More and more tornadoes in the Po Valley:how they are born and what the 'triple point' is

In 2021, on September 19, seven tornadoes formed in a few hours between Lombardy and Emilia-Romagna:this is why this phenomenon, which in the collective imagination makes us think of America, is happening more and more frequently in the Po Valley

BOLOGNA – It certainly won't be like Kansas or Iowa.But in recent years the Po Valley has also witnessed a increase in tornadoes, even of considerable intensity, in particular between Lombardy and Emilia-Romagna.Like the event of September 19, 2021, when they developed seven tornadoes in a few hours, causing serious damage in numerous locations in the Po Valley, of which four are at F2 level (the maximum is five).But how are they formed?Just the same way they are born in the USA.But on a smaller scale.

TORNADOES ARE BORN IN THE 'TRIPLE POINT'

Tornadoes over Northern Italy they are often born at a so-called 'triple point', that is, at the confluence of three air masses coming from different directions and with different characteristics.That is, humid, dry and cold.This was highlighted by a study conducted by the Institute of Atmospheric and Climate Sciences of the Cnr, in collaboration with the Universities of Bologna, Bari and Milan, published in the US scientific journal 'Monthly Weather Review'.

READ ALSO: Tornadoes increasingly frequent in Italy:what they are and why they are scary
READ ALSO: Hail, fires, winds and heat:if big data helps us defend ourselves from extreme events

HOW TORNADOS FORM

The Po Valley is considered a "hot-spot" for the development of tornadoes in Europe, due to the effects that the Alps and Apennines have on atmospheric flows in the lower layers.However, it was precisely the unusual sequence recorded in 2021 that pushed researchers to delve deeper into the physical mechanisms that led to the genesis of the vortices.“The study of ground observations during the event highlighted how tornadoes always developed no more than 20-30 kilometers away from a 'dryline', that is, from a front of dry air descending from the Apennines, and near a cold discontinuity generated by thunderstorms on the Alpine foothills - explains Vincenzo Levizzani of the Cnr - at the same time, very humid south-east currents they blew from the Adriatic Sea towards the Po Valley".Other storms that developed further away from the triple point that day “did not spawn tornadoes.”

HOW THE WIND ROTATES IN TORNADO FORMATION

They were also created for the study on tornadoes in the Po Valley numerical simulations high resolution with the 'Moloch' meteorological model, developed at the CNR in Bologna, to simulate the supercells that generated the tornadoes.“The model was able to correctly reproduce the development of the tornadic supercells and the complex interaction of surface flows that emerged from the observations,” explains Silvio Davolio, professor at the University of Milan and associated with the Cnr-Isac.“The model revealed a marked rotation of the wind near the 'dryline' in relation to the altitude - adds Mario Marcello Miglietta, professor at the University of Bari and researcher at the Cnr - from the south-east near the ground, to the south-west above the first kilometre.This peculiar wind profile generated the vorticity that leads to the development of tornadoes.Furthermore, a lot of humidity has accumulated near the triple point, which increases potential instability, another important element for the genesis of these violent phenomena."

PREDICTING TORNADOES IS IMPOSSIBLE

This model is “inspired by the dynamics observed in the United States in the so-called 'Tornado Alley', where tornadoes form at the confluence of moist air masses coming from the Gulf of Mexico, dry air masses from the Rocky Mountains and air masses coldest from Canada.In the case of the Po Valley, something similar is observed, but on a much smaller scale", comments Francesco De Martin, doctoral student at the Alma Mater of Bologna and first author of the article.This study, therefore, could "contribute to improving forecasts" of tornadoes, even if "a certain degree of uncertainty" remains.Even today, in fact, "it is impossible to know in detail if, where and when a tornado will develop, even a few hours after an event“.