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LAYER DEPENDENT ADVECTION IN CMAQ
Citation:
YOUNG, J. O. AND J. PLEIM. LAYER DEPENDENT ADVECTION IN CMAQ. Presented at 4th Annual CMAS Models-3 User's Conference, Chapel Hill, NC, NC, September 26 - 28, 2005.
Impact/Purpose:
The objectives of this task are to develop, improve, and evaluate EPA's Community Multiscale Air Quality (CMAQ) model, as an air quality management and NAAQS implementation tool. CMAQ is a multiscale and multi-pollutant chemistry-transport model (CTM) that includes the necessary critical science process modules for atmospheric transport, deposition, cloud mixing, emissions, gas- and aqueous-phase chemical transformation processes, and aerosol dynamics and chemistry. To achieve the advances in CMAQ, research will be conducted to develop and test appropriate chemical and physical mechanisms, improve the accuracy of emissions and dry deposition algorithms, and to develop and improve state-of-the-science meteorology models and contributing process parameterizations.
The model will be tested and evaluated to thoroughly characterize the performance of the emissions, meteorological and chemical/transport modeling components of the CMAQ system, with an emphasis on the chemical/transport model, CMAQ. Emissions-based models are composed of highly complex scientific hypotheses concerning natural processes that can be evaluated through comparison with observations, but not truly validated. Both operational and diagnostic evaluations, together with sensitivity analyses are needed to both establish credibility and build confidence within the client and scientific community in the simulation results for policy and scientific applications. The characterization of the performance of Models-3/CMAQ is also a tool for the model developers to identify aspects of the modeling system that require further improvement.
Description:
The advection methods used in CMAQ require that the Courant-Friedrichs-Lewy (CFL) condition be satisfied for numerical stability and accuracy. In CMAQ prior to version 4.3, the ADVSTEP algorithm established CFL-safe synchronization and advection timesteps that were uniform throughout all the vertical layers. The resulting small synchronization timesteps, required to satisfy the CFL condition for applications with high winds aloft, pose an undue computational restriction since we are modeling the PBL and not the regions aloft. A layer dependent horizontal advection scheme has been developed that allows for multiple advection steps per synchronization step in layers dominated by high winds. This presentation describes the method and shows results comparing the old scheme. Results from the new, explicit mass-conserving advection scheme are demonstrated.