Tracking 'Heavy Water' in the Sky Improves Weather Forecasts Up to Five Days Out

Weather forecasts rely on data such as temperature, humidity and wind. For the first time, researchers including those at the University of Tokyo have demonstrated that a long-theorized improvement to weather models, adding data on water isotopes in the atmosphere, actually works. Incorporating satellite measurements of water vapor isotope ratios into weather models improved forecasts of atmospheric conditions for up to five days, including predictions of heavy rainfall in many regions, according to the study published in Communications Earth & Environment.

Isotopes are alternate, usually heavier, forms of atoms with a different number of neutrons, and in water they can produce what is known as heavy water, which evaporates and condenses slightly differently from ordinary water. 'Water isotopes occur naturally in very small amounts. Their relative abundance changes slightly during processes such as evaporation and condensation. By taking advantage of these changes, we can obtain information about where water came from and what happened to it along the way,' said Kinya Toride, a researcher with the U.S. National Oceanic and Atmospheric Administration and the Institute of Industrial Science at the University of Tokyo.

The team combined many satellite observations from six-hour windows into a single high-quality measurement for each region, filtering out low-data areas to build a reliable global dataset, then fed it into a weather model through a technique called data assimilation. 'We found that this additional information improved our estimates of basic atmospheric conditions, such as winds, temperature and water vapor, which in turn led to more accurate weather forecasts,' Toride said.

Professor Kei Yoshimura of the University of Tokyo said this is the first study to show the benefit under near-operational conditions, but cautioned it cannot be introduced overnight since there is very little real-time isotope data today and current models are not designed to use it. The team's long-term goal is to integrate more accurate satellite observations of water vapor isotopes into operational forecasting systems, a prospect Yoshimura said is increasingly likely given the falling cost of satellite launches.