![[logo] US EPA](https://webarchive.library.unt.edu/eot2008/20081013073337im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Gas-Phase Chemistry in CMAQ
Research Programs
Realistic simulation of land surface and planetary boundary layer (PBL) processes are important for both meteorology and air quality modeling. Interactions between surface characterization, surface fluxes, and PBL processes are very tightly coupled. In addition, surface fluxes and PBL mixing of chemical constituents closely follow the meteorological processes. Therefore, research in this area involves both the meteorology and chemical transport models to develop realistic and consistent modeling of surface and PBL processes.
Parameterizations of vertical transport due to boundary layer turbulence are among the most important components of meteorology and air quality models. However, the PBL schemes employed in meteorology models and those used in air quality models are often quite different. Part of the reason for this is simply different model development histories, but it also seems that the kind of scheme that works well in meteorology models may not work so well in air quality models or vice versa. Mesoscale meteorology models typically include either simple non-local closure schemes or higher-order schemes that involve prognostic equations for turbulent kinetic energy (TKE) and sometimes other higher order terms such as turbulent dissipation or potential temperature variance. The non-local schemes, in particular, have been developed to address the inadequacies of local schemes that cannot produce realistic profiles of both first order quantities and their fluxes in convective conditions. Air quality models typically use simple local closure (eddy diffusivity), although both non-local and higher-order schemes have also been used. A difficulty for air quality models is that chemical profile data for evaluation are very sparse. Thus ground level concentration data are often used for evaluation which may be affected by many other processes.
Model Development
We have been experimenting with various PBL formulations as well as continuing development of new PBL schemes. Most notably, a new more advanced version of the Asymmetric Convective Model (ACM) has been developed and is currently being tested and evaluated. The new model (ACM2) is a combination of local and non-local closure. Specifically, ACM2 is a combination of the original ACM (Pleim and Chang, 1992) and eddy diffusion. The trick is to match the two schemes at a certain height, in this case the top of the lowest model layer, and apportion the mixing rate between the two schemes so that the resultant flux is identical to that produced by either scheme running alone.
The ACM2 has been implemented in MM5 and CMAQ. Evaluation is proceeding along two tracks. A series of MM5 and CMAQ simulations has been made for the summer of 2004. Operational evaluations of these runs have shown preliminary statistics for temperature, humidity, and winds, that are similar to previous MM5 simulations using ACM. Preliminary evaluation of CMAQ runs show improved simulation of ozone compared to the 2004 air quality forecasts. A more rigorous evaluation of the PBL model will be made involving comparisons of PBL heights and profiles of meterological and chemical parameters to ICARTT 2004 field measurements. The other evaluation track involves idealized case study simulations for various stability and wind conditions. Profiles of temperature, winds, and chemical tracers as well as their flux profiles are being compared to large eddy simulation results for identical conditions.
The data-assimilation scheme for the Pleim Xiu Land Surface Model (PX-LSM) was analyzed and evaluated against Southern Oxidant Study 1999 field experiment data. This scheme involves Newtonian nudging of surface and root-zone soil moisture according to model biases in 2-m air temperature and relative humidity. Sensitivity tests confirm the value of this scheme in improving temperature and surface flux simulations (Pleim and Xiu, 2003). The PX LSM has been an available option in the MM5 for several years. We are now working to implement the PX LSM in the WRF model.
REFERENCES
Pleim, J.E., and A. Xiu. Development of a land surface model. Part II: Data assimilation. Journal of Applied Meteorology 42:1811–1822 (2003).