Characteristics of Extratropical Cyclones That Cause Tornadoes in Italy: A Preliminary Study

Eigo Tochimoto,Mario Marcello Miglietta,Roberto Ingrosso,Hiroshi Niino,Leonardo Bagaglini

doi:10.3390/atmos12020180

Abstract

Characteristics of extratropical cyclones that cause tornadoes in Italy are investigated. Tornadoes between 2007 and 2016 are analyzed, and statistical analysis of the associated cyclone structures and environments is performed using the JRA-55 reanalysis. Tornadoes are distributed sporadically around the cyclone location within a window of 10° × 10°. The difference in the cyclone tracks partially explains the seasonal variability in the distribution of tornadoes. The highest number of tornadoes occur south of the cyclone centers, mainly in the warm sector, while a few are observed along the cold front. Composite mesoscale parameters are examined to identify the environmental conditions associated with tornadoes in different seasons. Potential instability is favorable to tornado development in autumn. The highest convective available potential energy (CAPE) in this season is associated with relatively high-temperature and humidity at low-levels, mainly due to the strong evaporation over the warm Mediterranean Sea. Upper-level potential vorticity (PV) anomalies and the associated cold air reduce the static stability above the cyclone center, mainly in spring and winter. On average, the values of CAPE are lower than for US tornadoes and comparable with those occurring in Japan, while storm relative helicity (SREH) is comparable with US tornadoes and higher than Japanese tornadoes, indicating that the environmental conditions for Italian tornadoes have peculiar characteristics. Overall, the conditions emerging in this study are close to the high-shear, low-CAPE environments typical of cool-season tornadoes in the Southeastern US.

Highlights

In the last few years, tornadoes are receiving growing attention in Italy
Tornado occurrences are taken from the dataset developed in MM18, which is largely based on the European Severe Weather Database (ESWD) produced by European Severe Storm Laboratory (ESSL), and covers the 10 years from 2007 to 2016, a period during which the number of tornadoes of enhanced Fujita category 1 or higher (EF1+) does not show any obvious trend (Figure 10 in MM18)
As discussed in MM18, spring cases are associated with the outbreak of cold mid-troposphere air crossing the Alps. They may occasionally occur along the coasts, the highest frequency is in the Po valley; we found that most of these tornadoes are EF1+

Summary

Introduction

In the last few years, tornadoes are receiving growing attention in Italy. This is due to a combination of different factors. Social media and the diffusion of smartphones have made it possible for a large audience of weather amateurs to share pictures and videos of tornadoes, contributing to an important crowdsourcing activity. Such documents are often posted in web journals and magazines, becoming popular among a large number of web surfers. The number of Italian tornado studies has increased in the last 20 years. In the 2000s, just a handful of papers were published in a decade, mainly concerning tornadoes that affected the northeastern Italian regions [1,2,3,4]; a historical chronicle of tornadoes in the extreme southeastern “heel-of-the-boot” (from 1546 onward) was published [5]

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Discussion

Conclusion