Extreme Landfalling Atmospheric River Events in Arizona: Possible Future Changes

The semiarid Salt and Verde River Basins in Arizona are susceptible to atmospheric river (AR)-related flooding. To understand the precipitation-related impacts of climate change on extreme ARs affecting Arizona, a pseudo-global warming method was used. High-resolution control and future simulations of five intense historical AR events that affected the Salt and Verde River Basins in Central Arizona were carried out using the Weather Research and Forecasting regional climate model. The pseudo-global warming approach for future simulations involved adding a temperature delta at different vertical levels to the historical initial and lateral boundary conditions of the input data while keeping constant relative humidity. The deltas were calculated using projected changes toward end of the 21st century from an ensemble of nine Global Climate Models for the Representative Concentration Pathway (RCP) 8.5. Future simulations showed an overall increase in vertically integrated transport of vapor and upward moisture flux at cloud base over the region for all events. The changes in precipitation at both domain and basin levels were highly spatially heterogeneous. Precipitation increased in all future simulations; but in general, this increase remained less than the increase in column-integrated water vapor. It was found that in most cases, cloud ice content decreased while cloud water content increased, indicating the increased role of warm-rain processes in producing precipitation in the future simulations. Freezing levels rose by more than 600 m, and this along with increased temperature and greater role of warm-rain processes led to a decrease of more than 80% in the amount of frozen precipitation during the events.