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). CPC US Unified Precipitation data were used. 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.
