The aim of this study is to identify the main mesoscale features and mechanisms responsible for the generation of a very intense precipitation and wind storm event, named “Vaia”, that affected the ...eastern Italian Alps on 27–29 October 2018. The event was characterized by extreme accumulated precipitation (up to 850 mm in three days) and exceptionally strong winds, causing severe and widespread impacts, such as floods, landslides, and extensive damages to forests and growing stock. The synoptic situation was characterized by a trough, which deepened over the eastern Atlantic, extending to France and Spain, driving a strong moist flow towards the Alpine region. At the surface, a wide cyclonic area developed over the western Mediterranean, east of the trough axis, and moved, deepening, towards northwestern Italy. The storm is investigated using a comprehensive dataset composed of both observations and numerical simulations by means of two models, namely WRF and MOLOCH, at convection-permitting resolution. The analysis highlights that the storm was characterized by two consecutive phases with strong precipitations, both fed by an intense moist southerly flow. In particular, the second phase was also marked by strong wind gusts in the Alpine area, exceeding 50 m s−1 at some weather stations. It is found that these extreme wind gusts were connected to the presence of an intense southerly low-level jet immediately ahead of a cold front, displaying an average wind speed of 35 m s−1 at 1500 m MSL. The comparison between observations and numerical results shows that the main characteristics of the storm are well simulated by both models, confirming the high predictability of this kind of events, typically associated with well-defined large-scale forcing. Also local scale features are reasonably captured by the simulations, despite the high complexity of the Alpine orography. However, WRF significantly underestimates total precipitation amounts over the most affected areas, while wind speed is overestimated by both models in the inner Alpine sectors.
•The Vaia storm is analyzed with observations and convection-resolving simulations.•WRF and MOLOCH reasonably reproduce precipitation and wind gusts over the Alps.•A strong low-level jet ahead of the cold front is detected by both observations and simulations.•The variability in the numerical results induced by different global analyses is small.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The two most intense precipitation events that occurred over northern and central Italy in the last century are analyzed in this study: the November 1966 flood and the Vaia storm in October 2018. ...These two events share a similar large-scale evolution, characterized by a vigorous baroclinic wave, deepening on the western Mediterranean basin and slowly evolving eastward. Although this is a common synoptic setting for severe weather over Italy in autumn, in these cases the interaction between incoming Rossby wave packets produced a particularly strong downstream development over the Mediterranean. In both events, the large-scale dynamics was able to focus towards the Mediterranean basin an exceptional high amount of moisture transported from the Tropics in the form of Atmospheric Rivers (ARs), although with local and remote differences related with the sea and atmospheric state conditions characterizing these two episodes occurring 52 years apart. As a result, the precipitation patterns, in terms of both duration, intensity and distribution, were quite different: while in 1966 heavy rainfall affected for 48 h mostly Tuscany region (infamous Florence flood) and north-eastern Italy, in 2018 almost the entire Alpine chain, as well as Liguria region and central Italy, were hit by severe events during almost three days. Only over the north-eastern Italian Alps the rainfall fields look similar in the two events. The mesoscale dynamics and the moisture supplies are investigated in detail, highlighting peculiarities and common aspects. It is found that the different characteristics of the ARs (intensity and steadiness) partially explain the rainfall patterns, but the complete picture has to take into account also local (e.g. Mediterranean) sources of moisture and smaller scale circulation features that turned out to be relevant. Through a relative comparison, this study points out some important aspects of the genesis of extreme precipitation events over Italy, identifying important precursors and moisture sources. In an operational context, this may help to recognize the environmental conditions potentially leading to the most severe precipitation episodes.
•Vaia storm and 1966 flood are investigated with datasets, reanalysis and numerical simulations.•The events share the same large scale-configuration.•Presence in both cases of an Atmospheric River that explains the spatial precipitation pattern.•Water budget analysis is computed to evaluate main contributing fluxes.
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Abstract
On November 12, 2019, an exceptional flood event took place in Venice, second only to the one that occurred on November 4, 1966. The sea level reached a peak value of 1.89 m above the local ...datum determining the flooding of almost 90% of the pedestrian surface of the historical city. Several processes concurred to raise the water level in Venice and the northern Adriatic Sea on November 12, 2019. Among these, a fast‐moving mesoscale cyclone travelled at about 12 m s in the northwestward direction over the northern Adriatic Sea, raising the sea levels at the shore in front of the Lagoon of Venice. High‐resolution numerical simulations indicated that atmosphere–ocean resonance occurred on November 12, 2019, generating a meteotsunami‐like wave that contributed significantly to the extreme sea level in Venice. The relative contributions of the wind and air pressure to the peak sea level were also estimated. Additional numerical experiments were performed to prove the occurrence of Proudman resonance and to determine a transfer function of such high‐frequency sea‐level perturbations for the Lagoon of Venice.
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On 12 November 2019, an exceptional flood event took place in Venice, second only to the one that occurred on 4 November 1966. Moreover, with four extremely high tides since 11 November 2019, this ...was the worst week for flooding in Venice since the beginning of sea level records (1872). The event that struck Venice and the northern Adriatic Sea on 12 November 2019, although having certain conditions seemingly typical of the events causing exceptional high waters, had some peculiar characteristics not observed before, which deserved an in-depth analysis. Several factors made this event exceptional: the in-phase timing between the peak of the storm surge and the astronomical tide; a deep low-pressure cyclone over the central-southern Tyrrhenian Sea that generated strong Sirocco (south-easterly) winds along the main axis of the Adriatic Sea, pushing waters to the north; a fast-moving local depression – and the associated wind perturbation – travelling in the north-westward direction over the Adriatic Sea along the Italian coast, generating a meteotsunami; very strong winds (28 m s−1 on average with 31 m s−1 gusts) over the Lagoon of Venice, which led to a rise in water levels and damages to the historic city; and an anomalously high monthly mean sea level in the Adriatic Sea, induced by a standing low-pressure and wind systems over the Mediterranean Sea, that was associated with large-scale low-frequency atmospheric dynamics. In this study, the large set of available observations and high-resolution numerical simulations have been used to quantify the contribution of the mentioned drivers to the peak of the flood event and to investigate the peculiar weather and sea conditions over the Mediterranean Sea during the Venice floods of November 2019.
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•On the 12 November 2019, an exceptional flood event took place in Venice.•Low-frequency disturbances of air pressure and wind provided the long-term precondition for floods.•The resonant coupling of the mesoscale atmospheric forcing and the sea generated a meteotsunami.•Very strong local winds led to a further rise in sea levels and damages to the historic city.
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The Adriatic basin is regularly affected by cold, strong and gusty bora winds blowing from the northeast, especially during winter. These events are characterized by intense air–sea interactions and ...produce important meteorological effects not only over the eastern Adriatic basin, where bora originates and attains its maximum intensity, but also downstream over the Italian peninsula where heavy rainfall and snowfall can occur.
The present study aims at evaluating the impact of surface fluxes of sensible and latent heat, that characterize air–sea interaction during a bora episode, on wind strength and profiles over the Adriatic Sea, in relation to intense precipitation affecting the Apennines and the Italian coast. High‐resolution numerical simulations are used in order to assess the role of surface fluxes in modulating the atmospheric water balance, modifying the thermodynamic characteristic of the boundary layer and, in turn, the dynamics of the orographic flow regime.
Results show that while surface evaporation is responsible for a relatively small contribution to the total atmospheric water budget over the Adriatic area, surface fluxes still have a remarkable impact on precipitation via dynamical processes. Both sensible and latent heat fluxes modify the speed, temperature and moisture profiles of the low‐level bora wind, sensibly changing the dynamical characteristics of interaction of the flow with the downstream orography. The orographic flow regime determines the intensity and location of orographically induced uplift and hence precipitation. Therefore, the picture that directly associates the precipitation amount upstream and over the Apennines with the degree of moistening of air during its passage over the sea and with orographic uplift is shown to be too simplistic. The variations of the wind speed and static stability due to surface fluxes involve complex and nonlinear effects, changing the flow regime in response to the orographic forcing and thus determining amount and location of heavy precipitation.
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Forecasting flash floods some hours in advance is still a challenge, especially in environments made up of many small catchments. Hydrometeorological forecasting systems generally allow for ...predicting the possibility of having very intense rainfall events on quite large areas with good performances, even with 12–24 h of anticipation. However, they are not able to predict the exact rainfall location if we consider portions of a territory of 10 to 1000 km2 as the order of magnitude. The scope of this work is to exploit both observations and modelling sources to improve the discharge prediction in small catchments with a lead time of 2–8 h. The models used to achieve the goal are essentially (i) a probabilistic rainfall nowcasting model able to extrapolate the rainfall evolution from observations, (ii) a non-hydrostatic high-resolution numerical weather prediction (NWP) model and (iii) a distributed hydrological model able to provide a streamflow prediction in each pixel of the studied domain. These tools are used, together with radar observations, in a synergistic way, exploiting the information of each element in order to complement each other. For this purpose observations are used in a frequently updated data assimilation framework to drive the NWP system, whose output is in turn used to improve the information as input to the nowcasting technique in terms of a predicted rainfall volume trend; finally nowcasting and NWP outputs are blended, generating an ensemble of rainfall scenarios used to feed the hydrological model and produce a prediction in terms of streamflow. The flood prediction system is applied to three major events that occurred in the Liguria region (Italy) first to produce a standard analysis on predefined basin control sections and then using a distributed approach that exploits the capabilities of the employed hydrological model. The results obtained for these three analysed events show that the use of the present approach is promising. Even if not in all the cases, the blending technique clearly enhances the prediction capacity of the hydrological nowcasting chain with respect to the use of input coming only from the nowcasting technique; moreover, a worsening of the performance is observed less, and it is nevertheless ascribable to the critical transition between the nowcasting and the NWP model rainfall field.
A large number of intense cyclones occur every year in the Mediterranean basin, one of the climate change hotspots. Producing a broad range of severe socio-economic and environmental impacts in such ...a densely populated region, Mediterranean cyclones call for coordinated and interdisciplinary research efforts. This article aims at supporting these efforts by reviewing the status of knowledge in the broad field of Mediterranean cyclones. First, we focus on the climatology of Mediterranean cyclone tracks, their relationship to large-scale atmospheric circulation and their future trends. Second, we discuss the dynamics and atmospheric processes that govern the genesis and development of Mediterranean cyclones. Then, we present the different subtypes of Mediterranean cyclones, devoting special attention to medicanes, i.e. cyclones with tropical characteristics and subjects of numerous recent studies. In a subsequent section, we review the state of the art in forecasting cyclones and relevant high-impact weather, and we discuss in detail the challenges and recent efforts to increase their forecast skill. Finally, we discuss the main impacts produced by cyclones, namely heavy precipitation, windstorms, dust transport, storm surges and sea wave extremes. In the last section of this review article, we thoroughly outline the future directions of research that would advance the broader field of Mediterranean cyclones.
Heavy precipitation (HP) constitutes a major meteorological threat in the western Mediterranean (WMed). Every year, recurrent events affect the area with fatal consequences for infrastructure and ...personal losses. Despite this being a well-known issue widely investigated in the past, open questions still remain. Particularly, the understanding of the underlying mechanisms and the modeling representation of the events must be improved. One of the major goals of the Hydrological Cycle in the Mediterranean Experiment (HyMeX; 2010-2020) has been to advance knowledge on this topic. In this article, we present an overview of the most recent lessons learned from HyMeX towards an improved understanding of the mechanisms leading to HP in the WMed.
A high‐impact meso‐beta vortex in the Adriatic Sea Miglietta, Mario Marcello; Buscemi, Federico; Dafis, Stavros ...
Quarterly journal of the Royal Meteorological Society,
January 2023 Part A, 2023-01-00, 20230101, Volume:
149, Issue:
751
Journal Article
Peer reviewed
Open access
On the evening of November 12, 2019, an exceptional high tide – the second‐highest in the ranking since sea‐level data have been recorded – hit the city of Venice in northern Italy and its entire ...lagoon, damaging a large part of its historical center. A small warm‐core mesoscale cyclone, which formed in the central Adriatic Sea and intensified during its northwestward movement toward the Venice lagoon, was responsible for the event. The cyclone was preceded by intense northeasterlies (Bora) in the northern Adriatic, which turned to southeasterlies (Sirocco) and then southwesterlies after its passage. Simulations with different initialization times were carried out with the Weather Research and Forecasting (WRF) model. Simulation results show a strong sensitivity to the initial conditions, since the track (and strength) of the cyclone was determined by the exact position of an upper‐level potential vorticity (PV) streamer. The factors responsible for the cyclone development and its characteristics are also investigated. The pre‐existence of positive low‐level cyclonic vorticity, associated with the convergence of the Sirocco and Bora winds in the central Adriatic, made the environment favorable for cyclone development. Also, the interaction between the upper‐level PV anomaly and the low‐level baroclinicity, created by the advection of warm, humid air associated with the Sirocco, was responsible for the cyclone's intensification, in a manner similar to a transitory (stable) baroclinic interaction at small horizontal scales. Sensitivity experiments reveal that convection, latent heat release and sea‐surface fluxes did not play a significant role, indicating that this cyclone did not show tropical‐like characteristics, notwithstanding its low‐level warm core. Thus, the warm‐core feature appears mainly as a characteristic of the environment in which the cyclone developed rather than a consequence of diabatic processes. Lastly, the cyclone does not fall into any of the existing categories for Adriatic cyclones.
At about 2100 UTC, November 12, 2019, a small warm‐core cyclone, formed in the central Adriatic Sea, made landfall near Venice and was responsible for an exceptional high tide. Convection and sea‐surface fluxes did not play a significant role in the cyclone development, notwithstanding its low‐level warm core. Conversely, the interaction between the upper‐level PV anomaly and the low‐level baroclinicity was responsible for its intensification, in a manner similar to a transitory (stable) baroclinic interaction at small horizontal scales.
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