The contribution of subtropical moisture within an atmospheric river on moisture flux, cloud structure, and precipitation over the Salmon River Mountains of Idaho using moisture tracers

The impact of an atmospheric river (AR) on the flux of subtropical moisture across Idaho's Salmon River Mountains and precipitation over the mountains is evaluated using the Weather, Research, and Forecasting model with water vapor tracers (WRF-WVT). The AR impacted Idaho between 17 and 19 January 2017 during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) campaign. WRF-WVT is configured to isolate the subtropical moisture contribution to the AR, the moisture flux, and precipitation. Subtropical water vapor advected by the AR into Idaho is tagged and tracked in three-dimensional space throughout the run. This allows the contribution of the subtropical moisture to the vertical distribution of water vapor and the precipitation to be directly calculated. The simulated cloud structure is compared with airborne radar data collected during two SNOWIE intensive operation periods. This study found that more than 70% of the moisture flux and more than 80% of the precipitation across the Idaho Mountains during SNOWIE IOP 4 could be attributed to subtropical moisture within the AR. Nearly all of the moisture flux in the upper cloud and 50% of the moisture in the lower cloud was attributable to the subtropical moisture. The subtropical moisture contribution within the AR to precipitation ranged from 35% in northern Idaho to more than 90% in southern Idaho. Across the entire period of impact of the AR, more than 60% of precipitation in Idaho was attributable to the subtropical moisture within the AR, with this percentage increasing toward the south across the state.Plain Language Summary Atmospheric rivers (ARs) are streams of strong moisture transport that often form along and ahead of the cold front of extratropical cyclones. They often bring heavy precipitation to the U.S. west coast resulting in hazardous conditions and flooding. States like Idaho are dependent on ARs for snowpack enhancement to prevent water shortages and support the creation of hydroelectric power. ARs often draw moisture into the west coast from the tropics and subtropics. This analysis presents a case study showing how the moisture flux from subtropical latitudes associated with an AR varied over Idaho's Salmon River Mountains during the course of several days. This analysis traced water vapor that originated south of 35 degrees N within the AR. More than 60% of precipitation in Idaho during this event was directly attributable to subtropical moisture within the AR, with this percentage increasing toward the south across the state. This analysis also examines how subtropical moisture transport varied with depth within the troposphere over the Salmon River Mountains and how the moisture distribution impacted cloud structure, as measured by an airborne vertically pointing radar. This study contributes a better understanding of how Pacific ARs impact water resources over the mountains of Idaho.