Picture: Crackling thunderstorm. Credit: Bogdan Radu (Pixabay)

Extreme weather events like cyclones have enormous economic and social impacts, making it vital to improve our understanding of how they form and develop. One of the hardest parts of these storms to forecast are small cyclone-like structures that embed themselves in major storms and cause significant unexpected damage around the larger storm’s centre.

Surprisingly, one of the best places to investigate these sub-cyclones is the Mediterranean Sea, because it has the highest frequency of damaging storm events in the world and is in a well-observed region. A recent study by CLEX researchers and colleagues investigated a Mediterranean storm system that developed on October 31, 2012, which included one of these destructive secondary cyclone structures. That secondary structure produced heavy rain near Catalonia, powerful winds in northern Minorca and eventually made landfall around Genoa bringing heavy rainfall to parts of Italy.

The Mediterranean storm system that developed on October 31, 2012

Through observations from satellite and land-based measurements, the researchers were able to follow the sub-cyclone’s formation and development closely. This allowed them to examine three key characteristics –  latent heat release; the dynamic upper atmosphere forces that influenced the storm’s formation and development; and the effects of the terrain below.

The results showed the upper atmosphere contributed to the development of storm through potential vorticity and intensified as rain droplets formed making the upper part of the sub-cyclone colder and stronger. The storm’s intensity was then amplified again by the terrain below. Together these produced a stable, long-lasting sub-cyclone that by itself caused significant damage.

Understanding how each of these characteristics contributed to the development of this storm within a storm can help us to recognise the features that create and sustain these sub-cyclones. With this knowledge, we hope to improve our ability to forecast them and give residents in their path more time to prepare for their impact.

  • Paper: Carrió, D. S., V. Homar, A. Jansà, M. A. Picornell, and J. Campins. “Diagnosis of a High-Impact Secondary Cyclone during HyMeX-SOP1 IOP18.” Atmospheric Research 242 (September 15, 2020): 104983. https://doi.org/10.1016/j.atmosres.2020.104983.