How a thunderstorm supercell forms and why it's dangerous
The persistent heat waves and a set of phenomena that occur in the Po Valley are at the origin of the development of thunderstorm supercells who are scourging Northern Italy these days. The studies of Marcello Miglietta, research manager of the Institute of Atmospheric and Climate Sciences, focus precisely on supercells and tornadoes, as well as on the so-called "Medicanes", the Mediterranean cyclones similar to tropical ones. The expert helps us to better define the characteristics of supercells.
How a supercell develops
“Let's start from a normal storm cell – explains Miglietta – which develops because the warm air that rises from the ground collides with the cold air at high altitude. When cumulonimbus saturation is reached, rain occurs. There thunderstorm supercell differs because it has an extra ingredient, the wind is not constant with altitude but tends to intensify. In the normal thunderstorm, in the dissolving phase the warmer air is dragged downwards and cools the colder air, in the supercell this does not happen because the wind has a transversal inclination, called wind shear”.
The difference in duration
It is this rotation of the wind that makes phenomena related to the supercell last longer. “By virtue of the wind shear, rotational movements of air occur inside the supercell, called mesocyclones. This rotating system of inclined winds – simplifies Miglietta – feeds the cloud and therefore the phenomena of the supercell can last even hours”.
Can you recognize the arrival of a supercell?
“Our increasingly sophisticated instruments, such as Doppler radars, allow us to check wind speed and quickly identify the nature of a phenomenon. However, with the naked eye, some experience is needed – continues the expert – a rotation, a more extensive shape, a different color of the sky with shades other than white and gray can be identified and since hail precipitations are usually associated with it, a more whitish front can be seen”.
What are the dangers?
“Supercells are more dangerous because they are associated with more violent phenomena: wind shear is associated with hail, tornadoes and downbursts, i.e. descending gusts of wind with vertical motion”.
Do supercells mostly affect the plains?
“A thesis signed by Francis DeMartin of the University of Bologna, which is being worked on for publication and has already received numerous awards, he clarified because three particularly favorable conditions for the formation of supercells are created in the Po Valley – explains Miglietta -. In this area, hot dry wind descending from the Apennines, colder air from the North and warm humid air from the Adriatic collide, thus creating a particularly favorable configuration for the appearance of this phenomenon”.
Is the formation of supercells more frequent in recent years?
“Scientific interest in these phenomena has certainly increased, and we are able to describe them more and more accurately also thanks to technological progress. Supercells have always existed, but it is clear that with heat waves and above all with the significant increase in sea temperatures more energy develops and therefore more intense phenomena”.
