Last July 7, in the evening, the Venetian lagoon area was affected by a sudden storm that put those at sea in serious difficulty. A Costa Crociera ship, the Costa Deliziosa, skidded dangerously in the harbor at the mercy of 40-50 knot gusts, under total hail.
The Mediterranean is an unpredictable sea, we know this well: that is why it is necessary to know how to best interpret weather signals in order to avoid being in the midst of a “downburst” (i.e., a weather phenomenon consisting of a strong downdraft reaching the surface, accompanied by a violent thunderstorm) as sudden as the one in Venice.
Together with experts from Meteomed, we used the Venetian gale as a “case history” to understand, in detail, how a gale is born (and how to avoid it by catching its clues in advance).
WHAT HAPPENED IN VENICE
On Sunday, July 7, 2019, the synoptic-scale situation over northeastern Italy saw the presence of an anticyclonic field of subtropical nature in the process of retreating toward central and southern Italy, under the push of more unstable North Atlantic currents aloft. The huge energy reservoir in the lower layers of the atmosphere (humid warm conditions over the entire Veneto and Friuli lowlands), accumulated in the previous days, thus favored the initiation of the first thunderstorms as early as mid-day(ill. 1, ill. 3).
An initial thunderstorm front, generated shortly after noon over eastern Trentino (Val di Fiemme-Pale di San Martino), invested the Valbelluna and part of the Agordino area, extending to the Trevigiano area and then aimed at the Sandonatese and the northern Venetian coast around 2 to 3 p.m. (specifically the Jesolo area). The carrier flow of currents aloft, predominantly from the west-northwest, renewed a second thunderstorm line developing in the second part of the afternoon, much more extensive than the previous one, which involved much of the Venetian plains from west to east-southeast. Heavy downpours, also associated with hail phenomena, involved parts of the provinces of Verona (including the Garda area), Vicenza, Padua, and Venice.
By early evening, around 7-8 p.m., the storm system vented its maximum intensity mainly over the Venetian lagoon area and toward the open sea, causing significant difficulties to maritime navigation due to the strong winds, reduced visibility, and rapidly rising swell generated by the storm (and particularly the downburst to and related to it (ill. 5, ill. 6, ill. 7), a phenomenon we will explore later). Winds at that juncture reached 60-70km/h at weather stations in Venice (Cavanis Institute), Mestre, Marghera and Chioggia, locally touching peaks even close to 80-100km/h considering the damage locally recorded in some areas of the Lagoon.
During Sunday, July 7, the atmosphere over the northeastern regions underwent rapid and profound instabilization. Therefore, let us analyze in detail the atmospheric conditions that led to the genesis of violent thunderstorm phenomena with particular reference to the lowland and coastal areas of Veneto. Taking a look at the macroscale synoptics, we observe a weakening of the anticyclonic field that affected northern Italy in the previous days, which is associated with an increased interference of the North Atlantic perturbative flow in the Central-European region(ill. 1). In the summer quarter such synoptic evolution is worthy of attention as it is favorable for the development of thunderstorm phenomena of high intensity since Northern Italy comes to be in the zone of interaction between the warm humid air masses inherited from the anticyclonic context and the cooler and more unstable ones from the North Atlantic flow; in other words, the south-alpine compartment becomes an area of strong contrast between different types of air masses.
Starting from this general context of propensity for atmospheric instabilization, in order to predict in detail the consequences on the Po Valley and the Upper Adriatic, it is necessary to carefully analyze the mode of entry of the western Atlantic flow. In fact, the interaction of this flow with the complex physiography of northern Italy plays a primary role in the southalpine compartment to identify the areas most at risk in terms of type, distribution, intensity, duration and spatiotemporal evolution of the phenomenology. To this must be added even more local dynamics related to the configuration of low-level winds and breezes, which also can play a decisive role in stimulating or not stimulating convection(ill. 3).
Keep in mind that the initiation of convective phenomena needs two main ingredients: firstly, the presence of latent instability, i.e., the available potential energy defined by the CAPE parameter (e.g., related to the presence of particularly warm and humid air in the lower layers), and secondly to a set of atmospheric conditions that allow this energy to be converted into kinetic energy i.e., vertical upward motions that underlie any convective phenomenon.
Analyzing the thermo-dynamic profile at 2 p.m. local time extrapolated from the atmospheric radiosounding performed at Udine Rivolto Airport(ill. 2) and the derived instability indices, these conditions were fully met. Indeed, we observe CAPE values above 2000, values that the literature associates with a high risk of rapid, deep and intense convection. Indices analyzing the probability of this energy actually being released are also favorable: these include LIFT INDEX -8, TOTAL TOTALS 57, CIN -38, SWEAT 380.
In the face of a very favorable synoptic and thermo-dynamic picture, as already mentioned, the most intense phenomena tend to be concentrated in circumscribed areas within larger storm systems(ill. 3, ill. 6). This fact is not random but related to low-level dynamics (by low-level we mean the layers of the atmosphere near the ground) that vary from time to time. In the case study at hand, the synoptic evolution was such that a confluence area, i.e., a narrow zone where winds from different directions meet, came into existence over the central plains of the Veneto. The ground wind chart at 5 p.m. local time (ill.4), elaborated by our LAM (high-resolution weather model), shows the wind situation over Veneto: moderate westerly winds over the provinces of Verona, Rovigo, and Padua that are arrested in the Venetian area by the breakout of Bora winds from the Treviso area.
Such convergence of winds can be defined as the spark needed to permanently set the thermo-convective machine in motion. How? Along the band of wind convergence, an accumulation of air mass takes place, which can only find natural venting upward, thus generating updrafts; in this way, the last resistances in the atmosphere to the release of CAPE are overcome, and convection is thus stimulated and enhanced.
These types of low-level confluences are very frequent and of critical importance in forecast analysis; without them, convective potential may often not be expressed, in other words, in the face of high potential in fact storms are not triggered. Such confluences may have synoptic reasons but are often related to the more classic dynamics of diurnal thermal breezes affecting coastal areas the immediate hinterland of both Veneto and Friuli-Venezia Giulia.
Typical, for example, is the convergence of sea breezes in the early afternoon hours that occurs in the immediate backshore areas. To sum up: synoptic analysis and the main thermo-convective and thermo-dynamic parameters, both in forecasting and post-analysis, showed a very favorable configuration for the development of severe thunderstorms over most of the Veneto and Friuli plain, identifying an area most at risk between the provinces of Treviso, Padua and Venice. It also identifies a moderate-to-high risk of violent events such as hailstorms or downbursts, a phenomenon that actually occurred in the lagoon area.
As mentioned in the previous paragraphs, the storm winds observed in the lagoon area can be attributed to the downburst phenomenon. The downburst is a strong downward current that orginates within the thunderstorm cloud (cumulonembus) and, like a waterfall, pours downward until it reaches the ground. Arriving on the ground, this cold air current often accompanied by torrential rain and sometimes hail, rapidly fans out from the point of impact generating linear but also turbulent wind gusts (given the interaction with the land and urban fabric) that in a few minutes can involve large portions of the territory given the wind speeds that can even exceed 100 km/h.
Therefore, this phenomenon results in a sudden and drastic change in weather conditions that can become extreme within a few minutes, thus being very dangerous for navigation as well. From a forecasting point of view, we have seen that through scrupulous analysis it is possible to identify the areas most at risk to high intensity phenomena, sometimes even with good confidence, but only through tools of nowcasting analyzed by meteorologists (analysis of current weather and forecast in the immediate following hours) it is possible to identify the areas that will actually be affected by phenomena capable of causing problems to human activities and navigation and therefore proceed with possible and timely warnings.
Even the highest resolution and best performing models are able to detect dynamics prior to the initiation of convection (e.g., convergence of winds on the ground ill. 4, ill. 7) but struggle to model the events of downburst as they are deeply related to dynamics within the storm cloud acting at scales below the resolution of the model itself. At these junctures, very different marine weather conditions, albeit transient, can be observed in terms of wind and wave height than predicted by the model. Therefore, the meteorologist’s contribution is still of paramount importance in forecasting to indicate the risk of occurrence in these phenomena.
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