UC Berkeley

Abstract This study investigates the factors influencing runoff response to atmospheric rivers (ARs) over the U.S. West Coast. We focused on runoff time series variations impacted by AR characteristics (e.g., category and frequency) and land preconditions during Northern Hemisphere cool seasons in the period of 1940–2023. Results show that high‐category ARs significantly increase local runoff with higher hourly precipitation rates leading to a greater incremental rate and peak runoff. Extreme runoff increases greatly with the AR category with an increase rate up to 12.5 times stronger than non‐extreme runoff. Besides the AR category, land preconditions such as soil moisture and snowpack also play crucial roles in modulating runoff response. We found that runoff induced by weak‐category ARs is more sensitive to land preconditions than high‐category ARs, with high peak runoff occurring when soil is nearly saturated. Additionally, more than 50% of high‐peak‐runoff events in snow‐covered grid cells are associated with rain‐on‐snow events particularly for the events associated with weaker ARs. Regression analysis reveals that AR precipitation and land preconditions jointly influence runoff, emphasizing the importance of including soil moisture and snowpack levels in AR impact assessments. The study also highlights the intensified runoff response to back‐to‐back ARs with short intervals, which may become more frequent with climate warming, posing increased flood risks via facilitating wet soil conditions. Our findings have significant implications for AR risk predictions and the development of prediction models for AR‐induced runoff. Plain Language Summary Atmospheric rivers are narrow bands of concentrated moisture in the atmosphere that can bring heavy rain when they reach land. Although it is known that stronger ARs carry more moisture and could lead to higher flood risk, the specific factors that influence these effects are not fully understood. For example, is a single strong AR as capable of producing flooding as a tightly sequenced set of less intense ARs, and how do antecedent land surface conditions (e.g., soil moisture and snowpack) attenuate or accentuate flood risk? This research investigates the relationship between AR characteristics, namely their intensity and landfall coupling frequency and the resulting runoff response on land at different AR landfall interval times and antecedent land surface conditions. We showed that weaker ARs can also cause significant runoff if they occur in conditions where the soil is already wet or the snow is ready to melt. Additionally, when ARs occur close together in time, the runoff from the second AR is often much higher because the land has not had enough time to release the water brought by the first one. This “back‐to‐back” effect is expected to become more common as the climate warms, potentially increasing flood risks. Key Points High‐category atmospheric rivers (ARs) and land preconditions significantly influence runoff response Weaker ARs can cause extreme runoff given saturated soil or high snowmelt Back‐to‐back ARs intensify runoff response and may increase flood risks