Downscale linkage of global model output for regional chemical transport modeling: Method and general performance

Abstract

The goal of this study is to assess the impact of the downscale linkage of global chemistry model output on the simulated regional scale O-3 concentration and its vertical and horizontal structure over the continental US. For the global chemistry model, the NASA LaRC-University of Wisconsin Real-time Air Quality Modeling System (RAQMS) with a global ozone assimilation framework was used. RAQMS simulation results were downscaled with a RAQMS-CMAQ linkage tool (RAQ2CMAQ) to provide dynamic lateral boundary conditions for regional air quality simulations with the EPA Models-3 Community Multiscale Air Quality (CMAQ) Model. We performed two sets of CMAQ simulations; (1) with the predefined lateral boundary condition profile representing relatively clean air conditions in the eastern-half of the United States, and (2) with the lateral boundary conditions generated from RAQMS 6 hourly analyses, for the 1999 Southern Oxidant Study (SOS-99) period (15 June-14 July 1999). When compared and evaluated with available ozone soundings and nation wide surface observations (AIRS/EPA) in U. S., CMAQ simulations with the lateral boundary condition from the predefined profile failed to simulate the so-called "chemical tropopause'' above which O-3 concentration rapidly increases with height. On the other hand, the simulations with the lateral boundary condition provided with the global RAQMS output not only improved diurnal variations and daily maxima of surface O-3 concentrations, but also produced better agreement with the vertical structure of O-3 measured in the middle and upper troposphere. The tracer mode CMAQ simulations showed that, at the upper troposphere, more than 85% of the O-3 concentration difference associated with the lateral BCs was caused by the transport and diffusion processes.