Atmospheric rivers (ARs) affect surface hydrometeorology in the US West; Coast and Midwest. We systematically sought optimal AR indices for; expressing surface precipitation impacts within the Atmospheric River; Tracking Method Intercomparison Project (ARTMIP) framework. We adopted a; multifactorial approach. Four factors—moisture fields, climatological; thresholds, shape criteria, and temporal thresholds—collectively; generated 81 West Coast AR indices and 81 Midwest indices from January; 1980 to June 2017. Two moisture fields were extracted from the MERRA-2; data for ARTMIP: integrated water vapor transport (IVT) and integrated; water vapor (IWV). Metrics for precipitation effects included two-way; summary statistics relating the concurrence of AR and that of; precipitation, per-event averaged precipitation rate, and per-event; precipitation accumulation. We found that an optimal AR index for; precipitation depends on the types of impact to be addressed, associated; physical mechanisms in the affected regions, timing, and duration. In; West Coast and Midwest, IWV-based AR indices identified the most; abundant AR event time steps, most accurately associated AR to days with; precipitation, and represented the presence of precipitation the best.; With a lower climatological threshold, they detected the most; accumulated precipitation with the longest event duration. Longer; duration thresholds also led to higher accumulated precipitation,; holding other factors constant. IWV-based indices are the overall choice; for Midwest ARs under varying seasonal precipitation drivers. IVT-based; indices suitably capture the accumulation of intense orographic; precipitation on the West Coast. Indices combining IVT and IWV identify; the fewest, shortest, but most intense AR precipitation episodes.
