Citation:

Finkelstein, P L. COMPARISONS OF SPATIAL PATTERNS OF WET DEPOSITION TO MODEL PREDICTIONS. Presented at NADP 2004 Scientific Symposium and Technical Committee Meeting, Halifax, Nova Scotia, September 21-24, 2004.

Impact/Purpose:

The goal of this task is to thoroughly characterize the performance of the emissions, meteorological and chemical/transport modeling components of the Models-3 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 validated. Both performance and diagnostic evaluation together with sensitivity analyses are needed to establish credibility and build confidence within the client and scientific community in the simulations 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 Community Multiscale Air Quality model, (CMAQ), is a "one-atmosphere" model, in that it uses a consistent set of chemical reactions and physical principles to predict concentrations of primary pollutants, photochemical smog, and fine aerosols, as well as wet and dry deposition. The model is being used to develop new federal regulations, state implementation plans, and air quality forecasts. As a part of a comprehensive evaluation of CMAQ, this study compares the spatial prediction of yearly total wet deposition of sulfate, nitrate, and ammonium across the country to measurements made by NADP. In order to develop spatial maps of wet deposition it is necessary to interpolate between monitoring sites. However it has been shown that rainfall fields are very spatially discontinuous and non-stationary. Therefore, spatial interpolation of wet deposition is problematic. To overcome this obstacle others have proposed using spatially interpolated precipitation-weighted concentration of the pollutant in rainfall along with a more detailed rainfall field derived from the more dense rainfall networks. We take that idea a step further, by considering two possible sources of data. One is a recently available National Precipitation Analysis. This dataset has been developed by NOAA's National Center for Environmental Prediction with the Office of Hydrology. The analysis merges two data sources, 3000 automated raingage observations with the digital precipitation estimates from the WSR-88D weather radar. The radar bias is corrected using the gage network. The results are generated onto a 4 km grid. For this analysis, the grid has been relaxed to 36 km to match it to the CMAQ grid. The other precipitation source is the NOAA cooperative observer network, with more than 6000 sites in the lower 48 states. The paper will discuss the advantages and disadvantages of these precipitation datasets. The NADP concentration measurements are interpolated to the same grid as the precipitation using a statistical model. Deposition is computed at each cell. The computed spatial fields of total deposition for sulfate, nitrate and ammonium are then compared to the CMAQ model output, and the similarities and differences are noted. Consideration is given to model biases caused by inaccurate precipitation inputs to the model as well as inherent model biases.

Although this work was reviewed by EPA and approved for publication, it may or may not necessarily reflect official Agency policy at this time .

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