Characterizing the size of Atmospheric Rivers using a perspective independent from the detection algorithm

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, 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 create a composite North American landfalling ARs with origin in the North Pacific Ocean in the 1980-2017 period, and we develop and implement five independent size estimation methods to characterize the size of ARs and reduce the large range of estimated sizes from the Atmospheric River Tracking Method Intercomparison Project (ARTMIP)algorithms. Moreover, we study the difference in size between ARs originating in theNorthwest Pacific (WP) (100 E-180 E) and those from the Northeast Pacific (EP)(180 E-240 E). Additionally, we analyze how the size and geometry (length, width, area, and angle of orientation) of ARs change through their life cycle.We found that WP ARs have larger sizes and are more zonally oriented than those from the EP. In general, ARs become smaller through their life cycle, mainly due to reductions in their width, and they also become more meridionally oriented towards the end of their life cycle. Overall, the size estimation methods proposed in this work provide a range of AR areas (between 7x1011m2 and 1013m2) that is several orders of magnitude narrower than the range of sizes reported by algorithms in the ARTMIP Tier 1 dataset. Finally, we apply these methodologies to simulations from the CMIP5 and CMIP6 multi-model ensembles to get objective insight about how AR geometry might change under different climate scenarios.