Triggering and evolution of a deep convective system in the Mediterranean Sea: modelling and observations at a very fine scale

Abstract

Gaining a deeper physical understanding of the high-impact weather events which repeatedly affected the Western Mediterranean Basin in recent years on the coastal areas of easternSpain, southern France and northern Italy is strongly motivated by the social request to reduce the casualties and the economical impacts due to these highly localized and hardly predictable phenomena.In October 2014, an extreme event hit Genoa city centre, less than 3 years after a very similar event, which occurred in November 2011.Taking advantage of the availability of both observational data and modelling results at the micro-α meteorological scale, this article provides insights about the triggering mechanism and the subsequent spatio-temporal evolution of the Genoa 2014 back-building Mesoscale Convective System. The major finding is the effect of a virtual mountain created over the Ligurian Sea by the convergence of a cold and dry jet outflowing from the Po valley and a warm and moist low-level southeasterly jet within the planetary boundary layer.

Role of SST Patterns on Surface Wind Modulation of a Heavy Midlatitude Precipitation Event

“…The reason why such rainfalls are so intense is because multiple convective cells continually regenerate in the same position at the same rate at which they are advected away (Chappell, ; Ducrocq et al, ; Schumacher & Johnson, , , ). This condition can be reached in a scenario where in a quasi‐stationary synoptic system, a low‐level moist and conditionally unstable jet, typically blowing from the sea, is lifted by the presence of some orographic barrier (Ducrocq et al, ) or some cold dry continental air mass (Davolio et al, ; Fiori et al, ). Those four elements (conditionally unstable atmosphere, low‐level moisture, slow‐evolving synoptic conditions, and mesoscale lifting) are crucial ingredients in the development of such MCSs (Fiori et al, ; Nuissier et al, ; Ricard et al, ).…”

Ensemble cloud-resolving modelling of a historic back-building mesoscale convective system over Liguria: the San Fruttuoso case of 1915

“…Panel (a) also shows, by means of the green dotted cross section (45 • ), the thin potential temperature layer (virtual mountain) in front of the actual Liguria topography (panel b). This acts, as described in Fiori et al (2017), to produce strong convective cells in panel (c) (updraft velocity above 10 m s −1 ), with the apparent backbuilding on the western side (less mature and intense cells around 8.4 • latitude). The main updraft produces vertical advection of water vapour (panel d), thus resulting in significant production of rainwater (panel e), snow (panel f, significantly advected inland by the upper-level south-westerly winds) and graupel (panel g).…”

“…The physical mechanism responsible for the generation of the back-building mesoscale convective systems in this area has been recently explained by Fiori et al (2017). Taking advantage of the availability of both observational data and modelling results at the micro-α meteorological scale, Fiori et al (2017) provide insights about the triggering mechanism and the subsequent spatiotemporal evolution of the Genoa 2014 back-building MCS. The major finding is the important effect of a virtual mountain created on the Ligurian sea by the convergence of a cold and dry jet outflowing from the Po Valley and a warm and moist low-level south-easterly jet within the planetary boundary layer.…”

“…The ARW-WRF model is configured for this case study based on the results achieved in the ARF-WRF modelling of the Genoa 2011 and Genoa 2014 v-shape convective structures (Fiori et al, 2014(Fiori et al, , 2017. Three nested domains (Fig.…”

“…Furthermore, the Thompson scheme was shown to perform the best for the Genoa 2011 and Genoa 2014 studies (Fiori et al, 2014(Fiori et al, , 2017. With regard to the results in Fiori et al (2014) about the role of the prescribed number of initial cloud droplets -N t c -created upon autoconversion of water vapour to cloud water and directly connected to peak rainfall amounts, a maritime value corresponding to a N t c of 25 × 10 6 m −3 has been adopted.…”