Atmospheric Rivers in the CESM: Validation, Connections to Extreme Precipitation, and Projections for the Future

Atmospheric Rivers (ARs) are long, narrow, intense, and evolving filamentary structures responsible for transporting significant amounts of moisture from the tropics to mid-latitudes and are often associated with extreme winter-time precipitation for regions such as the West Coast of the United States. The Community Earth System Model (CESM) captures these synoptic scale structures as well as their extreme precipitation. The CESM version employed utilizes high resolution atmosphere/land components (0.5^o) coupled to the standard (1^o) ocean/ice components. The high resolution atmosphere is able to more accurately represent extreme, regional precipitation.

Cataloguing AR events in long-term climate simulations where centuries of data are produced requires an automated AR-identification algorithm that captures the spatial and temporal structures of these events. These algorithms may vary in complexity but all necessitate the inclusion of water vapor content thresholds. The adoption of observationally based empirical threshold values may not translate well to future climate projections as increased temperatures change the background water vapor content (Clausius-Clapeyron relationship).

Here, we evaluate spatial and temporal characteristics of Western U.S. ARs in CESM and compare them to observations. A variety of threshold definitions will be discussed which span observationally based values of water vapor content to values relative to the background state (more appropriate for future climate). Model simulated extreme precipitation related to AR’s and results from the RCP8.5 future climate scenario will be shown.