Unravelling the sources of moisture for precipitation in atmospheric rivers

Atmospheric rivers (ARs) are filaments of enhanced moisture in the atmosphere, which often produce intense or even extreme precipitation when the enormous amounts of water vapor in them are forced upwards. In this sense, one of their most studied and debated properties is the origin of the moisture they transport. Although some studies have identified sources using different diagnostic tools 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 source diagnostics that can be employed to address this issue. Here we use the Lagrangian model FLEXPART together with an implementation of the Dirmeyer and Brubaker, (1999) methodology, which we previously validated using the WRF with Water Vapor Tracers (WRF-WVTs) model. This allows us to efficiently simulate air particle trajectories and compute moisture sources for precipitation within a wide range of ARs with a Lagrangian methodology, while maintaining consistency with the WRF-WVTs model, assumed to be one of the most accurate moisture tracking tools. 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 uptakes in ARs.

Dirmeyer, P. A. and Brubaker, K. L.: Contrasting evaporative moisture sources during the drought of 1988 and the flood of 1993, J. Geophys. Res. Atmospheres, 104, 19383–19397, https://doi.org/10.1029/1999JD900222, 1999.