UC Berkeley

Abstract Atmospheric rivers (AR) are large and narrow filaments of poleward horizontal water vapor transport. Because of its direct relationship with horizontal vapor transport, extreme precipitation, and overall AR impacts over land, the AR size is an important characteristic that needs to be better understood. Current AR detection and tracking algorithms have resulted in large uncertainty in estimating AR sizes, with areas varying over several orders of magnitude among different detection methods. We develop and implement five independent size estimation methods to characterize the size of ARs that make landfall over the west coast of North America in the 1980–2017 period and reduce the range of size estimation from ARTMIP. ARs that originate in the Northwest Pacific (WP) (100°−180°E) have larger sizes and are more zonally oriented than those from the Northeast Pacific (EP) (180°−240°E). ARs become smaller through their life cycle, mainly due to reductions in their width. They also become more meridionally oriented toward the end of their life cycle. Overall, the size estimation methods proposed in this study provide a range of AR areas (between 7 × 10 11 and 10 13  m 2 ), that is, several orders of magnitude narrower than current methods estimation. This methodology can provide statistical constraints in size and geometry for the AR detection and tracking algorithms, and an objective insight for future studies about AR size changes under different climate scenarios. Plain Language Summary Atmospheric rivers (AR) are a meteorological phenomenon with strong poleward water vapor transport. ARs have positive and negative impacts over the regions where they make landfall, particularly for West Coast US water resources. AR size (length, width, and area) is an important characteristic that needs to be studied, and could directly relate to the impacts of AR over land. There are large differences in size estimation between current detection methods, with areas varying over several orders of magnitude. Our study focuses on characterizing AR size using five different methods independent of the AR detection algorithm. We find that North American landfalling ARs originated in the North Pacific have areas between 7 × 10 11 and 10 13  m 2 (between 1 and 11 times the area of California), and their lengths are on average four times their widths. ARs originating from the Northwest Pacific (WP) (100°−180°E) are bigger and more parallel to the equator than those from the Northeast Pacific (EP) (180°−240°E). Our methods provide a narrower range of size estimation than the current methods and could be used to constrain current and future AR detection methods, and it could be used in future studies to understand how AR size could vary under climate change scenarios. Key Points Multiple independent estimates show atmospheric rivers areas are between 7 × 10 11 and 10 13  m 2 , with lower sizes range than ARTMIP estimates Landfalling atmospheric rivers from the northwest Pacific are larger and more zonally oriented than those from the northeast Pacific In general, atmospheric rivers tend to decrease their size and become more meridionally oriented through their life cycles