Atmospheric Rivers in Regional Climate Models for the Western U.S

Atmospheric Rivers (ARs) serve as both a crucial water resource and the cause of extreme flooding in the Western U.S. However, because of the fine-scale nature of these features, the impacts of ARs are not well-captured in large-scale future global climate simulations. Thus, NARCCAP datasets (50 km grid size) and finer-scale DRI-Regional Climate Modeling simulations (36 and 12 km) were used to better characterize the impact of ARs and to analyze current and future impacts of ARs in the Western U.S. First, an AR climatology has been constructed using an algorithm that is based on those parameters that capture the most defining characteristics of the AR (i.e. northeastern Pacific upper-level moisture flux maximum) and are readily available from the forcing dataset; RCM modeling output composites are later constructed based on this objective classification algorithm. Secondly, a model sensitivity study was performed to determine if grid geographical boundaries, specifically the western edge, impact simulation of fine-scale AR features in atmospheric models. This sensitivity study also evaluated how environmental features, such as the ITCZ and extratropical cyclones, interact with ARs, and to what extent AR events features are adequately transferred from GCM lateral boundaries to RCM downscaled simulations. Our results show the impact that defining the RCM's geographical boundaries has on AR event impacts.