From large-scale weather variability to localized extremes: Researchers at the European Centre for Medium-Range Weather Forecasts and TU Freiberg develop a framework to better predict extreme rainfall events in Mediterranean countries.

The researchers make use of the general (synoptic scale) picture of weather variability over the Mediterranean to better predict extreme precipitation in the region some days and weeks in advance. As the team reports in the current issue of the Quarterly Journal of the Royal Meteorological Society, this method could support decision making of various sectors, and assist in mitigating the negative consequences of these natural hazards.

“It is extremely challenging to forecast many days in advance when and where heavy rainfall will occur. Thus, researchers strive to develop new tools to better predict these phenomena allowing for early warnings and adequate mitigation strategies”, explains first author Nikolaos Mastrantonas. That is why, in a previous study, the PhD student analysed weather data from 1979 to today, grouping the daily weather into nine patterns of distinct atmospheric characteristics over the Mediterranean.

How the new information helps to further develop forecasting models

This new study showed that there is a strong relation between these nine patterns and the occurrence of extreme rainfall in the Mediterranean. “In fact, we can use this information to better predict extreme rainfall one week or longer in advance”, says Prof. Jörg Matschullat of TU Bergakademie Freiberg. The geoecologist supervises Nikolaos Mastrantonas’ PhD and adds: “The precipitation forecasts are, in general, reliable up to 7–8 days ahead. With our proposed methodology, we can use the forecasted weather patterns and their connections with extreme precipitation and provide skilful predictions over 10 days in advance, extending the forecasting horizon by half a week in many locations.” For locations on mountainous or coastal areas, like parts of western Turkey, the western Balkans, the Iberian Peninsula, and Morocco, we can even have reliable forecasts up to 14 days ahead, the team reports.

Forecasting tools that can be run on a laptop

Because the team takes into account data that are already provided by the global forecasting models the new tool can be easily implemented and can be run with limited computation resources, even on a laptop. “That is why we believe that such information can be promising for various users; for example, the agricultural sectors, emergency response units, and insurance companies”, Nikolaos Mastrantonas says. According to the scientists, the methodology can now be further assessed for other regions as well as other extreme weather phenomena.

To this end, the team is now examining possible interrelations of the newly identified weather patterns over the Mediterranean to other modes of atmospheric variability such as the El Niño or the North Atlantic Oscillation. They are interested in finding out, whether and how the Mediterranean patterns are influenced by these phenomena and how this influences their predictability. “Our ultimate intention is to incorporate these findings into new forecasting products informing about extreme weather”, Prof Jörg Matschullat clarifies.

Background: The CAFE research project

CAFE, Climate Advanced Forecasting of sub-seasonal Extremes, is an EU-funded project with ten international partners across Europe and Latin America, including ECMWF and TU Bergakademie Freiberg. The project is funded with more than 3 million Euros within the EU research framework programme "Horizon 2020" and the Marie Sklodowska-Curie Actions. The ultimate goal of the CAFE project is to improve the sub-seasonal predictability of extreme weather events through the interdisciplinary training of 12 young researchers in aspects such as climate science, complex networks and data analysis.

Original publication: Nikolaos Mastrantonas, Linus Magnusson, Florian Pappenberger, Jörg Matschullat: What do large-scale patterns teach us about extreme precipitation over the Mediterranean at medium- and extended-range forecasts?, Quarterly Journal of the Royal Meteorological Society.

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Cloud formations over mountains in Northern Spain. Photo: TU Bergakademie Freiberg