Sources of the Norwegian winter season snow pack constrained by stable water isotopes

Water vapour transport in the atmosphere contributes substantially to the uncertainty in weather and climate predictions. Better understanding of the water cycle is important for better preparedness against precipitation extremes, and for the management of renewable energy resources. In the SNOWPACE project, we investigated where in the ocean the water originates that gets deposited as snow over the Norwegian mountains. To this end, we used a specific chemical property of water molecules, their natural stable isotopes, to find the link between the ocean sources, atmospheric moisture transport, and mountain snowfall. From field measurements over ocean and land, from rain, snow and water vapour, the SNOWPACE project established a broad observational basis to test and improve the water cycle of atmospheric models using stable water isotopes. A new method was developed in the laboratory to correct water vapour analyzers in low humidity conditions. The SNOWPACE project thereby enabled higher accuracy for measurements in cold regions, at high elevations, during winter, and from airborne campaigns. From two dedicated field campaigns at Finse, long-term measurements in Bergen, and joining campaigns of other projects, a broad new observational data basis has been obtained at the end of the North Atlantic storm track. Thereby, we were able to acquire the first water isotope dataset focused on a regional water cycle, set up during specific wintertime weather events, so-called marine cold-air outbreaks. A wide collection of numerical tools was applied to unravel the interplay of moisture sources and transport and the water isotope composition. Water tracer modelling revealed that during marine cold-air outbreaks in the Norwegian sea water is in the atmosphere for less than 3 days between evaporation and precipitation. Assessment of vapour and precipitation isotopes showed that the spatial representativeness of water isotope measurements is substantially larger than that of precipitation alone. Isotope parameters vary at different time scales than humidity, and reflect the dominant weather patterns. Transfer of the water vapour isotope signal to the seasonal snow pack in Norway was found to be influenced by atmospheric conditions during snowfall, and changes of the snow pack in response to atmospheric and soil parameters thereafter. A comprehensive climatology of the moisture pathways for Scandinavia revealed previously unmapped spatial gradients of land and ocean sources of precipitation. Both the mean state and the variability are important background information for understanding variations in the hydropower resources. A citizen science experiment with snow sampling during Easter 2019 involved more than hundred volunteers to observe the isotopic footprint of weather events in the Norwegian mountains. The legacy of SNOWPACE will be the new knowledge, and the collection of open-access data sets, enabling new approaches to improve weather and climate models within the next years.

From only a few precipitation isotope samples in Norway before SNOWPACE, thousands of data points are now available from precipitation, snow, and water vapour over several years. Future validation studies can focus on the North Atlantic storm track utilizing SNOWPACE observations. We showed the utility of stable water isotopes on meteorological time scales as a constraint on processes of the hydrological cycle. From a complex combination of model tools we isolated the influence of moisture source and transport variations onto the Scandinavia, and showed the potential of combining model information with isotope measurements. The SNOWPACE project website will serve as the lasting platform to present all information from the project, including the available datasets. By developing a citizen science initiative, we could initiate conversations about the changing atmospheric water cycle with an interested public, which we intend to continue in the future by similar citizen science efforts.

Current climate and weather prediction models provide information to better prepare against precipitation extremes, and to manage hydropower resources, now and in a future climate. The atmospheric water cycles contribute largely to uncertainty in model simulations. Models continue to use established parameterization for processes such as evaporation and cloud microphysics while increasing to every higher resolution. This implies an urgent need for validating these model's water cycle with new and additional observations. In SNOWPACE we address this need through a bold approach, leading to significant scientific renewal: we propose to employ stable water isotope measurements to constrain atmospheric moisture transport from source to sink. This will be achieved from dedicated field sampling of evaporating sea water, water vapor from a network of stations in the North Atlantic region, and of snowfall and snow cores of the Norwegian winter snow pack. SNOWPACE uses the new national infrastructure FARLAB and a combination of sophisticated numerical modeling tools partly developed by the PI to provide new constraints of the atmospheric water cycle from these measurements. The project is closely tied into the international stable water isotope community. Exchange with other scientific communities and potential users will lead to the expansion of disciplinary knowledge. SNOWPACE provides an important and unique scientific innovation both for Norway and internationally. The new knowledge will pave the way to constrain processes and improve parameterizations in weather and climate models, and for the management of natural resources in a changing climate. Through clearly targeted dissemination of new knowledge, and embedding a partner from energy industry, we establish communication channels towards future applications of the fundamental knowledge gained in SNOWPACE. The legacy of the project will be an open-access data set of all measurements collected during the project.