The role of morphology in the spatial distribution of short-duration rainfall extremes in Italy

Abstract

Abstract. The dependence of rainfall on elevation has frequently been documented in the scientific literature and may be relevant in Italy, due to the high degree of geographical and morphological heterogeneity of the country. However, a detailed analysis of the spatial variability of short-duration annual maximum rainfall depths and their connection to the landforms does not exist. Using a new, comprehensive and position-corrected rainfall extreme dataset (I2-RED, the Improved Italian-Rainfall Extreme Dataset), we present a systematic study of the relationship between geomorphological forms and the average annual maxima (index rainfall) across the whole of Italy. We first investigated the dependence of sub-daily rainfall depths on elevation and other landscape indices through univariate and multivariate linear regressions. The results of the national-scale regression analysis did not confirm the assumption of elevation being the sole driver of the variability of the index rainfall. The inclusion of longitude, latitude, distance from the coastline, morphological obstructions and mean annual rainfall contributes to the explanation of a larger percentage of the variance, even though this was in different ways for different durations (1 to 24 h). After analyzing the spatial variability of the regression residuals, we repeated the analysis on geomorphological subdivisions of Italy. Comparing the results of the best multivariate regression models with univariate regressions applied to small areas, deriving from morphological subdivisions, we found that “local” rainfall–topography relationships outperformed the country-wide multiple regressions, offered a uniform error spatial distribution and allowed the effect of morphology on rainfall extremes to be better reproduced.

A Gauge-Based Sub-Daily Extreme Rainfall Climatology for Western Europe

“…As shown in other studies on the Italian peninsula and Italian Alps there is not always a consistent relationship between topography and sub-daily rainfall intensity and local factors play a large role in varying this relationship (Formetta et al, 2022;Mazzoglio et al, 2022). At durations below 1-h a reverse-orographic effect of decreasing intensity with elevation is well documented (Avanzi et al, 2015;Marra et al, 2021) but for durations between 2 and 6-h the effect is less clear, with intensity sometimes enhanced but not consistently (Formetta et al, 2022;Mazzoglio et al, 2022). Coastal effects may also be important for short-duration extremes (Marra et al, 2022) but this effect is not obvious here.…”

Orographic Precipitation Extremes: An Application of LUME (Linear Upslope Model Extension) over the Alps and Apennines in Italy

“…As already mentioned in the introduction, trying to find the best rainfall-elevation relation for a particular area at the event-based scale is not straightforward, confirming the difficulties encountered by [53,54]. In fact, the spatial and temporal variability of the precipitation, especially if the orographic effect can trigger convective thunderstorms, generally does not follow a linear rainfall-elevation function [21,27].…”

“…Unfortunately, this relation cannot be generalized to all seasonal periods and it is generally not valid during the summer when thunderstorm convection generally occurs [53]. A confirmation was made by the authors of [54], who carried out a study for the whole Italian Peninsula based on the Improved Italian-Rainfall Extreme Dataset (I2-RED) [62]. These authors have reported that it is rather difficult to find a generalized orographic precipitation gradient, although their analysis has not given any statistical evidence of a correlation between rainfall and elevation for the Italian Peninsula considering extreme rainfall events.…”

“…The orographic precipitation mechanism has been studied in the last decades, with a range of different approaches proposed [16,20,29,[52][53][54][55][56][57]. Some authors [53][54][55]58] have considered an empirical-statistical interpretation and proposed methodologies to correlate the recorded precipitation at rain gauges with the local elevation. In those cases, rainfall amounts and elevation were treated as independent variables where a statistically significant correlation was researched.…”

FOCA: a new quality-controlled database of floods and catchment descriptors in Italy

“…A discussion of the overall spatial and temporal variability of the rainfall extremes is available in Mazzoglio et al (2020Mazzoglio et al ( , 2022aMazzoglio et al ( , b, 2023. What is useful to comment on here is the result of the spatial averaging of the parameters a and n at the catchment scale (Fig.…”