A Scale to Characterize the Strength and Impacts of Atmospheric Rivers

Atmospheric Rivers (ARs) are often identified using integrated water vapor transport (IVT) ≥250 kg m^–1 s^–1 as a key threshold (Rutz et al. 2014, Cordeira et al. 2017). However, the range of AR-related IVT intensity along the U.S. West Coast extends far above this magnitude, with most locales having observed IVT ≥1250 kg m^–1 s^–1 at least once since 1980. These rare, but extreme values of IVT highlight the need for differentiation between ARs that barely clear a 250 kg m^–1 s^–1 threshold, and those featuring IVT magnitudes five times greater.

Furthermore, AR-related impacts are not only a function of IVT magnitude, but also the duration of AR conditions (Ralph et al. 2013), which must also be considered in differentiating the impacts associated with ARs. For this reason, AR events are divided arbitrarily into three categories on the basis of their duration: <24, 24–48, and >48 h. If the duration of AR conditions is < 24 h, the AR-related hazards are expected to be less severe. If the duration of AR conditions is > 48 h, the AR-related hazards are expected to be more severe.

Therefore, an AR Scale has been developed to categorize AR events based on the maximum instantaneous IVT and the duration of the event (i.e., the duration of IVT ≥250 kg m^–1 s^–1) at a given point. The AR Scale is intuitive, with 5 AR categories (AR Cats 1–5) arising as a function of intensity and duration. These categories provide a wide range of users with a baseline for gauging the potential impacts, both beneficial and hazardous, associated with an AR event at their location. This presentation will focus on describing the AR Scale, use and interpretation of this scale, and the spatiotemporal distribution of AR Cat 1-5 events during the cool season (October – April) during 1980–2017.