Strong and persistent low-level winds blowing over the Adriatic basin are often associated with intense precipitation events over Italy. Typically, in case of moist southeasterly wind (Sirocco), ...rainfall affects northeastern Italy and the Alpine chain, while with cold northeasterly currents (Bora) precipitations are localized along the eastern slopes of the Apennines and central Italy coastal areas. These events are favoured by intense air-sea interactions and it is reasonable to hypothesize that the Adriatic sea surface temperature (SST) can affect the amount and location of precipitation.
High-resolution simulations of different Bora and Sirocco events leading to severe precipitation are performed using a convection-permitting model (MOLOCH). Sensitivity experiments varying the SST initialization field are performed with the aim of evaluating the impact of SST uncertainty on precipitation forecasts, which is a relevant topic for operational weather predictions, especially at local scales. Moreover, diagnostic tools to compute water vapour fluxes across the Italian coast and atmospheric water budget over the Adriatic Sea have been developed and applied in order to characterize the air mass that feeds the precipitating systems. Finally, the investigation of the processes through which the SST influences location and intensity of heavy precipitation allows to gain a better understanding on mechanisms conducive to severe weather in the Mediterranean area and in the Adriatic basin in particular.
Results show that the effect of the Adriatic SST (uncertainty) on precipitation is complex and can vary considerably among different events. For both Bora and Sirocco events, SST does not influence markedly the atmospheric water budget or the degree of moistening of air that flows over the Adriatic Sea. SST mainly affects the stability of the atmospheric boundary layer, thus influencing the flow dynamics and the orographic flow regime, and in turn, the precipitation pattern.
•Numerical simulations of heavy precipitation associated with intense air-sea interaction over the Adriatic Sea•Sensitivity of forecast precipitation to SST uncertainty•The response of intense precipitation patterns to SST uncertainties is complex.•SST impact on dynamics and on orographic flow regime and, in turn, on precipitation
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Floods are among natural disasters which cause the largest damages worldwide each year, inducing fatalities of human lives, destruction of infrastructure and economical losses. Consequently, ...forecasting this type of events through hydro‐meteorological models is still of great importance from a civil protection point of view since it allows to reduce hydrological risk by means of early warning systems. Nevertheless, hydrological model initialization in ungauged basins, where there is lack of direct measurements of meteorological information, is a known issue affecting the entire prediction chain. The present study evaluates the possibility of using forecasts provided by the meteorological model MOLOCH developed by CNR‐ISAC forcing the FEST‐WB hydrological model developed by Politecnico di Milano to perform discharge simulations assuming that the forecasting errors are negligible when using the first 24 h of time horizon. The study is carried out in the urban catchments of Milan city, the Seveso‐Olona‐Lambro (SOL) river basins, located in northern Italy. The main hydro‐meteorological variables are analysed by comparing the spatialized and observed meteorological data, provided by an official regional network of weather stations plus a citizen scientists' contribution with the meteorological model forecasts. Moreover, a sensitivity analysis following the well‐known one‐factor‐at‐a‐time methodology is accomplished with the aim of defining which atmospheric forcing, beyond rainfall, mostly affects flowrate forecasts. Results generally show satisfactory correspondences between forecasts and observed data for the discharge variable at daily scale, although an underestimation of precipitation, particularly for severe events in summer, is present. Therefore, using meteorological forecasts to create daily initial conditions for hydrological model, instead of ground observations, might be a reliable and valuable approach, even if some considerations should be borne in mind when coupling the two models.
Dealing with discharge forecasting in ungauged basins, the main results have shown the following:
it is feasible to initialize hydrological models using meteorological forecasts,
considering the first 24 h of forecasts given by the MOLOCH model, an accurate discharge estimation is reached and
outcomes may be sensitive to the local land cover and land use conditions over the basins: urban areas where soil conditions have less influence than permeable territories denote higher scores.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
On 2–3 October 2020, a heavy precipitation event severely affected northern Italy and in particular the western Alps, with rainfall amount exceeding 600 mm over 24 h. This event was associated with ...an upper-level trough over the western Mediterranean basin, a large-scale configuration typical of heavy precipitation phenomena on the southern side of the Alps, since it induces a northward transport of large amounts of moisture impinging on the orography. The present study shows that a relevant amount of moisture moved towards the Mediterranean basin in the form of an atmospheric river (AR), a long and narrow filament-shaped structure crossing the whole Atlantic Ocean, characterized in the present case by a maximum Integrated Vapour Transport exceeding 1000 kg m−1 s−1. Therefore, in addition to the local contribution from the Mediterranean Sea, a relevant amount of moisture moved from the Tropics towards the Mediterranean, feeding the precipitation systems.
The presence of an AR represented a distinguishing aspect of the event, superimposed on the well-known dynamic-thermodynamic mechanisms of heavy precipitation over the Alps. High-resolution numerical simulations and diagnostic tools have been exploited to investigate in detail how the transport of water vapour associated with the AR has influenced the dynamics and favoured the severity of the heavy precipitation processes.
The results disclose the role of the AR and add further details to the theoretical framework of heavy precipitation mechanisms in the Alpine area, improving our understanding of the complex interaction between large-scale flows and mesoscale dynamics during extreme precipitation episodes. Due to the relatively fast evolution of the synoptic disturbance, the typical mesoscale mechanisms would have led only to an ordinary intense rainfall event. The contribution of the AR turned the event into a devastating flood.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This study focuses on the initiation of deep convection during a heavy precipitation episode, which occurred during the first Special Observation Period (SOP 1) of the Hydrological cycle in the ...Mediterranean Experiment (HyMeX). In the course of 14 and 15 October 2012 (Intensive Observation Period 13), intense convective events affected southern France, Corsica and several regions of Italy. Numerical simulations are performed with two state‐of‐the‐art numerical weather prediction models (COSMO and MOLOCH), driven by the same initial and boundary data and operated on an identical domain. With this set‐up, the sensitivity of the model results to horizontal grid spacing and terrain elevation is assessed. Furthermore, model outputs are compared with observations from rain‐gauges, radars and radiosondes made during the campaign. Although the higher resolution runs show a higher correlation with observed precipitation, the influence of model grid spacing on total precipitation amount or timing is rather weak. Since the overall performance of both mesoscale models is fairly good, they are used together to investigate the physical processes characterizing IOP 13. In particular, the differences in the location and timing of convection between the simulations are used to identify and explore those processes that need to be well represented in order to reproduce the mechanisms initiating heavy precipitation in the Mediterranean region adequately.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The autumn of 2014 was characterized by a number of severe weather episodes over Liguria (northern Italy) associated with floods and remarkable damage. This period is selected as a test bed to ...evaluate the performance of a rainfall assimilation scheme based on the nudging of humidity profiles and applied to a convectionpermittingmeteorologicalmodel at high resolution. The impact of the scheme is assessed in terms of quantitative precipitation forecast (QPF) applying an object-oriented verification methodology that evaluates the structure, amplitude, and location (SAL) of the precipitation field, but also in terms of hydrological discharge prediction. To attain this aim, themeteorologicalmodel is coupledwith the operational hydrological forecasting chain of the Ligurian Hydrometeorological Functional Centre, and the whole system is implemented taking operational requirements into account. The impact of rainfall data assimilation is large during the assimilation period and still relevant in the following 3 h of the free forecasts, but hardly lastsmore than 6 h.However, this can improve the hydrological predictions. Moreover, the impact of the assimilation is dependent on the environment characteristics, being more effective when nonequilibrium convection dominates, and thus an accurate prediction of the local triggering for the development of the precipitation system is required.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The first Special Observation Period (SOP1) of HyMeX (Hydrological cycle in the Mediterranean eXperiment) was held in fall 2012 and focused on heavy precipitation events (HPEs) and floods in the ...northwestern Mediterranean. Nine intensive observation periods (IOPs) involved three Italian target areas (northeastern Italy, NEI; Liguria and Tuscany, LT; central Italy, CI), enabling an unprecedented analysis of precipitation systems in these regions. In the present work, we highlight the major findings emerging from the HyMeX campaign and in the subsequent research activity over the three target areas by means of conceptual models and through the identification of the relevant recursive mesoscale features.
Abstract The Po Valley in northern Italy is a hotspot for tornadoes in Europe in spite of being surrounded by two mountain ridges: the Alps in the north and the Apennines in the southwest. The ...research focuses on the case study of 19 September 2021, when seven tornadoes (four of them rated as F2) developed in the Po Valley in a few hours. The event was analyzed using observations and numerical simulations with the convection-permitting Modello Locale in Hybrid Coordinates (MOLOCH) model. Observations show that during the event in the Po Valley, there were two surface boundaries that created a triple point: an outflow boundary generated by convection triggered in the Alpine foothills and a dryline generated by downslope winds from the Apennines, while warm and moist air advected westward from the Adriatic Sea east (ahead) of the boundaries. Tornadoes developed about 20 km northeast of the triple point. Numerical simulations with 500-m grid spacing suggest that the development of supercells and drylines in the Po Valley was sensitive to the elevation of the Apennines. Simulated vertical profiles show that the best combination of instability and wind shear for the development of tornadoes was attained within a narrow area located ahead of the dryline. A conceptual model for the development of tornadoes in the Po Valley is proposed, and the differences between tornado environments over a flat terrain and over a region with complex terrain are discussed. Significance Statement The Po Valley is a highly populated area where some of the most violent tornadoes in Europe have developed. We investigated a tornado outbreak that occurred on 19 September 2021 in this region, in order to identify its main environmental characteristics. High-resolution numerical simulations revealed that values of instability and wind shear were compatible with the development of several tornadoes only in a narrow area close to the intersection of two surface boundaries (a triple point). Moreover, the atmospheric environment during the tornado outbreak was strongly influenced by the presence of mountain ridges surrounding the plain. We have summarized our results in a conceptual model that can potentially be used for forecasting applications.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The first Special Observation Period (SOP1) field campaign of the HyMeX (HYdrological cycle in the Mediterranean EXperiment) program was held in fall 2012 and was devoted to the investigation of ...heavy precipitation and floods in the Western Mediterranean. Both the Italian research and operational meteorological communities actively participated by providing a valuable contribution through the organization of an ad hoc national Operational Centre. The paper presents an overview of this participation, resulting in a fruitful multidisciplinary experience able to build a bridge between academia, researchers, forecasters, end-users and decision makers. The benefits provided by the wide national involvement and the consequent possible national impacts and progress are discussed within the context of the complex organization of meteorology in Italy.
Coupling meteorological and hydrological models is a common and standard practice in the field of flood forecasting. In this study, a numerical weather prediction (NWP) chain based on the BOLogna ...Limited Area Model (BOLAM) and the MOdello LOCale in Hybrid coordinates (MOLOCH) was coupled with the operational hydrological forecasting chain of the Ligurian Hydro-Meteorological Functional Centre to simulate two major floods that occurred during autumn 2011 in northern Italy. Different atmospheric simulations were performed by varying the grid spacing (between 1.0 and 3.0 km) of the high-resolution meteorological model and the set of initial/boundary conditions driving the NWP chain. The aim was to investigate the impact of these parameters not only from a meteorological perspective, but also in terms of discharge predictions for the two flood events. The operational flood forecasting system was thus used as a tool to validate in a more pragmatic sense the quantitative precipitation forecast obtained from different configurations of the NWP system. The results showed an improvement in flood prediction when a high-resolution grid was employed for atmospheric simulations. In turn, a better description of the evolution of the precipitating convective systems was beneficial for the hydrological prediction. Although the simulations underestimated the severity of both floods, the higher-resolution model chain would have provided useful information to the decision-makers in charge of protecting citizens.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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