Potential Influence of Climate Change on Atmospheric Rivers Induced Heavy Precipitation and Flooding in the Western US

The western U.S. receives precipitation predominantly during the cold season when storms approach from the Pacific Ocean. Several studies in recent years have clarified the role of atmospheric rivers (ARs) in producing heavy precipitation and floods in the mountainous regions of the West. Atmospheric rivers are narrow bands of enhanced water vapor associated with the warm sector of extratropical cyclones over the Pacific and Atlantic oceans. Because of the strong winds and neutral stability, atmospheric rivers often lead to heavy precipitation due to large orographic enhancement during landfall on the U.S. west coast. This study aims to investigate how global warming may affect the frequency and water vapor fluxes of atmospheric rivers and the potential impacts on heavy precipitation and flooding in the western US.

As part of the North American Regional Climate Change Assessment Program (NARCCAP), simulations have been performed using the Weather Research and Forecasting (WRF) model for North America at 50 km horizontal resolution. Three sets of simulations, one driven by large-scale conditions from the NCEP/DOE global reanalysis for 1980-1999, and two driven by the Community Climate System Model (CCSM) for 1970 - 2000 and 2040 - 2070 have been performed for North America at 50 km grid resolution. Analysis of the large-scale environments simulated by CCSM for the current and future climate suggests a 27% increase in frequency of atmospheric rivers that make landfall in the US west coast. In addition, the atmospheric rivers carry 7 – 12% more atmospheric moisture in the future climate. Analysis, focusing on both the atmospheric and land surface influence on floods, is being performed using the downscaled simulations to assess the impacts of climate change on heavy precipitation and flooding in the western US.