How far into the future can we look?

How far into the future can we look?

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Weather-related disasters and extreme weather events, including river overflows and floods as well as heatwaves and droughts, cause massive loss of life and cost billions of dollars in property damage every year.

Therefore, weather forecasts and protection measures are of great importance and will become more important in the future. However, there is a natural limit to weather predictability, one that has not yet been reached.

“There is still great potential to improve weather forecasts for mid-latitude regions,” said Dr. Michael Rimmer from Johannes Gutenberg University Mainz (JGU). “But there is also a point beyond which reliable prediction is no longer possible.”

Reimer and his colleagues investigated the accuracy of weather forecasts under best-case scenarios. According to their calculations, it should be possible to extend the forecast period by up to four or five days. The study is published in Journal of Atmospheric Sciences.

Weather forecasts are becoming more reliable

Weather in mid-latitudes can be predicted with reasonable accuracy seven to 10 days in advance. This was not always the case. The quality of weather forecasts has improved significantly over recent decades. Seven-day forecasts are now as accurate as four-day forecasts 30 years ago. Improved computer performance and the availability of new data have contributed to this improvement, but the quality of diagnosis is still very poor in some cases.

Weaknesses and flaws in forecasting methods are part of the problem, but it is inherently difficult to predict certain weather conditions in chaotic environments. For example, large-scale tornadoes can be identified approximately seven days in advance, while the period is much shorter for thunderstorms. As the forecast period increases, the probability of errors increases.

Critical factors for the limits of predictability

Limitations of weather forecasting have been a topic of research since the 1960s. Unlike tides and planetary orbit, the atmospheric system has an intrinsic limit that represents a natural and final limit beyond which prediction is no longer possible. “Research has repeatedly come to the same conclusions: we can predict the weather 14 days in advance at best,” said Dr. Michael Reimer, a meteorologist at the JGU Institute for Atmospheric Physics.

In collaboration with colleagues Dr. Tobias Selz and Professor Georg Craig from LMU Munich, Rimmer confirmed the existence of a fundamental limit to predictability. The researchers also investigated the processes responsible. “Forecasts are currently the most affected by errors in our initial conditions,” Reimer added. “If we improve these initial conditions from which our computer models start their predictions, our predictions will also be more accurate.”

The butterfly effect in weather forecasting

Using quantitative estimates, the team demonstrated that it is necessary to more closely consider large-scale factors such as wind, wind pressure, temperature and jet stream. “We have to reduce the initial uncertainty by 80% to 90% to reach the intrinsic limit inherent in the system,” Reimer said. This could extend the reliable forecast period by another four to five days.

Once 90% error reduction has been exploited, the mechanisms involved will change and large-scale factors will no longer be critical. From this point on, the butterfly effect will dominate events. “Thunderstorms as the main driver of the butterfly effect will play a role.” However, this effect is so small that even severe thunderstorms will not affect the reliability of the current weather forecast for the next few days.

More accurate weather forecasts require increased allocation of resources for atmospheric monitoring

American meteorologist Edward Lorenz coined the term “butterfly effect” in the 1970s to describe how small variations in the initial conditions of a complex system, such as the atmosphere, can lead to large variations in the subsequent state, limiting the predictability of such systems. Even very small disturbances that are initially undetectable can accumulate and cause widespread changes in weather after a certain period of time.

“The individual thunderstorm cells in our study are like those butterflies,” Reimer added. “To improve our forecasts, we must focus on the key influencing factors.” The scientist recommends intensifying atmospheric observations and measurements, perhaps with the help of satellites. The potential of weather forecasting has not yet been exhausted and can be significantly improved in the coming decades.

more information:
Tobias Sells et al., Moving from Practical to Substantive Predictability of Midlatitude Weather, Journal of Atmospheric Sciences (2022). doi: 10.1175/JAS-D-21-0271.1

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