NEW ANALYSIS HAS revealed global patterns of extreme rainfall, which could lead to better forecasts and more accurate climate models.
Extreme rainfall – defined as the top five percent of rainy days – often forms a pattern at the local level, for example, tracking across Europe.
However, new research, published in the Nature journal, has revealed that there are also larger-scale global patterns to extreme rainfall events.
These patterns connect through the atmosphere rather than over land – for example, extreme rainfall in Europe can precede extreme rainfall in India by around five days, without extreme rain in the countries in between.
The research could help better predict when and where extreme rainfall events will occur around the world, according to the researchers. The insights could be used to test and improve global climate models, leading to better predictions.
The study was led by a team at Imperial College London and the Potsdam Institute for Climate Impact Research in Germany.
This research also provides a “baseline” for climate change studies, according to the authors. By knowing how the atmosphere behaves to create patterns of extreme weather rainfall events, scientists will be able to gain new insights into changes that may be caused by global warming.
“Uncovering this global pattern of connections in the data can improve weather and climate models,” lead author Dr Niklas Boers said.
This is especially true for the emerging picture of couplings between the tropics and the European and North American regions and their consequences for extreme rainfall, the authors noted.
“This finding could also help us understand the connections between different monsoon systems and extreme events within them. I hope that our results will, in the long term, help to predict extreme rainfall and associated flash floods and landslides in northeast Pakistan, north India and Nepal,” Boers said.
“There have been several hazards in recent years, with devastating consequences in these regions, such as the 2010 Pakistan flood.”
The research
To find patterns in extreme rainfall events, the team developed a new method rooted in complex system theory to study high-resolution satellite data of rainfall. The data comes from the Tropical Rainfall Measuring Mission.
By breaking the globe into a grid, the team could see where events happened and determine how “synchronous” they were – a statistical measure that assesses connections even if the events did not happen at exactly the same time.
The results from this “complex network” model, analysed using our understanding of the motion of the atmosphere, revealed a possible mechanism for how the events were connected.
The patterns appear to be created by Rossby wave – wiggles in fast-flowing currents of air high in the atmosphere, known as the jet streams, the authors noted.
Rossby waves have been connected by regular rainfall, but this study is the first to connect them to extreme rainfall event patterns.
Co-author Professor Brian Hoskins said: “This new technique applied to satellite data shows very surprising relationships between extreme rainfall events in different regions around the world.
For example, extreme events in the South Asian summer monsoon are, on average, linked to events in the East Asian, African, European and North American regions.
Although rains in Europe do not cause the rain in Pakistan and India, they belong to the same atmospheric wave pattern, with the European rains being triggered first.
“This should provide a strong test for weather and climate models and gives promise of better predictions.”
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