Moisture Sources Throughout the Life Cycle of an Atmospheric River: Storm Dennis Case Study

Abstract Moisture transport within atmospheric rivers is driven by a complex combination of processes, including the convergence of moisture from different sources, which change over the atmospheric river's life cycle. The water vapor budget (WVB) within an atmospheric river enables us to understand moisture sources and sinks (horizontal flux, evaporation, and precipitation). Here, we applied our new WVB approach throughout the life cycle of the exceptional atmospheric river associated with Storm Dennis, which led to record‐breaking precipitation on 15 February 2020. We used the WRF model to simulate the event and performed two sets of sensitivity experiments: one reducing tropical moisture and the other modifying ocean evaporation to assess how these two main moisture sources affect the water vapor balance within the atmospheric river. We analyzed changes in the atmospheric river, cyclone, and associated precipitation at landfall in the sensitivity experiments. In the Dennis case study, tropical moisture played a prominent role in the early stages of the atmospheric river, whereas ocean evaporation became critical later. Additionally, the reduction of evaporation and also of tropical moisture is related to a decrease in precipitation over Europe. This study offers a new approach to understanding the evolution of atmospheric rivers and highlights the importance of different moisture processes. It provides a case study that helps unravel feedback mechanisms and the impact of different perturbations on the water vapor balance of atmospheric rivers.