The hydrological cycle, with its feedbacks from water vapour and clouds, is one of the largest sources of uncertainty in weather prediction and climate models. In particular processes that occur on microscopic scales, such as evaporation from the oceans, and the formation of rain and snow, can cause forecast errors. Some errors can compensate one another, such as too much evaporation by too much rain, and thus be overlooked. Undetected compensating errors critically limit the understanding of water and weather extremes, how it responded to past climate changes thousands of years in the past, and how the water cycle responds to a warming climate. The overall objective of the ISLAS project was to find new evidence for how the water cycle works in weather systems from source to sink, ultimately allowing researchers to make better predictions of water and weather extremes.
To this end, we made use of highly resolved and precise measurements of stable isotopes in atmospheric water. These stable water isotopes are a naturally occuring, built-in tracer for water's sources, storing a record of atmospheric processing within rain and snow. The key hypothesis of ISLAS was that such measurements, if made during all stages of water transport along its journey from source to sink, can help us to identify error compensation in models. Our approach in the project has been to combine a comprehensive suite of field measurements with novel instrumentation, and with digital forecast tools for the atmosphere to determine the amount of mixing and transport all along on the way from evaporation sources to precipitation sinks.
During the project period, we made field experiments to follow water as it moved through the atmosphere within weather systems. We had found previously that cold-air outbreaks, a common type of weather system in the European Arctic during spring, would be ideally suited to observe water during all stages of its atmospheric life, like in a natural laboratory. Accordingly, cold-air outbreak episodes were the primary target of all three scientific field expeditions in 2020, 2021, and 2022. Campaigns were focused making stable isotope measurements in evaporation over the sea, within clouds, and from snow and rain falling on surrounding land regions. Utilizing a combination of research aircraft, research ship, an array of measurement stations, and help from citizen scientists, these field experiments provided truely unique and comprehensive datasets including stable isotope information, which are an important legacy from ISLAS.
Analysis of the dataset up to this point confirms our key hypothesis, namely that stable water isotope measurements in different parts of cold-air outbreaks provide evidence of how air masses are transformed and water is processed within the atmosphere during transport. By showing how water vapour interacts with different surfaces in the Arctic, we have been able to close an important knowledge gap with evidence from direct measurements. In conclusion, ISLAS has been successful in advancing towards the overall project aim of bringing a more reliable water cycle into atmospheric prediction models using stable isotopes.
