Dynamical Downscaling of NASA/GISS ModelE: Continuous, Multi-Year WRF Runs

The WRF Model is being used at the U.S. EPA for dynamical downscaling of the NASA/GISS ModelE fields to assess regional impacts of climate change in the United States. The ModelE fields were included in the IPCC Fourth Assessment Report, and updated science in the improved ModelE will contribute toward the IPCC Fifth Assessment Report. The dynamically downscaled climate fields from WRF ultimately will be used to predict the regional impacts of climate change on air quality and other regional environmental concerns. The WRF model has been successfully linked to the ModelE fields in their raw hybrid vertical coordinate, and continuous, multi-year WRF downscaling simulations have been performed.

The use of nudging for downscaled regional climate simulations has been somewhat controversial over the past several years but has been recently attracting attention. Several recent studies that have used reanalysis (i.e., verifiable) fields as a proxy for GCM input have shown that nudging can be beneficial toward achieving the desired downscaled fields. In this study, the value of nudging will be shown using fields from ModelE that are downscaled using WRF. Several different methods of nudging are explored, and it will be shown that the method of nudging and the choices made with respect to how nudging is used in WRF are extremely critical to balance the constraint of ModelE against the freedom of WRF to develop its own fields.