In Search of the Optimal Atmospheric River Index for US Precipitation: A Multifactorial Analysis

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.

Automated summary

The study systematically explored the impact of atmospheric rivers on surface hydrometeorology in the US West Coast and Midwest. IWV-based AR indices proved to be the optimal choice for addressing seasonal precipitation drivers in the Midwest, while IVT-based indices were effective in capturing intense orographic precipitation on the West Coast. Combined indices of IVT and IWV identified the most intense AR precipitation episodes.