Identification and ranking of extraordinary rainfall events over Northwest Italy: The role of Atlantic moisture. Issue 5 (1st March 2013)

• Record Type: Journal Article
• Authors: Pinto, Joaquim G.; Ulbrich, Sven; Parodi, Antonio; Rudari, Roberto; Boni, Giorgio; Ulbrich, Uwe
• Is Part Of: Journal of geophysical research. Volume 118:Issue 5(2013:May)
• Page Start: 2085
• Page End: 2097
• Publication Date: 2013-03-01
• Subjects: extreme events -- moisture advection -- Western Mediterranean -- ranking -- rainfall annual maxima; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/jgrd.50179
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: [1] 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. Key Points: A objective ranking of extreme rainfall events for NW Italy is performed Local rainfall intensity and spatial extension are considered for ranking Six clusters are identified, also with different synoptic characterisation