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Heavy rainfall events are likely to become more frequent and severe in the next future, under the effects of a changing climate. In this context, the scientific literature is characterized by a relevant number of studies trying to identify the convective component of precipitation, since this kind of phenomena, due to their short duration and high intensity, may lead to an increased risk of flash floods or debris flows and, consequently, human life losses and economic damages. In this study, a separation between the convective and stratiform precipitation, starting from the subhourly precipitation time series recorded over Sicily (Italy), is provided on a monthly scale. Results show that the percentage of convective precipitation increases as the associated intensity increases, following what is generally known about convective events. Moreover, during the summer months, higher percentages of convective rainfall are generally reached for lower values of intensity, since this season favours the occurrence of these phenomena due to high temperature and relative humidity. In addition to the regional analysis, even an at‐site analysis has been carried out, through the definition of a critical intensity threshold to identify the predominant convective component for each rain gauge. This analysis has pointed out that such convective events have occurred mainly in the east part of the island mainly due to the complex orography of this area, which favours the occurrence not only of such phenomena but of flash floods and debris flows as well. [ABSTRACT FROM AUTHOR]

Green roofs have been widely recognized as sustainable nature-based solutions to mitigate floods in urban areas, which, in the last decades, are increasing due to the combination of intense worldwide urbanization and climate change. Besides flood mitigation, green roofs provide additional benefits for the urban environment (e.g., reducing the urban heat island and ensuring energy saving for the underneath building). Moreover, green roofs facilitate the increase of urban biodiversity, attracting different species of small animals, and upgrade the city aesthetic value. Among the different types of green roofs, multilayer blue-green roofs present an additional layer to store water during rainfall events. As part of the Polder Roof field lab project, prototypes of multilayer blue-green roof developed by the Dutch company Metropolder were installed in four Italian cities: Cagliari, Palermo, Perugia, and Viterbo. The four prototypes and the experimental set up are described and the potential benefits of this innovative solution are discussed. Preliminary analyses, from December 2020 to December 2021, enable to estimate runoff reduction and thermal properties of multilayer blue-green roofs, underlying the high potential of this nature-based solution, which allows to retain most of the rainfall events and to mitigate the daily temperature variability. [ABSTRACT FROM AUTHOR]

Understanding the structure of precipitation and its separation into stratiform and convective components is still today one of the important and interesting challenges for the scientific community. Despite this interest and the advances made in this field, the classification of rainfall into convective and stratiform components is still today not trivial. This study applies a novel criterion based on a clustering approach to analyze a high temporal resolution precipitation dataset collected for the period 2002–2018 over the Sicily (Italy). Starting from the rainfall events obtained from this dataset, the developed methodology makes it possible to classify the rainfall events into four different classes, which can be related to the convective and stratiform components of the events on the basis of their hyetograph shapes and average intensities. The results show that the occurrence of stratiform events is always much higher than the convective ones, especially in the winter and spring seasons, while from the summer to the mid-autumn the rainfall depth due to convective events results to be higher than that due to the stratiform events. Moreover, the comparison with a more widely accepted separation methodology demonstrates the physical consistency of the proposed methodology. [ABSTRACT FROM AUTHOR]

The atmospheric circulation can be recognized as one of the causes of severe rainfall events occurrence. Such events, especially when are characterized by short durations and high intensities, result in flood events in the Mediterranean area. It is very important to understand how these heavy rainfall events, which can be usually identified with convective rainfall, are related to the different types of atmospheric circulation. In order to do this, some weather circulation patterns (WPs), which have been derived for the Europe, have been first connected with the rainfall annual maxima (AMAX) recorded over the Sicily. The analyses allowed to identify those WPs that are more likely to result into the occurrence of the AMAX. Secondly, two ERA-Interim reanalysis indexes have been used to define a criterion to distinguish those AMAX mainly due to a convective component from those more related to a stratiform precipitation, also detecting a transient zone between these two types of events. Finally, the main results have been connected together with the aim to define a set of triggering factors of extreme rainfall events. [ABSTRACT FROM AUTHOR]

Abstract: Increasing precipitation extremes are one of the possible consequences of a warmer climate. These may exceed the capacity of urban drainage systems, and thus impact the urban environment. Because short‐duration precipitation events are primarily responsible for flooding in urban systems, it is important to assess the response of extreme precipitation at hourly (or sub‐hourly) scales to a warming climate. This study aims to evaluate the projected changes in extreme rainfall events across the region of Sicily (Italy) and, for two urban areas, to assess possible changes in Depth‐Duration‐Frequency (DDF) curves. We used Regional Climate Model outputs from Coordinated Regional Climate Downscaling Experiment for Europe area ensemble simulations at a ~12 km spatial resolution, for the current period and 2 future horizons under the Representative Concentration Pathways 8.5 scenario. Extreme events at the daily scale were first investigated by comparing the quantiles estimated from rain gauge observations and Regional Climate Model outputs. Second, we implemented a temporal downscaling approach to estimate rainfall for sub‐daily durations from the modelled daily precipitation, and, lastly, we analysed future projections at daily and sub‐daily scales. A frequency distribution was fitted to annual maxima time series for the sub‐daily durations to derive the DDF curves for 2 future time horizons and the 2 urban areas. The overall results showed a raising of the growth curves for the future horizons, indicating an increase in the intensity of extreme precipitation, especially for the shortest durations. The DDF curves highlight a general increase of extreme quantiles for the 2 urban areas, thus underlining the risk of failure of the existing urban drainage systems under more severe events. [ABSTRACT FROM AUTHOR]

Critical rainfall thresholds for landslides are powerful tools for preventing landslide hazard. The thresholds are commonly estimated empirically starting from rainfall events that triggered landslides in the past. The creation of the appropriate rainfall-landslide database is one of the main efforts in this approach. In fact, an accurate agreement between the landslide and rainfall information, in terms of location and timing, is essential in order to correctly estimate the rainfall-landslide relationships. A further issue is taking into account the average moisture conditions prior the triggering event, which reasonably may be crucial in determining the sufficient amount of precipitation. In this context, the aim of this paper is exploiting historical landslide and rainfall data in a spatial database for the derivation of critical rainfall thresholds for landslide occurrence in Sicily, southern Italy. The hourly rainfall events that caused landslides occurred in the twentieth century were specifically identified and reconstructed. A procedure was proposed to automatically convert rain guages charts recorded on paper tape into digital format and then to provide the cumulative rainfall hyetograph in digital format. This procedure is based on a segmentation followed by signal recognition techniques which allow to digitalize and to recognize the hyetograph automatically. The role of rainfall prior to the landslide events was taken into account by including in the analysis the rainfall occurred 5, 15 and 30 days before each landslide. Finally, cumulated rainfall duration thresholds for different exceedance probability levels were determined. The obtained thresholds resulted in agreement with the regional curves proposed by other authors for the same area; antecedent rainfall turned out to be particularly important in triggering landslides. [ABSTRACT FROM AUTHOR]

Abstract: Flow duration curve provides an important synthesis of the relevant hydrological processes occurring at the basin scale, and, although it is typically obtained from field observations, different theoretical approaches finalized to its indirect reconstruction have been developed in recent years. In this study a recent ecohydrological model for the probabilistic characterization of base flows is tested through its application to a study catchment located in southern Italy, where long historical series of daily streamflow are available. The model, coupling soil moisture balance with a simplified scheme of the hydrological response of the basin, provides the daily flow duration curve. The original model is here modified in order to account for rainfall reduction due to canopy interception and stress its potential applicability to most of the ephemeral Mediterranean basins, where measurements of air temperature and rainfall often represent the only meteorological data available. The model shows a high sensitivity to two parameters related to the transport and evapotranspiration processes. Two different operational approaches for the identification of such parameters are explored and compared: by the first approach, these parameters are considered as time invariant quantities, while, in the second approach, empirical relationships between such parameters and the underlying climatic forcings are first derived and then adopted in the parameters calibration procedure. The model ability in reproducing the empirical flow duration curves and the model sensitivity to climate forcings, here referred as elasticity of the model, are investigated and it is shown how the adoption of the second approach leads to a general improvement of the model elasticity. [Copyright &y& Elsevier]

• Reconstruction of a rainfalldatabase of annual maxima for southern Italy. • Investigation of existing trends on annual maxima. • Southern Italy is experiencing an increase of annual maxima at short durations. The growing number of extreme hydrological events observed worldwide has raised the level of attention towards the impact of climate change on the rainfall process, which is difficult to quantify given its strong spatial and temporal heterogeneity. Therefore, the impact of the climate change should be determined ranging from individual hydrological series to a regional and/or district scale. With this context, the present study aims to identify trends and dynamics in extreme sub-daily rainfall patterns in southern Italy from 1970 to 2020. A comprehensive database of annual maxima was constructed using all available rainfall data and further expanded through the implementation of the gap-filling procedure, specifically the spatially-constrained ordinary kriging method. This expanded dataset was utilized to explore regional and local trends in annual maxima, offering valuable insights into the evolving nature of rainfall patterns in recent years. While most of the observed trends did not reach statistical significance, a significant number of locations exhibited upward tendencies for shorter durations. It is noteworthy that these trends tended to disappear over longer durations. [ABSTRACT FROM AUTHOR]

Warmer air has the potential to hold more water vapour and, therefore, to provide more water to rainfall events. Studying the relationship between rainfall and temperature represents an emerging issue in hydrology and meteorology, since it can be considered fundamental for evaluating the effects of global warming on future precipitation. Various approaches have been tested across different parts of the world, in many cases observing an intensification of extreme precipitation at higher temperatures consistent with the well-known thermodynamic Clausius-Clapeyron relation (CC-scaling rate of 6–7%°C−1). However, at different locations for hourly time-scales, the temperature-extreme rainfall scaling can be higher (super-CC) or lower (sub-CC). This study contributes to the understanding of the scaling relationship between extreme rainfall and temperature under climate conditions characteristic of Mediterranean semi-arid regions, rarely explored in the past. The role of different factors, such as rainfall characteristics and climatic seasonality, modelling framework and rainfall accumulation period are investigated through an application to Sicily (Italy). In particular, the suitability of different types of regression models used to interpret the relationship between hourly and sub-hourly extreme rainfall and surface temperature is explored. We find overall a sub-CC scaling for most of the island of Sicily. However, the rainfall-temperature scaling relationship is not constant over the temperature range and may be dependent on the season. The different results obtained highlight the importance of modelling choices for analyses in regions characterized by semi-arid climates. More specifically, we observe increasing scaling rates for decreasing rainfall accumulation periods, and significant sensitivity of scaling rates to the selected extreme rainfall quantile. Our novel use of piecewise and locally-weighted scatter plot smoothing regression-based approaches allow the accurate characterization of the temperature dependence of extreme rainfall in Sicily. This identifies a peak-like structure for the drier season, not detected by the simple application of the commonly-used exponential regression based approach. Unlabelled Image • The extreme rainfall-temperature relationship in a Mediterranean area is explored. • An overall sub-CC (<7%°C−1) scaling rate for most of Sicily (Italy) can be observed. • The scaling rate can vary over the temperature range and depends on the season. • The scaling rates increase with decreasing rainfall accumulation periods. • Piecewise and LOESS based approaches can capture the dry season peak-like structure. [ABSTRACT FROM AUTHOR]

This study aims to assess the reliability of satellite-precipitation products from the Global Precipitation Measurements ( GPM ) mission in regions with complex landscape morphology. Our analysis is carried out in the European mid-latitude area, namely on the two major islands of Mediterranean Sea, i.e. Sardinia and Sicily (Italy). Both islands experience precipitation originating from the interaction of steep orography on the coasts with winds carrying humid air masses from the Mediterranean Sea. The GPM post real-time IMERG (Integrated Multi-satellitE Retrievals from Global Precipitation Measurement) “Final” run product at 0.1° spatial resolution and half-hour temporal resolution have been selected for the two-year 2015–2016 period. Evaluation and comparison of the selected product, with reference to raingauge network data, are performed at hourly and daily time scales using statistical and graphical tools. The influences of morphology and land-sea coastal area transition on the reliability of the GPM product have been analysed. Confirming previous studies, results showed that GPM satellite data slightly overestimate rainfall over the study areas, but they are well correlated with the interpolated raingauge data. Metrics based on occurrences above a given threshold and on total volume above the same threshold were applied and revealed better performances for the latter ones. Applying the same metrics we show how GPM performances improve as the temporal aggregation increases. Several drawbacks were detected in the coastal areas, which were characterized by worse performances than internal areas. Statistics are generally very similar for the two considered case studies (i.e., Sardinia and Sicily) except for correlation between topography and accuracy of GPM products, which was slightly higher for Sardinia. [ABSTRACT FROM AUTHOR]

Different low impact development measures have been proposed to make cities more flood-resilient, and recent literature is paying great attention to the evaluation of their direct benefits in terms of flood risk mitigation and the numerous co-benefits that they may offer. This study describes an experimental prototype of a technologically advanced multilayer green roof installed in a Mediterranean urban area (i.e., Palermo, Italy) and explores the results of an analysis of data collected over a one-year monitoring period by a complex sensors network. Multilayer green roofs, or "blue-green" roofs (BGRs), are characterized by a high water retention capacity compared to traditional green roofs due to the presence of an additional storage layer (blue layer), usually equipped with a valve that allows for regulating discharge outflow and water storage. Due to their recent development, BGRs are still scarcely explored in literature and have never been tested before in semi-arid environments, where they could represent valid measures to counter possible climate change and growing urbanization effects. In this study, the hydrological effectiveness of the experimental BGR is quantitatively evaluated by using appropriate indicators, based on the comparison between the hydrological response of the system and an equal size benchmark "grey" roof. The analyses are prevalently focused on the system's stormwater retention function, also investigating the relative contributions of the green layer and the storage layer to the overall retention capacity through the introduction of new BGRs specific indicators. Results emphasize the high impact of storms characteristics, antecedent soil moisture of the green layer, and initial water storage in the blue layer on the system's retention capacity. The overall mean retention rate for the experimental BGR, on average equal to 77% at the daily scale and 61% at the event scale, is comparable to the typical values of traditional extensive green roofs and could be further improved through a "retention-oriented" management of the outflow valve. The system was able to entirely retain almost half of the rainfall events occurred during the monitoring period and, for all the others, it was however extremely effective in reducing runoff peaks and delaying the hydrograph produced. [Display omitted] • Blue-green roofs (BGRs) combine advantages of traditional GRs and RHSs. • Appropriate indices can quantitatively evaluate the hydrological benefits of BGRs. • We analyzed data collected from an experimental system over about one-year period. • On average, runoff is de-peaked (−80%), reduced in volume (−61%) and delayed (4 h). • Rainfall properties and initial conditions strongly impact BGRs retention function. [ABSTRACT FROM AUTHOR]