Regional climate model downscaling of the U.S. climate and change

A state-of-the-art integrated modeling system has been developed to provide credible information at regional to local scales on climate and air quality, including their variability, change and impact. It consists of a regional climate model (RCM), an emissions model, and an air quality model and is driven by general circulation model (GCM) simulations of the present and future climates. The RCM is the climate extension of the MM5 and uses two cumulus parameterizations that produce distinctly different downscaling skills with strong regional dependence. Simulations from two GCMs under the IPCC SRES A1Fi, B1, A2, B2 emissions scenarios are used to drive the regional modeling system that produce downscaling of the U.S. present climate, future change and their impacts on air quality as well as the uncertainty of the results. It is demonstrated that the RCM, with its finer resolution and more comprehensive physics, simulates the present U.S. climate more accurately than the driving GCMs, especially for precipitation and surface temperature, including summer means, diurnal cycles and daily frequency distributions. Consequently, the RCM generates very different patterns of the U.S. climate change projection than the driving GCMs. This study suggests that the RCM downscaling can significantly reduce GCM biases in simulating the present climate and this improvement has important consequences on the future projection of regional climate changes. The study will also quantify and understand the individual and combined impacts of global climate and emission changes on U.S. air quality, from the present to 2050 and 2100.