Classification of atmospheric river events on the U.S. west coast using a trajectory model

Abstract:

We investigate transport pathways of water vapor associated with landfalling atmospheric river (AR) events that result in precipitation along the west coast of the U.S. for winter of 1997–2010. The water vapor transport pathways are determined by computing back-trajectories with a trajectory model using the MERRA reanalysis dataset. The majority of AR events (about 86%) over the west coast of the U.S. are associated with three trajectory types. We designate the first type as Ascending near landfall and of Tropical Origin (AT), the second type as Ascending near landfall and of Extratropical Origin (AE), and the third type as Descending or parallel near landfall and of Extratropical Origin (DE). The magnitude and spatial distribution of precipitation of a given AR event are found to be strongly determined by the mixture of its type of trajectories. AR events composed of both AT and AE trajectories have more frequent and intense precipitation over a broad region of the western U.S. (particularly in the northern California) than AR events dominated by a single trajectory type (AT) or other combinations (e.g. AT and DE). In addition, different patterns of trajectory types among AR events are closely linked to the location and the shape of potential vorticity intrusions near the tropopause, which emphasizes that lower tropospheric air is interacting with upper tropospheric air during the developing rainfall system.