The Mediterranean basin is recognized as a primary climate change “hot-spot” and a region highly vulnerable to severe hydrometeorological extremes, including Heavy Precipitation Events (HPEs) and devastating flash floods . Acting as a transition zone between midlatitude and tropical climate regimes, the Mediterranean Sea serves as a reservoir of heat and moisture. When this moist air interacts with the complex and steep orography surrounding the basin, it frequently triggers highly energetic and stationary Mesoscale Convective Systems (MCSs). Recent findings from the HyMeX program demonstrate that the severity of these events is mostly driven by large-scale atmospheric patterns. Synoptic structures, such as cut-off lows and Mediterranean cyclones, trigger intense air-sea interactions. Through evaporation and surface heat fluxes, the Mediterranean Sea moistens and destabilizes the lower atmosphere, building up potential energy. Low-level winds then transport this marine air toward the coastlines. The complex topography surrounding the basin forces these incoming flows to rise, ultimately triggering intense convective storms.

In this context, the Mediterranean Sea is particularly susceptible to these dynamics due to the proximity of the Alps and Apennines to the warm surrounding sea . A prominent example of this vulnerability is the Marche region, located in central Italy (see Fig. ). Geographically, the region is bounded by the Adriatic Sea to the east and the central Apennines to the west, whose western slopes approach the Tyrrhenian coast. Its topography is characterized by a series of narrow, steep river valleys oriented perpendicular to both the coastlines and the mountain barrier. When low-level moist atmospheric flows originating from the Mediterranean Sea impinge on the steep Apennine ridge, they undergo orographic lifting and make the region susceptible to severe weather events. Furthermore, the numerous small and medium-sized steep catchments exhibit a rapid hydrological response to heavy precipitation, frequently resulting in sudden flash floods .

A recent example of this kind of dynamics is the flood that struck the Marche region on 15 September 2022. This event was characterized by a quasi-stationary, V-shaped MCS capable of generating extreme precipitation within just a few hours. Previous studies on similar events in Italy highlighted that the stationarity and intensity of such systems are often linked to low-level convergence lines, induced by both the synoptic-scale circulation and the local flow deflection by the terrain , together with the advection of very moist air over even modest orography e.g..

Compared to previous studies, the present research exploits the higher grid resolution of current numerical weather prediction models to better resolve small-scale terrain features. This improved resolution allows models to explicitly disentangle the meso-gamma scale, which plays a fundamental role in airflow dynamics near complex topography. Consequently, this helps us better reproduce localized, severe orographic precipitation events, which proposed are governed by three core factors: the large-scale synoptic forcing, which dictates the characteristics of the impinging airmass, including wind velocity, stability, and humidity; the terrain interaction at mesoscale, which determines whether the flow goes over or around the orographic obstacle; microphysical processes, which control the efficiency of cloud development and whether moisture falls as precipitation.

The objective of this study is to investigate the atmospheric conditions and multi-scale dynamical mechanisms that drove the September 2022 Marche event using the Weather Research and Forecasting (WRF) model . In this context, the use of high-resolution numerical models becomes essential to improve our understanding of the atmospheric drivers associated with severe hydrogeological impacts in the complex terrain of the Apennines. It is worth noting that the scope of this research extends beyond the numerical reproduction of the rainfall; instead, it aims to better understand the mechanisms triggering this extreme event. Consequently, we focus on the simulation that provided the most accurate representation of the storm's localization and intensity, which has been subsequently used as the baseline for the physical analysis. This high-resolution simulation allows for the explicit representation of convective processes and their interaction with the complex orography, providing a tool for diagnosing and understanding such an extreme meteorological event.

The paper somewhat explores the predictability of localized heavy rainfall events, which is related to a fundamental challenge in numerical weather prediction: the scale-dependency of atmospheric predictability. The ECMWF-EPS (Ensemble Prediction System) initial perturbations are tuned for large-scale synoptic patterns. This presents a critical limitation when predicting localized heavy rainfall: different simulations, which are nearly indistinguishable at larger scales at initial times, may show some differences in mesoscale features, which may dramatically affect the subsequent evolution, since localized convective systems depend heavily on such small-scale dynamics. Therefore, different ensemble members can produce nearly identical synoptic forecasts while generating greatly different local precipitation totals. showed that moist convective processes can cause a rapid upscale initial error growth in regions of moist instability so that the small amplitude error at small scales ultimately limits the predictability even at larger scales. Additionally, small changes in topography representation or moisture content can cause massive differences in precipitation amount, location, and structure.

The paper is structured as follows: Sect. 2 introduces the data and the model used for the analysis; Sect. 3 describes the synoptic environment; Sect. 4 presents the observational analysis and the numerical simulations; sensitivity to initial and boundary conditions is shown in Sect. 5, while conclusions are drawn in Sect. 6.

The numerical simulations in this study were performed using the Advanced Research WRF (WRF-ARW) model, version 4.6.1. WRF-ARW is a fully compressible, non-hydrostatic model that employs a hybrid sigma-pressure vertical coordinate system, designed to minimize errors over steep topography.

The WRF model was employed here to reproduce the Marche flood of September 2022, with the objective of understanding the atmospheric drivers and conditions that led to the event. Following a trial-and-error procedure, the configuration explained hereafter, initialized at 00:00 UTC on 15 September 2022 (24 h duration), was identified as optimal, yielding the best compromise in terms of precipitation localization and amount.

The simulation setup includes three one-way nested domains, as shown in Fig. .

The parent domain (d01) covers a wide area with a horizontal grid spacing of 10 km (200×200 grid points).

Vertically, the model atmosphere is discretized into 41 levels, extending from the surface up to a top pressure of 50 hPa. Initial and lateral boundary conditions were derived from the NCEP GDAS/FNL 0.25° Global Tropospheric Analyses and Forecast Grids. Boundary conditions were updated every 3 h using the analysis fields (f00), available every 6 h, as well as the short-term 3 h forecasts (f03) for the intermediate times. This high-frequency update indicates the diagnostic nature of this work, aiming to reconstruct the event dynamics using the setup emerging as the best available in terms of atmospheric boundary forcing.

A summary of the parameterization schemes adopted here is provided in Table ; they have been selected following the configuration used in . A crucial distinction in the setup concerns the cumulus parameterization. For the coarser domain (d01), the Tiedtke scheme was activated to parameterize sub-grid scale convection. Conversely, for the intermediate and innermost domains (d02 and d03), a convection-permitting configuration was adopted.

The synoptic environment leading to the extreme event of 15 September 2022 was characterized by a combination of atmospheric patterns that facilitated the accumulation of high convective available potential energy and moisture over the central part of the Italian peninsula in the previous days .

The thermal structure (Fig. d–f) reveals a pronounced temperature gradient moving eastward over the western Mediterranean. The Marche region was positioned at the boundary between a ridge extending from the south, bringing warm air from North Africa on its western side, and a trough on its northwestern side, drawing in colder air from Northern Europe. By 15 September (panels c and f), the synoptic situation over the central Mediterranean evolved from a meridionally elongated pattern into a predominantly zonal flow pattern, with isohypses oriented perpendicular to the Apennine chain . The low-level warming and moisture accumulated during the previous days, due to the persistence of the anticyclonic configuration, combined with the advection of moist and cooler mid-level air, increased the potential instability in the area. This evolution is typical of severe convective storms in the Mediterranean region e.g. and ultimately facilitated the triggering of a stationary V-shaped system.

The upper-level dynamics also played a key role in sustaining the convective system. The rapid evolution of the geopotential at 300 hPa on 15 September (see Fig. S1 in the Supplement) reveals that the jet streak reached Northern Italy in the afternoon, placing the Marche region below the right entrance region (right-rear quadrant). This configuration is well known to induce upper-level divergence and low-level convergence , generating vertical lift that provides a favorable dynamic environment for deep convection development.

To evaluate the model's sensitivity to initial conditions (I.C.), boundary conditions (B.C.), and physical parameterizations, several sensitivity experiments were performed. The configurations are summarized in Table , and the resulting 24 h accumulated precipitation is shown in Fig. . Figure a represents the “reference simulation” (REF) (initialized at 00:00 UTC, 15 September and forced with NCEP/GDAS analysis and forecast fields as explained in Sect. ), which is the control run extensively discussed in the previous sections. The authors believe that this sensitivity analysis is relevant to understand the spread in the model outputs at a relatively local scale. Indeed, dramatic differences emerge in terms of precipitation amount and distribution, as shown in Fig. .

A comparison between the REF simulation (Fig. a) and the run initialized 6 h later (Fig. b) reveals a significant decrease in the accumulated rainfall over Cantiano and the loss of the secondary precipitation band located further south. This result illustrates the key role of the initial conditions in correctly modulating the rainfall in the areas affected by the heavy rainfall event. Conversely, the third run, using boundary conditions updated only every 6 h (Fig. c) increases the rainfall intensity and extends the area affected by intense rainfall, but erroneously displaces the precipitation peak north of Cantiano. Overall, these latter changes appear relatively minor, as the high impact of the event is clearly identified in the region, although the location is not perfectly represented. Furthermore, the choice of the parameterization schemes plays a crucial role in the spatial distribution and intensity of the precipitation; as shown in Fig. d, changing the model physics package – by simultaneously modifying the cumulus convection, microphysics, radiation, and land surface schemes – significantly alters the precipitation: this sensitivity run shifts the rainfall pattern northward and decreases the rainfall amount compared to the control simulation. These results suggest a significant sensitivity to the initial conditions and the model physics, making a limited area model ensemble approach (including both the sensitivity to initial conditions and model physics) highly recommended to correctly evaluate the possibility of severe events affecting a limited area. This is especially true for this specific event for which most real-time simulations missed the severity of the rainfall.

Using the NCEP GFS historical archive as I.C. and B.C. (Fig. e) – thus changing both the initial and boundary conditions with respect to REF – results in a primary precipitation band that is less elongated in the southwest–northeast direction, with lower accumulation peaks compared to the REF simulation; conversely, the southern rainband appears more intense and longer than in REF. To isolate the impact of the initial and boundary conditions alone, an additional simulation was performed using the REF initial conditions (GDAS dataset) but driven by GFS forecast boundary conditions (Fig. f). Compared to Fig. e (thus exploring the effect of changing the I.C.), this configuration produces a less elongated northern rainband and more concentrated precipitation near Cantiano, while the southern rainband appears weaker. Comparing with Fig. a (thus exploring the effect of changing the B.C. with respect to REF), the impact of the boundary condition appears a secondary one, although not negligible.

Finally, Fig. g and h demonstrate that driving the model with high-resolution (HRES) ECMWF operational data and ERA5 reanalysis, respectively, fails to accurately reproduce the accumulated precipitation for this case. The ECMWF-driven run completely shifts the precipitation pattern to the north of the Adriatic coast (offshore).

To understand this behavior, it is important to clarify the nature of the forcing datasets. The ECMWF initial and boundary conditions were provided as operational data on 20 standard pressure levels, whereas GDAS analyses included additional data and were available on 34 pressure levels. The coarser vertical resolution of the ECMWF fields might limit the accurate representation of the vertical thermodynamic structure, including the boundary layer and upper-troposphere dynamics. In contrast, the ERA5-driven run fails to generate the elongated banded structure, producing a weaker and incorrectly localized peak.

The analysis of the flood of 15 September 2022 in the Marche region highlights that the catastrophic event emerged from the simultaneous occurrence of different atmospheric forcings across multiple scales. Through the use of high-resolution WRF model simulations and observational data, this study identifies the key ingredients that caused the initiation and maintenance of the stationary convective system.

At the synoptic scale, the continuous transport of humid air masses from the western Mediterranean acted as the primary fuel, sustaining convection for several hours. While these dynamic factors drove the intensity of the phenomenon, the Apennine topography played the critical role of anchoring the convective system. The analysis reveals a primary convergence line that partially overlapped with the main precipitation band. Other isolated convergence nuclei were present inland and co-located with localized rainfall peaks. This suggests that orographic forcing played a crucial role: the terrain deformed the horizontal wind field, forced the air to converge, and consequently triggered convection through strong vertical uplift. In addition, vertical cross-sections show alternating vertical motion patterns consistent with orographic gravity wave activity, which may have locally modulated the updrafts leeward of the mountain barrier.

The comparison between the model output and the observations offers critical information on the capabilities and limitations of numerical simulations. Although the WRF model suggests a qualitative spatial agreement in reproducing the dynamical chain of events and the geometry of the precipitation band, including its orientation and position, it showed evident limitations in quantifying the exact rainfall accumulation. Nevertheless, the model's ability to correctly identify the structural precursors – such as the capping inversion, the convergence line, and the moisture transport – highlights its utility in representing the essential physical mechanisms driving this extreme convective event.

Sensitivity experiments emphasized the role of initial and boundary conditions, as well as physical parameterizations, in accurately predicting the event. While the reference GDAS-driven simulation and the runs with the same initial conditions but different boundary conditions captured the mesoscale features of the event, the model run forced with ECMWF data resulted in a significant northeastward displacement of the precipitation band. A comparative analysis identified the primary drivers of this discrepancy: despite an almost identical large-scale synoptic forcing in the mid-troposphere, the ECMWF-driven run failed to reproduce the correct low-level mesoscale dynamics for this case. Specifically, an underestimated meridional pressure gradient led to a weaker low-level wind field and altered the moisture transport. Furthermore, the ECMWF initial and boundary conditions showed an early lower-tropospheric (850 hPa) warm and moist bias over central Italy. This induced a temporal mismatch between the thermodynamic instability peak and the dynamic triggers, preventing the system from releasing the accumulated potential energy at the correct location and timing. This demonstrates that the predictability of such extreme Mediterranean storms is constrained by the model's ability to resolve the dynamics of the lower troposphere and their timing in interacting with the local topography as well as with the upper level features. This is particularly relevant in the context of climate change, as the warmer SST in the Mediterranean area is affecting the air-sea interaction, hence the thermodynamics and dynamics of the lower troposphere.