A conceptual model for the development of tornado in the Po Valley

Why is the Po Valley a hot spot in Europe for tornadoes? In this study the authors propose an explanation to this issue, studying a tornado outbreak that affected the Po Valley on 19 September 2021. During that event seven tornadoes (four of them ranked as F2 according to the Fujita scale) developed between Lombardia and Emilia-Romagna regions in a few hours. Although tornadoes are not rare in Italy, so many tornadoes in such a short time are an unusual event. 

The case study was analysed by means of observations and numerical simulations obtained with the convection permitting MOLOCH model. Observations showed that during the event there were two low-level boundaries in the Po Valley: a cold front coming from the Alps and a dry line generated by the downslope winds from the Apennines. These two boundaries created a triple point, like those observed during tornado outbreaks in the US MidWest, but on a smaller scale. Observations proved a strong correlation between tornado developments and low-level boundaries.

Numerical simulations with 500 m grid spacing showed that a warm and moist air tongue from the Adriatic Sea played a fundamental role in generating the supercells, causing an advection of vorticity and favouring instability conditions. Moreover, through numerical experiments, it has been proved that this moist air tongue was sensitive to the Froude number of the south-westerly flow from the Apennines: the greater the Froude number, the further north and narrower was the tongue of air, with impacts on the development of supercells. Along the cold front large amounts of streamwise vorticity were generated by the buoyancy gradient. Furthermore, the dry line played a key role in the generation of tornadoes, creating locally large amounts of instability and strong wind veering near the surface: kinematic and windshear parameters were comparable to those observed in US-tornado events only along a narrow path near the dry line. 

Comparing these results with previous papers, the presence of thermal boundaries and dry lines represents a typical pattern during tornado-events in the region. Then, in conclusion, a conceptual model also useful for forecasting applications is proposed for the development of tornadoes in the Po Valley, which explains why tornadoes are relatively common in Northern Italy.