Impacts of atmospheric river reconnaissance dropsondes on <scp>ECMWF</scp> Integrated Forecasting System precipitation forecasts

Abstract Atmospheric rivers (ARs) are responsible for much of the wintertime precipitation over the U.S. West Coast. Their associated precipitation can be beneficial, by replenishing water resources, and can be detrimental, by triggering flooding and landslides. Therefore, accurate forecasts of ARs are crucial for water management and hazard preparedness. To help bring about improvements in downstream precipitation forecasts in the western United States, the AR Reconnaissance (AR Recon) field campaign has been set up, which deploys research aircraft to sample essential upstream atmospheric structures using dropsondes and airborne instruments. This study examines the impacts of AR Recon dropsonde data on precipitation forecasts over the western United States in the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS), for the AR Recon field seasons of 2022/23 and 2023/24. Observing‐system experiments demonstrate that dropsonde observations bring moderate, yet statistically significant improvements at various lead times. Over California, the additional observations enhance precipitation forecasts at lead times of 24–36 hours and 72–96 hours, which is likely linked to aircraft operating from two bases, that is, the U.S. West Coast and Hawaii. For the Pacific Northwest region, dropsonde observations mainly improve forecast skill at lead times of 48–72 hours. The timing differences in improvements between the two regions in 2022/23 are associated with the southward propagation of error reductions in the large‐scale flow. The smaller dropsonde impacts in 2023/24 compared to 2022/23 can be partly explained by the long data assimilation window used in 2023/24. The dropsonde impacts in the Global Forecast System (GFS) are consistent with those in IFS, although the impacts are larger in the GFS. Despite the overall benefits of dropsonde assimilation, slight degradation is also observed at certain lead times, such as 60 hours over California for two seasons combined, warranting further investigation.