The role of Dry Intrusions in the formation and intensification of Atmospheric Rivers in the North Atlantic

Extra-tropical storms over the North Atlantic often leads to socio-economic impacts over Western Europe, associated with strong winds and precipitation. Such storms can be associated with so-called Atmospheric Rivers (ARs) which are relatively narrow regions of concentrated water vapor (WV) and strong winds where intense horizontal moisture transport can take place. In turn, the moisture availability along the cyclone path and the ARs lifetime can be impacted by boundary layer processes. For example, the occurrence of Dry Intrusions (DI) associated with previous cyclones can strongly destabilize the planetary boundary layer (PBL) leading to enhanced moisture uptake over the ocean. This can support the formation and/or intensification of the ARs themselves.

The objective of this study is to understand the influence of DI on the moisture uptake in the PBL and transport associated with ARs impacting France.

With this aim, an adapted version of the detection algorithm developed by Ramos et al. (2015), was applied to ERA-5 reanalysis targeting events impacting the Atlantic coast of France. A total of 300 AR-events were detected over the extended winter (ONDJFM) spanning the years 1979 to 2023. Indeed, the most intense landfalling ARs are associated with intense precipitation and high wind speeds over western France.

For a subset of these AR-events, occurring between 1992 and 2022, the Lagrangian FLEXPART model using ERA5-data was applied to calculate the moisture sources for these events. This approach allows for the tracking of air masses 10 days backward in time from the target region (5°W to 0.5°E and 43.75°N to 50°N).

Additionally, the occurrence of DI outflows (from 1979 onward) was based on its Lagrangian detection in ERA5 to assign possible DI outflows overlapping with the source regions of moisture uptake.

Our results suggest a relationship between the areas of DIs and moisture uptake, indicating the possibility of the DI exerting influence on the formation and intensification of ARs. Overall, this work serves as a preliminary investigation for the upcoming North Atlantic Waveguide, Dry Intrusion, and Downstream Impact Campaign (NAWDIC) recently endorsed by the World Weather Research Programme (WWRP).