Downscaling Hydroclimate Change Over Western US Based on CAM Subgrid Scheme and WRF Regional Climate Simulations

Global and regional climate simulations have been performed to compare two dynamical downscaling methods for simulating orographic effects and projecting the hydrologic impacts of climate change in the western U.S. The first approach applies a subgrid parameterization in a global climate model (the Community Atmosphere Model: CAM3) to simulate orographic effects. The second approach uses a regional climate model (Weather Research and Forecasting: WRF) to explicitly resolve the effects of orography on clouds and precipitation. Two 10-year simulations were completed for the present (1993-2003) and future (2039-2049) with CAM3 applied at 1x1.25 degree spatial resolution with the subgrid orographic precipitation scheme. Downscaling was performed using WRF driven by the CAM3 simulation for the two 10-year periods at 15 km spatial resolution for the western US. Precipitation, temperature, runoff, and snowpack simulated by CAM3 and WRF for 1993-2003 were evaluated using observations. We also compared the surface water budgets changes as well as extreme precipitation and runoff changes between 2039-2049 and 1993-2003. The large scale spatial distributions of precipitation changes are generally consistent between the WRF and CAM3 simulations. However, the WRF simulation indicates larger changes of precipitation along the coastal mountains (Cascades in the Northwest and Sierra Nevada in California) than CAM3. As the WRF model explicitly simulates the interactions of regional atmospheric circulation and the underlying topography, changes in winds in the future climate can lead to larger changes in orographic precipitation than that caused by changes in atmospheric moisture and temperature alone. The 95th percentile precipitation change is 3-5 times larger than the mean precipitation change in the winter. With the annual mean temperature increase of 0.5-2°C over western US, snowpack is significantly reduced by 40-60% over the mountain areas in the Pacific Northwest. Driven by the combined effects of precipitation change and snowmelt change due to warmer temperature, the runoff change shows a complicated spatial and seasonal variability. Our simulation experiments suggest significant impacts of greenhouse warming on mountain precipitation, snowpack and runoff that influence water resources. This study has also identified weaknesses in both downscaling methods and directions for future improvements.