Recurring features of extreme autumnall rainfall events on the Veneto coastal area

Abstract

Abstract. Recent recurring episodes of heavy flash floodproducing rainfall events on the Veneto coastal area have renewed the interest in documenting the frequency and key dynamical ingredients of such events. A climatological analysis of the precipitation in Veneto reveals that, in comparison with the rest of the region, the coastal area is characterized by fewer rain days, lower rainfall accumulations, yet more days with heavy precipitation. If set in relation to the yearly rainfall, daily accumulation can reach values as high as 40 % of the yearly total rainfall, more regularly between 15 % and 30 %, often in periods of 12 h or less.Four such heavy rainfall events were analyzed and synthetically described to highlight key ingredients which appear instrumental in producing the high rainfall accumulations. These comprise an upper-level trough elongating or cutting off into the Western Mediterranean basin after a period of one to two weeks of anticyclonic fair weather conditions with temperatures above normal. The moisture supply over the Adriatic onto north-eastern Italy is favoured by above normal sea surface temperatures, enhanced advection by a surface low in the Gulf of Genoa, and in three of the four cases, an additional surface low over southern Italy. The air flows associated with the upper-level trough for the cases discussed were of moderate to weak intensity, and convectively conditionally unstable. The flow intensity was such that the lower tropospheric portion was blocked by and forced to flow around the Alpine barrier, i.e. manifesting as a north-easterly, low-level flow over much of the north-eastern Italian plains. This blocked flow seemed to interact with the larger-scale synoptic flow to form a distinct and persistent low-level convergence in the area of the Veneto coast.It is suggested that these low-level convergence patterns are key in releasing the convective instability present in the larger-scale flow just on the Veneto coastal area. Hereby, it is the synoptic rather than the convective setting which dictated the observed timescales of intense rainfall. Therefore, the convective rainfall rates paired with the synoptic durations combine to produce the exceptionally high rainfall accumulations observed. Cases like these are significant contributors to forming the coastal precipitation climatology, which for this area is found to be distinctly different than for the rest of the region in terms of precipitation concentration.

Intense air-sea exchange and heavy rainfall: impact of the northern Adriatic SST

“…Sometimes they produce a daily accumulation that can reach values as high as 40 % of the mean annual amount, even in less than 12 h, thus leading to severe flash floods, damages and human casualties (Barbi et al, 2012). Heavy precipitation over the Alps is directly (e.g., by orographic uplift) or indirectly (e.g., by orographic cyclogenesis) related to the influence of mountain ranges on atmospheric motions.…”

Overview of the first HyMeX Special Observation Period over Italy: observations and model results

“…These sites were selected because they have weather systems representative of those re- sponsible for most of the heavy precipitation and flood events affecting the Italian territory, which can be related to specific large-scale patterns . For example, the severe weather events that occurred over the Alps in recent years (Rotunno and Houze, 2007;Mariani et al, 2009;Barbi et al, 2012) or in Liguria and Lazio during the autumn of 2011 Ferretti et al, 2012;Parodi et al, 2012;Rebora et al, 2013), are directly (e.g., by orographic precipitation) or indirectly (e.g., by cyclogenesis: see Tibaldi et al, 1990) related to the influence of mountain ranges on atmospheric motions. The steep slopes of the Alps and the Apennines in the vicinity of large coastal areas of the Mediterranean, and the sea itself that acts as a large source of moisture and heat, are the key factors in determining the convergence and the rapid uplift of moist and unstable air responsible for triggering condensation and convective instability processes (Benzi et al, 1997;Rotunno and Ferretti, 2001).…”

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

“…The examination of four heavy rainfall events that occurred in the coastal area of northeastern Italy, a region fringed by the Alpine barrier to the north of the Adriatic Sea, revealed that synoptic rather than convective settings controlled the observed timescales of these intense rainfall events. The combined convective rainfall rates paired with the synoptic durations led to the exceptionally high rainfall accumulations observed (Barbi et al, 2012). 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).…”