Meteorology – forecast: more thunderstorms

In Italy, various regions of severe rainfalls were devastated from the Alps to ankle sales last week. Countless trees were knocked over, houses were covered and landslides were triggered. Parts of Switzerland, Slovenia and France, where several wind pants or tornados occurred, were also affected by the storms. The Central and Southern Italian regions, Abruzzo, Molise and Puglia were reported on particularly large hailstorms of usually two to three centimeters in diameter. According to the British newspaper “Guardian”, the largest was registered with a diameter of almost ten centimeters from the village of Guglionesi in the Molise region.

Like the tornados, the particularly violent and destructive hail is a sign that some of the precipitation was connected to so -called super cells, that is, with particularly severe thunderstorms. Even if these thunderstorm cells obviously appeared in the pack this time, they usually only have a few kilometers of diameter – including the super cells and are therefore not yet grasped by the climate models. However, meteorologists can read relatively well in their models for the weather forecast, where thunderstorms occur. However, the models also improve in climate sciences, as a new study shows, which also predicts the increase in this particularly severe storm in a warming world.

The number of super cell storms could increase by up to 52 percent on the alpine north side.

A predictive model for the weather of tomorrow or the climate of the next decades can be imagined as follows: Over the entire planet or even individual regions, 30 to 90 layers are placed on bars and the state of the atmosphere (temperature, air humidity, wind speed and direction and so on) is calculated with the various physical equations for a sequence of times. Accuracy and meaningfulness of the results depend on the mesh size of the grids and the size of the time steps. In general, the smaller, the better. The limiting factor is the computer output, because the number of operations necessary is enormous even for the major computers used.

For the fourth report by the 2007 World Clime Commerce IPCC, most global models were still calculated with a mesh size of around 200 by 200 kilometers. In the meantime, earth system models can now work with 50 kilometers resolution and 95 vertical layers.

If the calculations are limited to individual regions such as Europe, it can be expected much more small, as Swiss researchers have recently done in order to learn more about super cells in times of climate change. The work was held last week in the journal “Science Advances” published. At the University of Bern and the renowned Federal Technical University in Zurich, the frequency of these particularly severe and often major damage could be simulated in detail with a mesh size of 2.2 kilometers. First, the frequency was determined with the current boundary conditions and compared with the European weather data for the years 2016 to 2021. The agreement was quite good, says co-author Monika Feldmann, who works at the Mobiliar Lab for natural risks and at the Bernese Oeschger Center for Climate Research. However, somewhat less heavy thunderstorms were simulated than actually performed. However, that is not surprising, says Feldman. »The model can only represent super cell thunderstorms that have an extension of more than 2.2 kilometers and take longer than an hour. However, some storms are smaller and last less long. ”In a second step, it was then simulated how many super cell storms in a world that is to be expected compared to the pre-industrial level by three degrees Celsius warmer world. The result: they occur on average by eleven percent more often, their number could increase by up to 52 percent, especially on the alpine north side. In the future, even more harvest and other weather damage will be expected in southern Germany.