event. [ABSTRACT FROM AUTHOR]

The Iberian Peninsula (IP) is often affected by extreme precipitation and severe winds that are mostly associated with extratropical cyclones and their atmospheric fronts. ERA5 reanalysis data are used to revise and update two existing rankings of extreme precipitation and extreme potential wind‐loss days for the extended winter months on the IP. A ranking of concurrent precipitation and wind extreme events is produced by matching the two individual rankings for the IP and for specific IP subregions, including its main river basins. The intensity and the variability at the interannual and decadal scale are evaluated for precipitation, wind and concurrent precipitation and wind events. A systematic analysis of the underlying synoptic conditions is pursued for the top100 most extreme concurrent events including (a) cyclones' trajectories, (b) the presence of an Atmospheric River (AR), (c) recurrent patterns in the locations affected by precipitation and wind extremes and (d) possible long‐term trends. Results show that 85% of concurrent precipitation and wind extreme events are clearly associated with a cyclonic feature and most of these cyclones either cross the IP or pass to the northwest. Concurrent events are more likely occurring in the northwestern sectors of the IP on wintertime as in recent decades a downward trend for the events' frequency is observed in autumn. ARs are more likely associated with the strongest events occurring on the western sector of IP. Accordingly, the areas that mostly experience concurrent precipitation and wind extremes related to those systems are located in the northwestern sector of the peninsula. [ABSTRACT FROM AUTHOR]

In this work we performed an analysis on the impacts of blocking episodes on seasonal and annual European precipitation and the associated physical mechanisms. Distinct domains were considered in detail taking into account different blocking center positions spanning between the Atlantic and western Russia. Significant positive precipitation anomalies are found for southernmost areas while generalized negative anomalies (up to 75 % in some areas) occur in large areas of central and northern Europe. This dipole of anomalies is reversed when compared to that observed during episodes of strong zonal flow conditions. We illustrate that the location of the maximum precipitation anomalies follows quite well the longitudinal positioning of the blocking centers and discuss regional and seasonal differences in the precipitation responses. To better understand the precipitation anomalies, we explore the blocking influence on cyclonic activity. The results indicate a split of the storm-tracks north and south of blocking systems, leading to an almost complete reduction of cyclonic centers in northern and central Europe and increases in southern areas, where cyclone frequency doubles during blocking episodes. However, the underlying processes conductive to the precipitation anomalies are distinct between northern and southern European regions, with a significant role of atmospheric instability in southern Europe, and moisture availability as the major driver at higher latitudes. This distinctive underlying process is coherent with the characteristic patterns of latent heat release from the ocean associated with blocked and strong zonal flow patterns. We also analyzed changes in the full range of the precipitation distribution of several regional sectors during blocked and zonal days. Results show that precipitation reductions in the areas under direct blocking influence are driven by a substantial drop in the frequency of moderate rainfall classes. Contrarily, southwards of blocking systems, frequency increases in moderate to extreme rainfall classes largely determine the precipitation anomaly in the accumulated totals. In this context, we show the close relationship between the more intrinsic torrential nature of Mediterranean precipitation regimes and the role of blocking systems in increasing the probability of extreme events. [ABSTRACT FROM AUTHOR]

In this work we reassess the impacts of blocking patterns on precipitation regimes in the Iberian Peninsula, distinguishing between north Atlantic and European blocking. For this we take full advantage of the recently developed high-resolution datasets for the Iberian countries. Precipitation anomalies during blocking events obtained with this dataset allow a much finer regional characterization of the impacts in both average and extreme daily precipitation, particularly when compared to widely used low-resolution reanalysis datasets. Blocked patterns induce a negative-positive dipole of precipitation anomalies from northwest to southeast Iberia. Increases are widespread during Atlantic blocks and pronounced in southern and eastern areas of Iberia, while during European blocks they are more spatially restricted, with increases above 50 % in coastal Mediterranean areas, which represents a considerable fraction of the annual precipitation. Blocking impacts in precipitation are nearly opposite to those found during strong zonal flow situations, but there are also some asymmetries in the precipitation responses. A significant increase in cyclones and cut-off lows frequency southwards of blocking structures is related to precipitation excesses over southern and eastern areas, where dynamical factors and local processes play a crucial role. On the contrary, precipitation deficits in northwest Iberia during blocking episodes are better explained by a reduction in north Atlantic frontal activity and simultaneous decreases in large-scale moisture advection towards northern Iberia. We show that these anomalies during blocking result from changes in precipitation amount rather than from increases in rainy days, pointing to more extreme rainfall regimes, particularly in southeastern Iberia. Finally, an Extreme Value Analysis was performed, fitting Generalized Pareto Distributions to precipitation extremes. Results show that the different extreme precipitation regimes of northwest and Mediterranean regions are partially determined by opposite anomalies of the zonal flow. Thus, heavy precipitation events in Mediterranean areas are usually short-lived and frequently associated with blocking conditions, while in northwest Iberia the total accumulations during rainfall episodes are more important for triggering extreme events and they are mainly related to strong westerly flows. [ABSTRACT FROM AUTHOR]

The relationship between circulation weather types (WTs) and the spatial variability of precipitation across the Iberian Peninsula were studied using a high density, quality controlled, homogenized monthly precipitation database with approximately 3000 stations and interpolated to a 10 km grid. The circulation WTs were computed using an objective version of the Lamb classification centered on the Iberian Peninsula. A total of 26 WTs were selected for the period 1948-2003. Daily WTs were grouped to obtain their monthly frequencies, and used as potential independent variables in a linear least-square non-negative regression model with a forward stepwise selection. Results show the impact of each WT on precipitation in the Iberian Peninsula with a spatial resolution which had never been achieved before and additionally were obtained on a monthly scale highlighting the large seasonal cycle observed in each class, and including significantly different patterns in winter and summer. Nevertheless, results confirm that most of the precipitation in the Iberian Peninsula is produced by just a few WTs, with W, SW and C being the most influential. The association between WTs and precipitation is more robust in winter months and for the western IP areas, while it is lower during summer months and for the eastern IP areas. Spatial analysis revealed that precipitation on the Mediterranean coastland is mostly related with easterly flows (NE, E, SE and their hybrid counterparts), while on the Cantabrian coastland. N and NW flows are the most influential WTs. In general, cyclone-related types are the least frequent ones and also the most efficient in generating precipitation; while anticyclone-related types have the highest frequencies, but also the lowest contribution to total monthly rainfall in the Iberian Peninsula. [ABSTRACT FROM AUTHOR]

An analysis of the frequency of cyclones and surface wind velocity for the Euro–Atlantic sector is performed by means of an objective methodology. Monthly and seasonal trends of cyclones and wind speed magnitude are computed and trends between 1960 and 2000 evaluated. Results reveal a significant frequency decrease (increase) in the western Mediterranean (Greenland and Scandinavia), particularly in December, February, and March. Seasonal and monthly analysis of wind magnitude trends shows similar spatial patterns. We show that these changes in the frequency of low-pressure centers and the associated wind patterns are partially responsible for trends in the significant height of waves. Throughout the extended winter months (October–March), regions with positive (negative) wind magnitude trends, of up to 5 cm/s/year, often correspond to regions of positive (negative) significant wave height trends. The cyclone and wind speed trends computed for January–March are well matched by the corresponding trends in significant wave height, with February being the month with the highest trends (negative south of lat 50°N up to −3 cm/year, and positive up to 5 cm/year just north of Scotland). Trends in European precipitation are assessed using the Climatic Research Unit data set. The results of the assessment emphasize the link with the corresponding tendencies of cyclone frequencies. Finally, it is shown that these changes are associated, to a large extent, with the preferred phases of major large-scale atmospheric circulation modes, particularly with the North Atlantic Oscillation, the eastern Atlantic pattern, and the Scandinavian pattern. [ABSTRACT FROM AUTHOR]

The extratropical cyclones that originate in the North Atlantic and propagate towards Europe are one of the major natural hazards in mid-latitudes. In December 2019, three consecutive extratropical cyclones named Daniel, Elsa, and Fabien affected Portugal. In this study, the synoptic and upper-level dynamic conditions associated with these systems during their impact in mainland Portugal are evaluated. The persistent intense zonal flow that crossed the entire Atlantic revealed by the integrated vapor transport and the vertically integrated moisture flux favored these hydro-meteorological systems. The patterns of mean sea level pressure, geopotential, potential vorticity, total column water, and convective available potential energy were used to characterize the influence of every system over mainland Portugal. A cluster analysis of monthly precipitation permitted the classification of the country into four main regions named the Northwest, Centre West, Northeast and Centre East, and South region on which the analysis was focused. The three storms affected every region on consecutive days by the middle of December, producing extreme precipitation events and significant effects on the accumulated rainfall and runoff, particularly in the Northwest, Centre West, Northeast and Centre East regions. As consequence, multiple incidences of damage were reported along mainland Portugal. However, an assessment of the Standardized Precipitation Index (SPI) and the Standardized Precipitation–Evapotranspiration Index (SPEI) on time scales of 1, 3, 6, and 12 months revealed a positive impact of rainfall increase on the attenuation of short and long term accumulated drought conditions, particularly in the center and north regions. [ABSTRACT FROM AUTHOR]

Variability and predictability of extreme weather events are research topics for which significant advances are strongly demanded by the scientific community. The socio-economic impacts of these events as fatalities, proprieties damages, rebuilding and reinsurance costs are becoming critical in the context of ongoing climate change. Within this framework, precipitation extremes associated with extra-tropical cyclones that affect the Iberian Peninsula are considered in this study. The aim of this research is to provide for an automated methodology to objectively assign precipitation and atmospheric fronts to the corresponding cyclones in order to study the characteristics of winter storms associated with extreme precipitation in the Iberian Peninsula.Firstly, a method to objectively assign the amount of precipitation to mid-latitude fronts at sub-daily basis is used (Hénin et al., 2018a). The method relies on the co-location of 6-hourly accumulated precipitation and fronts (both warm and cold fronts) inside a search box of predefined dimension. Results for the period 1979-2016 show that wintertime frontal precipitation accounts for up to 70% of total precipitation over the western European extremities. Secondly, an automated procedure is developed to objectively assign, at sub-daily basis, the fronts to the center of the parent cyclone. The approach is similar to the one used to relate precipitation and fronts: a search box is used to look for the cyclone center in the neighborhood of a frontal feature. A catalog of storms is obtained for the Northern Hemisphere from 1979 to date, based on mean sea level minima for cyclones' center's detection and on the nearest neighbor search for cyclones tracking (Trigo, 2006).Results show that the assignment of fronts to cyclones is not always straightforward and it strongly depends on the different mechanisms driving the overall weather pattern. For example, local pressure minima may form in the edge of a pre-existing cyclone making ambiguous to state which cyclone each front is attending. Therefore, a subset of extreme events in Iberia is analyzed taking into account three different situations: precipitation extremes, wind extremes and compound events due to extreme precipitation and wind. Different precipitation sources, including the new ERA5 reanalysis product by the European Centre for Medium-Range Weather Forecasts (ECMWF) currently available from 2000 to date, are considered, according to the assessment of satellite-based estimates and high-resolution precipitation datasets over Iberia presented in Hénin et al. (2018b). REFERENCESHénin, R.; Ramos, A.M.; Schemm, S.; Gouveia, C.M. Liberato, M.L.R. Assigning precipitation to mid-latitudes fronts on sub-daily scales in the North Atlantic and European sector: Climatology and trends. Int. J. Climatol. 2018a, 1-14. DOI: 10.1002/joc.5808.Hénin, R.; Liberato, M.L.R.; Ramos, A.M.; Gouveia, C.M. Assessing the Use of Satellite-Based Estimates and High-Resolution Precipitation Datasets for the Study of Extreme Precipitation Events over the Iberian Peninsula. Water. 2018b, 10(11), 1688. DOI: 10.3390/w10111688.Trigo, I. Climatology and interannual variability of storm-tracks in the Euro-Atlantic sector: a comparison between ERA-40 and NCEP/NCAR reanalyses. Climate Dynamics. 2006, 26: 127–143. DOI 10.1007/s00382-005-0065-9.ACKNOWLEDGMENTSThis work is supported by the Portuguese Science and Technology Foundation (Fundação para a Ciência e a Tecnologia – FCT), under the projects UID/GEO/50019/2013 – Instituto Dom Luiz and CMU/CS/0012/2017 – "eCSAAP - expert Crowdsourcing for Semantic Annotation of Atmospheric Phenomena". FCT is also providing for R. Hénin doctoral grant (PD/BD/114479/2016) and A.M. Ramos postdoctoral grant (FCT/DFRH/SFRH/BPD/84328/2012). [ABSTRACT FROM AUTHOR]