Examining Dynamically Downscaled Extreme Events Using 12-km WRF Simulations

Continued improvements in the speed and availability of computational resources have allowed dynamical downscaling of global climate model (GCM) projections to be conducted at increasingly finer grid scales and over extended time periods. The implementation of dynamical downscaling with increased resolution has the potential to better simulate extreme events, as smaller mesoscale and topographic features can be resolved by a finer grid. However, use of finer grid spacing must be investigated to assess whether the increased resolution and its associated computational demands are justified by adding value to simulations of regional climate conditions.

Research within EPA’s National Exposure Research Laboratory has been conducted for more than eight years to develop and refine a methodology to dynamically downscale the Weather Research and Forecasting (WRF) model at 36-km grid spacing. Those studies included a focus on simulating extreme events, methods of constraining WRF to the GCM, improving the representation of convective cloud cover, and improving the representation of inland water bodies in downscaled simulations. That research led to several dynamically downscaled climate change simulations that were used for air quality and human health studies.

In this study, dynamical downscaling using the WRF model is extended to a 12-km grid covering the contiguous U.S. The 12-km simulations are updated to use a current version of WRF and several scientific advances that have been made within the WRF model. Sensitivity to the use of mosaic land use information with the Noah land surface model is investigated, as is the sensitivity to the use of an alternative planetary boundary layer scheme. A global reanalysis dataset, the 0.75° × 0.75° ERA Interim, is used as a proxy GCM for evaluation of historical WRF simulations conducted with a 36- and 12-km nested grid configuration over a 1-year period. The added value of the 12-km grid will be assessed based upon whether its results are more accurate than those of the coarser parent grid. The ability of the simulations to accurately simulate extremes in minimum and maximum daily 2-m temperatures, as well as heavy rainfall events, will be investigated.