Unravelling moisture sources for precipitation in atmospheric rivers

Atmospheric rivers (ARs) are filaments of enhanced moisture in the atmosphere, typically located in subtropical and mid-latitudes oceanic regions. These structures are of extreme importance for the climate of the western coasts, as they provide a large fraction of the annual precipitation recorded in these areas. In addition, ARs can eventually cause extreme rainfall when the enormous amounts of water vapor they carry are abruptly forced upwards. As a result, much research has been done on ARs in recent years, and one of their most studied and debated properties is the origin of the moisture in them. Although some studies have identified sources using different moisture transport models for specific AR cases, it remains unclear whether tropical or extratropical contributions are generally more prevalent, and even the AR definition in the Glossary of Meteorology reflects this lack of consensus.

To fill this gap, a climatology of moisture sources for precipitation in ARs is needed. There are a variety of moisture transport models that can be employed to address this issue. However, they tend to produce considerably different results, and the lack of observations hampers any validation. Here we use the WRF with Water Vapor Tracers (WRF-WVTs) model as a proxy for reality to validate a Lagrangian moisture transport technique based on the FLEXPART model for selected cases. This allows us to minimize the discrepancies with the WRF-WVTs model, assumed to be the most accurate moisture tracking method, while efficiently simulating air particle trajectories within a wider range of ARs with a Lagrangian tool.

Preliminary results reveal a wide diversity of moisture sources, including both oceanic and continental regions, with substantial variability in their contributions across different AR cases. Importantly, our findings also indicate a less relevant role of tropical moisture than previously known. Ultimately, this highlights the complexity of the moisture uptake process in ARs.