Citation:

Ching, J.K S., J A. Herwehe, AND J L. Swall. PARADIGM USING JOINT DETERMINISTIC GRID MODELING AND SUB-GRID VARIABILITY STOCHASTIC DESCRIPTION AS A TEMPLATE FOR MODEL EVALUATION. Presented at 2004 Models-3 Conference, Chapel Hill, NC, October 18-20, 2004.

Impact/Purpose:

The objective of this task is to develop and evaluate numerical and physical modeling tools for simulating ground-level concentrations of airborne substances in urban settings at spatial scales ranging from ~1-10 km. These tools will support client needs in the areas of air toxics and homeland security. The air toxics tools will benefit the National Air Toxics Assessment (NATA) program and human exposure modeling needs within EPA. The homeland security-related portion of this task will help in developing tools to assess the threat posed by the release of airborne agents. Both sets of tools will consider the effects induced by urban morphology on fine-scale concentration distributions.

Description:

The goal of achieving verisimilitude of air quality simulations to observations is problematic. Chemical transport models such as the Community Multi-Scale Air Quality (CMAQ) modeling system produce volume averages of pollutant concentration fields. When grid sizes are such that significant within-grid variability is not resolved, the comparison against one or more point measurements is not, in general, expected to be equal, except under very special limited circumstances. Yet, since models are judged by such comparative analyses, there is an implicit assumption (and a hope) that the point measurement is representative of the grid model prediction, and vice versa.

In this paper, we accept that there will always be a component of within-grid concentration variability to grid models, even at fine scale grid resolution. Such variability arises from the distribution of emission sources within individual gird cells. Additionally, there may be significant variability arising from coupled chemical and turbulent interactions. On the other hand, general guidance for locating monitors may not necessarily mean that such measurements will be representative of the grid model fields. This desired goal is only possible for idealized horizontally homogeneous fully dispersed, uniform source distributions situation. In fact, especially in urban areas even with an ideally placed monitor, it is a rare case in which the monitors and grid model output are really expected to be completely comparable.

In the presence of inherent within-grid variability, we suggest that it is more reasonable to reformulate the comparative basis between models and monitoring information by recognizing and accepting the a priori presence of within-grid variability

Examples using CMAQ at neighborhood scales and the corresponding model outputs that represent sub-grid variability, along with observations, are presented to illustrate this paradigm.

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. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.

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