Mechanisms producing different precipitation patterns over north‐eastern Italy: insights from HyMeX‐SOP1 and previous events

Abstract

During the first HyMeX Special Observation Period (SOP1) field campaign, the target site of north‐eastern Italy (NEI) experienced a large amount of precipitation, locally exceeding the climatological values and distributed among several heavy‐rainfall episodes. In particular, two events that occurred during the last period of the campaign drew our attention. These events had common large‐scale patterns and a similar mesoscale setting, characterised by southerly low‐level flow interacting with the Alpine orography, but the precipitation distribution was very different. During Intensive Observing Period IOP18 (31 October–1 November 2012), convective systems were responsible for intense rainfall mainly located over a flat area of the eastern Po Valley, well upstream of the orography. Conversely, during IOP19 (4/5 November 2012), heavy precipitation affected only the Alpine area. In addition to IOP18 and IOP19, the present study analyses other heavy‐precipitation episodes that display similar characteristics and which occurred over NEI during the autumn of recent years. A high‐resolution (2 km grid spacing) non‐hydrostatic NWP model and available observations are used for this purpose. The two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the Alps. Depending on the thermodynamic profile, convection can be triggered when the impinging flow is forced to rise over a pre‐existing cold‐air layer at the base of the orography. In this situation a persistent blocked‐flow condition and upstream convergence are responsible for heavy rain localized over the plain. Conversely, if convection does not develop, flow‐over conditions are established and heavy rain affects the Alps. Numerical parameters proposed in the literature are used to support the analysis. Finally, the role of evaporative cooling beneath the convective systems is evaluated. It turns out that the stationarity of the systems upstream of the Alps is mainly attributable to persistent blocked‐flow conditions, while convective outflow slightly modifies the location of precipitation.

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

“…The three selected cases are: 14-16 September 2006 (Chievolis), 30 October-2 November 2010 (Vicenza) and 10-12 November 2012 (Piancavallo). For the last two cases, a more detailed description can be found in Davolio et al (2016). Chievolis event occurred in the Friuli Venezia Giulia (FVG) region in late summer and was characterized by convective precipitation exceeding 300 mm in 24 h. The other two cases (Piancav- allo and Vicenza), occurred in autumn, were responsible for floods and affected both FVG and Veneto regions with orographic rainfall up to 250 and 300 mm in 24 h, respectively (Fig.…”

“…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. Heavy precipitations over NEI are often associated with Sirocco or Bora winds (Manzato, 2007;Davolio et al, 2015Davolio et al, , 2016, thus involving intense air-sea interactions. In this situation, the Adriatic Sea acts as a source of moisture and heat for the atmosphere and contributes to destabilize the air mass in the boundary layer (PBL), which is then transported toward the mountains where convective instability is released.…”

Overview of the first HyMeX special observation period over Croatia

“…Previous studies (e.g. Buzzi and Foschini, 2000;Davolio et al, 2016) have shown that the largest component of the mountain-rangescale precipitation appears to be due to the orographic lifting of moist and impinging low-level flows. Consequently, the vertical uplifts forced by the Dinaric Alps area were favourable for the initiation and maintenance of convection.…”

“…Additional details on the synoptic situation are described in Ferretti et al (2014) and Davolio et al (2016). More rainfall was recorded on rain gauges on the north-eastern Adriatic coast.…”

GPS – Zenith Total Delay assimilation in different resolution simulations of a heavy precipitation event over southern France

“…The prediction of Heavy Precipitation Events (HPEs) that typically strike the western Mediterranean region by late summer (Lee et al, 2016;Davolio et al, 2016) is still a challenge for current Numerical Weather Prediction (NWP) models. Under a weak synoptic forcing, the location and time of the triggered convective cells is determined by mesoscale temperature and humidity inhomogeneities that define unstable regions prone to lifting.…”