Persistent landfalling atmospheric rivers over the west coast of North America

• Published in: Journal of geophysical research. Atmospheres 2016-11, Vol.121 (22), p.13,287-13,300
• Main Authors: Payne, Ashley E., Magnusdottir, Gudrun
• Format: Article
• Language: English
• Subjects: Analogs; Anomalies; Atmosphere; Atmospheric precipitations; atmospheric rivers; Atmospherics; Climatology; Coastal environments; Contours; Correlation; Datasets; Drying; Duration; Dynamic height; Dynamics; Extreme weather; Flooding; Geophysics; Geopotential; Geopotential height; Heavy precipitation; Height anomalies; Hourly; Hydrology; Identification; Moisture content; moisture transport; persistent events; Planetary waves; Potential vorticity; Precipitation; Rivers; Rossby wave breaking; Similarity; Transport; Troposphere; Vorticity; Wave breaking; west coast
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/2016JD025549

Landfalling atmospheric rivers (ARs) are linked to heavy precipitation and extreme flooding, and are well known along the western coast of North America. The hydrological impacts of ARs upon landfall are correlated with their duration and magnitude. In order to improve the forecast of these hydrologically significant landfalling events, a better understanding of how they differ from other landfalling events must be established through an investigation of the mechanisms leading to their development prior to landfall. A subset of persistent landfalling AR events between 30°N and 50°N is identified in 3‐hourly Modern‐Era Retrospective Analysis for Research and Applications reanalysis and validated against existing data sets. These events are identified as features in the low troposphere with high moisture transport and extended geometry that persist over a limited region of the coastline for longer than 63 h (85th percentile of AR duration). A composite analysis shows that persistent events have distinct thermodynamical and dynamical characteristics compared to all AR events. They are characterized by greater moisture content, suggestive of Pineapple Express‐type events, a perturbed upper level jet and anticyclonic overturning of potential vorticity contours associated with anticyclonic Rossby wave breaking. Moreover, the location of the Rossby wave breaking is shifted inland compared to all AR events. Analogue analysis of the 500 hPa geopotential height anomalies is used to find nonpersistent events with similar dynamical characteristics to persistent events. Despite their similarity to persistent events, nonpersistent analogues show very little shift toward longer duration. A comparison of the development of persistent and nonpersistent analogues shows that persistent events have much greater moisture content. Key Points Landfalling AR duration along the western U.S. coastline is quantified Spatially and temporally persistent ARs have distinct dynamical and thermodynamical characteristics from AR climatology Moisture content rather than dry dynamics appear to be the main contributor to AR persistence