Both the horizontal and vertical grid and coordinate systems used in the WRF-NMM are different from those traditionally employed in CMAQ. Because the difference of coordinate and grid structure between NAM and CMAQ, the initial linkage between the models was based on interpolation of meteorological data from the NAM's coordinate and grid structure to that employed by CMAQ; this configuration is hereafter referred to as loose coupling. To reduce errors associated with loose coupling, modifications were introduced in PREMAQ so that CMAQ governing equations and calculations employ the same vertical coordinate system, i.e. the hybrid sigma-P coordinate, as WRF-NMM. The updated system, referred to as tight coupling hereafter, provides more accurate representation of the 3-D meteorological fields and is expected to reduce mass inconsistency errors in chemistry/transport calculations. Also, since the WRF-NMM vertical top extents to 2mb and the number of vertical layers can differ from those used in the chemistry transport calculations in CMAQ, flexibility is built in to examine the impact of different layer structures.
This paper will review the formulation and implementation of the hybrid sigma-P vertical coordinate system in CMAQ. Diagnostic analysis of model results to examine mass-consistency in the new system will be discussed. Results from tests with different layer structures and their impact on pollutant distribution forecast will be presented. Finally, it will describe and explain reasons for differences in predicted surface ozone between the loose and tight coupling configurations.
Disclaimer: The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.