Finding the sources of atmospheric water
The hydrological cycle is a complex system. Some processes occur on scales far smaller than can be represented in weather prediction models. In addition, some processes can compensate one another, making their calculation difficult. These ‘undetectable compensating errors’ limit our understanding of the hydrological cycle, making it the largest source of uncertainty in weather prediction and climate models. One example is the balance between evaporation and precipitation. Evaporation makes the atmosphere more wet, while precipitation makes it drier. As observing wetness is easier than evaporation and precipitation, too much evaporation can be compensated by too much precipitation. “This is a problem, because for one we care a lot about precipitation, but also because there is a lot of energy turnover tied to both of these processes, so getting this correct is important for weather forecasts,” says Harald Sodemann(opens in new window), professor in Meteorology at the University of Bergen. Stable water isotopes can help, as they are sensitive to phase transitions (such as liquid to vapour on the ocean, and from vapour to liquid or ice in clouds). Any stable water isotope measurement in the air can also inform about how much moisture was added or removed in the days before. In the EU-funded ISLAS(opens in new window) project, Sodemann and his team explored the use of stable isotopes in revising our view of the atmospheric water cycle. The idea was to use the Nordic Seas as a natural laboratory to study the underlying processes at a large scale.
Following the history of water vapour from source to sink
The Nordic Seas are home to a particular weather phenomenon during winter and spring called cold-air outbreaks (CAOs). During CAOs, freeze-dried Arctic air moves over the ice edge and onto open water, and as the ocean is warmer this leads to strong evaporation into the lower atmosphere. “We know precisely where the source of the moisture is in that case. That is why we consider this region a natural laboratory,” notes Sodemann. The team used instruments on a research aircraft, a research vessel and land to intercept the CAO air mass during different stages. This allowed them to see how much moisture was inside the atmosphere and how much had been removed. As a final step, the team measured stable isotopes in snow on the ground.
Adopting stable isotopes into model predictions
The most direct, hands-on outcome from the project, and its legacy, is the dataset from three measurement campaigns. The team also had one unexpected scientific highlight from their 2020 measurements over Svalbard, which helped to resolve a controversy about stable isotopes in the Arctic. Yet Sodemann stressed the knowledge from the project is much bigger than achieving an objective or providing a deliverable. “This concerns how we think about the water cycle, as a connected sequence of events, rather than an isolated state,” he explains. The team could confirm that the concept of using stable water isotopes to trace the history of water vapour from source to sink works. “This underscores the value of including stable isotopes into model predictions, at least for research purposes,” Sodemann adds.
Incorporating citizen science
The ideas from the project will continue in the EU Water4All framework, in a highly interdisciplinary project named ISOSCAN(opens in new window) that will explore the use of stable isotope measurements in surface snow for improving river run-off forecasts. “To that end, we will employ citizen science to collect surface snow over large areas, and involve locals and tourists in the scientific process,” says Sodemann.
