CS24D - Concurrent Session 24D: Dynamical Downscaling to Quantify Changes in Extreme Precipitation Events Over the Western United States in Historical and Future Climate Scenarios

The spatial distribution of rainfall is crucially important to Western United States water managers due to the strongly heterogeneous nature of precipitation events and topography in this region. In the Western United States, the changing climate is expected to impact flood frequency via changes in the timing and magnitude of extreme rainfall, which can change the hydrologic response. In fact, some regions of CONUS have already observed an increase in the intensity of extreme precipitation events with decreases in light or moderate events. These changes in precipitation are expected to result in changes in flood frequency, which decisionmakers use (among other information) to inform flood risk potential. High resolution atmospheric modeling is required to accurately simulate these extreme precipitation events to provide hydrologic models with the high spatiotemporal resolution forcing data for flood risk applications.

To that end, the National Center for Atmospheric Research (NCAR) and the Bureau of Reclamation Dam Safety Office (DSO) have embarked on a project to evaluate downstream effects of future climate extremes through dynamical downscaling of extreme precipitation events over two mountainous case study regions in the Western United States. First, the Community Earth System Model Large Ensemble (LENS2) is used to identify large scale features of historical and future precipitation extremes over various accumulation thresholds (1- day, 3-day, 7-day, 15-day). Then ten ensemble members of the LENS2 for the historical and SSP3-7.0 scenarios provide six-hourly forcing data at 1-degree spatial resolution. The Weather Research and Forecasting model (WRF) is then used to downscale this coarse data to 4-km, hourly resolution over the two test basins for hundreds to thousands of events, allowing for statistical evaluation of many precipitation events to more reliably quantify precipitation changes due to a changing climate. Intensity-Duration-Frequency (IDF) curves and event sequencing are developed and assessed for extreme precipitation events from historical to future climate scenarios, providing water managers crucial information for identifying facilities that might face increasing flood risk in the coming decades.