Extreme landfalling atmospheric river events in Arizona: Possible future changes

Authors: SINGH, I. - University Of Illinois; DOMINGUEZ, F. - University Of Illinois; Demaria, Eleonora; WALTER, J. - Salt River Project

Submitted to: Journal of Geophysical Research Atmospheres
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2018
Publication Date: 7/17/2018
Citation: Singh, I., Dominguez, F., Demaria, E.M., Walter, J. 2018. Extreme landfalling atmospheric river events in Arizona: Possible future changes. Journal of Geophysical Research Atmospheres. 123:7076-7097. https://doi.org/10.1029/2017JD027866.
DOI: https://doi.org/10.1029/2017JD027866

Interpretive Summary: Within the Salt and Verde River basins in the semiarid northeastern Arizona, natural ecosystems and humans compete for over-allocated water resources. The basins are highly dependent on winter precipitation for ecosystem functioning, water supply, fire suppression, and in the case of extreme events, for water quality, flood control and dam safety. Atmospheric Rivers (ARs) which are narrow corridors of concentrated water vapor that bring copious amount of rainfall from the Pacific into the region and contribute to its hydrology by increasing snow pack and replenishing soil moisture in the basins. ARs are also responsible for extreme precipitation and streamflow events. We use a Pseudo-Global Warming (PGW) method to simulate future temperature scenarios in the basins. The PGW adds temperature deltas (differences between historical and future climate model projections) to historical meteorological data which is used as an input to a regional climate model which has a higher spatial resolution. Our results show increases in winter precipitation and a rise on the melting levels (elevations in the basin with solid precipitation) by more than 600 meters which suggests changes in precipitation from solid to liquid in the future.

Technical Abstract: The semi-arid 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 (PGW) 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 WRF regional climate model. The PGW 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 from an ensemble of nine General Circulation Models for the Respective Concentration Pathway 8.5 (RCP8.5) scenario. Future simulations showed an overall increase in integrated vertical 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 level were highly spatially heterogeneous. Precipitation increased in all future simulations, but the increase remained sub-Clausius-Clapeyron. Melting levels rose by more than 600m in all future simulations and this led to a decrease of more than 80% in the fraction of frozen precipitation falling during the events.