Identification and ranking of extraordinary rainfall events over Northwest Italy: The role of Atlantic moisture

Abstract

An objective identification and ranking of extraordinary rainfall events for Northwest Italy is established using time series of annual precipitation maxima for 1938–2002 at over 200 stations. Rainfall annual maxima are considered for five reference durations (1, 3, 6, 12, and 24 h). In a first step, a day is classified as an extraordinary rainfall day when a regional threshold calculated on the basis of a two‐components extreme value distribution is exceeded for at least one of the stations. Second, a clustering procedure taking into account the different rainfall durations is applied to the identified 163 events. Third, a division into six clusters is chosen using Ward's distance criteria. It is found that two of these clusters include the seven strongest events as quantified from a newly developed measure of intensity which combines rainfall intensities and spatial extension. Two other clusters include the weakest 72% historical events. The obtained clusters are analyzed in terms of typical synoptic characteristics. The two top clusters are characterized by strong and persistent upper air troughs inducing not only moisture advection from the North Atlantic into the Western Mediterranean but also strong northward flow towards the southern Alpine ranges. Humidity transports from the North Atlantic are less important for the weaker clusters. We conclude that moisture advection from the North Atlantic plays a relevant role in the magnitude of the extraordinary events over Northwest Italy.

Review Article: Atmospheric conditions inducing extreme precipitation over the eastern and western Mediterranean

“…Associations between the Pacific El Niño/Southern Oscillation (ENSO) and Mediterranean precipitation have been described by several researchers (e.g., Ropelewski and Halpert 1987;Rodo et al, 1997;Price et al, 1998;Mariotti et al, 2002b). Over the WM, several studies have demonstrated statistical associations between Iberian region precipitation and ENSO (Rodo et al, 1997;Mariotti et al, 2002b;Park, 2004;Rimbu et al, 2001;Pozo-Vazquez et al, 2005;Vicente-Serrano, 2005;Sordo et al, 2008). Shaman and Tziperman (2011), based on observational analyses, show that ENSO modulates rainfall over southwestern Europe (Iberia, southern France, and Italy) during the SeptemberDecember wet season.…”

“…Even for events in the central Mediterranean, a significant role of moisture import into the basin was detected. Pinto et al (2013) found that moisture advection from the north Atlantic plays a relevant role in the magnitude of the extraordinary rainfall events over northwest Italy, while humidity transports from the north Atlantic are less important for weaker events in the same region.…”

“…7). The anomalous atmospheric circulation associated with this moisture advection as obtained for a particular stage of the ENSO cycle plays a relevant role in the magnitude of the rainfall events in fall (Pinto et al, 2013).…”

“…In some particular cases, daily amounts of precipitation of 300, 400, and even 800 mm have been reported in that particular region (see, for example, Romero et al, 1997;Ramis et al, 1998). Extreme rainfall events over northwest Italy were classified, taking local station intensities and rainfall durations simultaneously into account (Pinto et al, 2013). The two clusters containing the strongest events were found to be associated with strong and persistent upper air troughs, inducing not only moisture advection from the North Atlantic into the WM but also strong northward flow towards the southern Alpine ranges.…”

<span>The Mediterranean moisture supply in the genesis of climatological and extreme monthly continental precipitation</span>

“…Large-scale precipitation events in the western, central and eastern Mediterranean are related with local anomalies of high moisture over the Mediterranean Sea [9], and those are influenced by remote positive sources anomalies [10]. Many works pointed out the important role of remote sources for extreme precipitation events in the area of Mediterranean basin [10][11][12][13].…”