Heavy rainfall episodes over Liguria in autumn 2011: numerical forecasting experiments

Abstract

Abstract. The Liguria coastal region in Italy was affected by two heavy rainfall episodes and subsequent severe flooding that occurred at the end of October and the beginning of November 2011. In both cases, the very large accumulated precipitation maxima were associated with intense and quasi-stationary convective systems that developed near the coast, both related to orographic lift and similar low-level mesoscale flow patterns over the Ligurian Sea, giving rise to pronounced convergence lines.This study aims at analysing the main dynamical processes responsible for the onset, lifecycle, intensity and localisation/propagation of the precipitating systems, using the ISAC convection-permitting model MOLOCH applied at different spatial resolutions and comparing model output fields with available observations. The ability of the model in quantitative precipitation forecasting (QPF) is tested with respect to initial conditions and model horizontal resolution. Although precipitation maxima remain underestimated in the model experiments, it is shown that errors in QPF in both amount and position tend to decrease with increasing grid resolution.It is shown that model accuracy in forecasting rainfall amounts and localisation of the precipitating systems critically depends on the ability to represent the cold air outflow from the Po Valley to the Ligurian Sea, which determines the position and intensity of the mesoscale convergence lines over the sea. Such convergence lines controls, together with the lifting produced by the Apennines chain surrounding the coast, the onset of the severe convection.

Statistical modelling of rainfall-induced shallow landsliding using static predictors and numerical weather predictions: preliminary results

“…On the other hand, from Table 3 (top), we can state that the model is able to capture the characteristics of a heavy rainfall event in the area of interest. In particular the maximum value achieved by the model in the whole area of interest (218 mm (24 h) −1 ) is similar to that obtained by Buzzi et al (2014), who analysed the same rainfall event with the ISAC convection-permitting MOLOCH model (Buzzi et al, 2004). In their paper, the authors found a maximum rainfall amount of 286 mm (24 h) −1 .…”

“…4a and b. The general underestimation of NWP model forecast in predicting precipitation maxima during intense precipitation events is a well known issue and was recently addressed for this specific rainfall event by Buzzi et al (2014). On the other hand, from Table 3 (top), we can state that the model is able to capture the characteristics of a heavy rainfall event in the area of interest.…”

“…From a meteorological point of view, the 25OCT2011 rainfall event was deeply investigated by Buzzi et al (2014) using a numerical weather model and by Rebora et al (2013) using the measurements available from a large number of sensors, both ground based and space borne. In this latter paper, the authors concluded that the large-scale features of the event and the complex geographical characteristics of the area determined the conditions for the persistence of heavy precipitations over the same region, i.e.…”

“…In recent years, Tuscany region and nearby areas (central Italy), have been hit by noticeably heavy rainfall events (Parodi et al, 2012;Sacchi, 2012;Avanzi et al, 2013;Rebora et al, 2013;Fiori et al, 2014;Buzzi et al, 2014;Perna et al, 2015). In particular the northwestern part of the Tuscany region is very prone to frequent and severe rainstorms, due to its geographical position and its steep orography close to the sea, making this area one of the wettest in northern and central Italy (Brunetti et al, 2009).…”

Shallow landslide initiation on terraced slopes: inferences from a physically based approach

“…Due to self-regenerating storm cells or to persistent cyclonic Tyrrhenian circulation (Crosta 1998;Cevasco et al 2009), extreme precipitation can occur, between late summer and mid-autumn, along the coast of eastern Liguria (van Delden 2001;Silvestro et al 2012Silvestro et al , 2015Rebora et al 2013;Buzzi et al 2014;Cevasco et al 2015;Cassola et al 2016). Usually these rainstorms have short duration (<24 h), but rainfall intensities can reach or exceed values of 100 mm h ¡1 .…”