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Better forecasting of extreme rainfall in Mediterranean countries

  • Better forecasting of extreme rainfall in Mediterranean countries
  • An EU-funded study has analysed ways to improve forecasts of extreme rainfall events in the Mediterranean. It found that most occur in autumn in the west and in winter in the east.

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Extreme rainfall is one of the most frequent natural hazards in the Mediterranean. It often results in floods and landslides, with severe consequences for the environment, society and the economy. Because of this, scientists have been striving to understand this natural hazard and find ways to improve forecasts. However, it’s “extremely challenging to forecast many days in advance when and where exactly heavy rainfall will occur,” explains Nikolaos Mastrantonas of Germany’s Technische Universität Bergakademie Freiberg (TUBAF) in a news release posted on the ‘EurekAlert!’ website. “Thus, researchers strive to develop new tools to better predict extreme weather phenomena allowing for early warnings and adequate mitigation strategies.” Mastrantonas is the lead author of a study exploring how to improve forecasts of extreme precipitation events (EPEs) in Mediterranean countries. The study’s novelty is twofold. First, it used “a systematic, Mediterranean‐wide, and year‐round approach” in which identification of such rainfall events is based on the latest ERA5 data set. Second, it quantified the link between EPEs and large‐scale atmospheric flow patterns over the Mediterranean. The results showed a distinct spatial division in the occurrence of EPEs, with most events occurring during winter in the eastern Mediterranean and autumn in the western Mediterranean.

Atmospheric flow patterns

With support from the EU-funded CAFE project, the research team analysed EPEs that occurred between 1979 and 2019. Daily weather was grouped into nine distinct atmospheric flow patterns over the Mediterranean: Atlantic Low, Biscay Low, Iberian Low, Sicilian Low, Balkan Low, Black Sea Low, Mediterranean High, Minor Low and Minor High. “When it comes to climate, the Mediterranean Sea is a particularly interesting region as it is surrounded by large continents and mountain ranges. The regional climate of the area is also dependent on large-scale patterns over the Atlantic Ocean, the Balkans and the Black Sea,” remarks co-author Prof. Jörg Matschullat of CAFE project partner TUBAF. The nine patterns correspond to unstable conditions such as cut-off lows and troughs or are associated with cyclonic and anticyclonic conditions. “Such conditions lead to extreme precipitation events at different subregions of the Mediterranean,” comments Mastrantonas. The ‘EurekAlert!’ news release provides an example: “A low-pressure system centred over the Bay of Biscay increases the probability of extreme rainfall over mountainous and coastal regions in Spain, Morocco, Italy, and even in the West Balkans more than sixfold.”

The role that mountains play

The study also showed how mountains create a strong link between distant locations. Over 30 % of EPEs in parts of western Italy happened on the same day as EPEs hundreds of kilometres away over the coast of Croatia and Montenegro. “This is a result of the Apennines that block a substantial part of the airflow, and frequently force the moisture to precipitate in the western part of Italy, and on the same day over Croatia,” Mastrantonas explains. Next, the researchers aim to establish how reliable the latest weather forecasting models are in predicting the nine atmospheric flow patterns. “Our intention is to incorporate such information into new forecasting products informing about extreme weather over the Mediterranean at sub-seasonal scales,” concludes Prof. Matschullat. The CAFE (Climate Advanced Forecasting of sub-seasonal Extremes) project began in March 2019 and has a duration of 4 years.

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