Upstream Evolution of Landfalling Atmospheric Rivers along the U.S. West Coast

Atmospheric Rivers (ARs) are responsible for a majority of global poleward moisture transport and have been linked to extreme precipitation events and flooding along the U.S. West Coast. ARs are long (>2000 km) and narrow (500–1000km) corridors of high values of vertically integrated water vapor (IWV) and integrated water vapor transport (IVT) that are typically found in the pre-cold frontal region of extratropical cyclones. Observational evidence suggests that ARs primarily transport IWV “directly” from the tropics, but may also “indirectly” transport IWV from midlatitude and subtropical locations. This study identifies “direct tap” and “indirect tap” landfalling ARs along the U.S. West Coast using 10 years of IVT magnitude and vector analyses derived from the NCEP Climate Forecast System Reanalysis (CFSR) and Global Forecast System (GFS) model analyses. Subsets of landfalling AR along the U.S. West Coast can be created by investigating the IVT vector magnitude and orientation during periods of heavy precipitation derived from the NCEP Stage-IV quantitative precipitation estimates. The upstream dynamical evolution of each subset of ARs will be shown via the terms in the water vapor budget [i.e., IWV tendency (IWVT)]. Preliminary results indicate that the “indirect tap” events are associated with water vapor source regions at higher latitudes and are maintained in westerly flow with higher evaporation rates, whereas “direct tap” events are associated with water vapor source regions at lower latitudes and are maintained in southwesterly flow with larger precipitation and IVT-convergence rates.