GPM Satellite Radar Observations of Precipitation Mechanisms in Atmospheric Rivers over the Northeast Pacific Ocean

Atmospheric rivers (AR) account for more than 90% of the total meridional water vapor flux in mid-latitudes, and 25-50% of the annual precipitation in the coastal western United States. Despite numerous studies identifying the importance of moisture transport in atmospheric rivers, the lifecycle of water vapor transport is difficult to investigate given the challenges of observing the frequency, location and intensity of precipitation and precipitation processes within ARs over the ocean. In this study, satellite radar reflectivity profiles from the Global Precipitation Measurement Dual-Frequency Precipitation Radar (GPM-DPR) are used to evaluate the precipitation characteristics of ARs over the northeast Pacific Ocean, as identified by integrated vapor transport (IVT) in reanalyses. Transects of 192 ARs with GPM-DPR-measured precipitation, occurring between 2014 and 2018, exhibited both abundant stratiform and convective precipitation. The primary mechanism that generated precipitation across these AR transects upstream of the warm-conveyor belt was forced ascent over the cold front. Additionally, despite low convectively available potential energy, convective precipitation was frequently observed in the vicinity of the moist-neutral low-level jet. The ratio of stratiform to convective precipitation in GPM-DPR observations of ARs was 75% to 25%, with stratiform precipitation exhibiting a characteristic peak on the cold-sector side of the IVT maximum and convective precipitation occurring more frequently in the warm sector. These results highlight the importance of observing precipitation to evaluating AR moisture transport, and demonstrate the utility of satellite radar observations in future investigations of water vapor budgets over the global oceans.