84th AMS Annual Meeting

Monday, 12 January 2004
Local to Regional to Global Pollutant Transport with Adaptive Grids
Hall 4AB
Ananthakrishna Sarma, SAIC, McLean, VA; and D. P. Bacon, N. N. Ahmad, T. J. Dunn, M. S. Hall, M. D. Turner, and T. R. Wait
Several regions of the world, such as Eastern Asia, are undergoing major political and economic changes, leading to heightened energy usage and significant increases in pollutant emissions. This is especially true of the major industrialized megacities (cities with population greater than 5 million people, e.g., Hong Kong, Shanghai, Beijing, and Tokyo). The energy consumption in Asia is expected to more than double by the year 2020 (an annual growth rate of ~3.7%) with a similar trend in carbon emissions. Without the imposition of controls, the anthropogenic NOx emissions from Asia will increase by 350% over the period 1990 - 2020. Within two decades, emissions from East Asia could account for roughly half of the sulfur and nitrogen and a third of the carbon emitted from anthropogenic sources worldwide. An important consequence of increasing NOx emissions in Asia is the potential impact on the surface ozone concentrations in the United States as well as on the aerosol burden and radiative balance that controls the global climate.

The pollutant plumes from several large cities in East Asia travel long distances over the Pacific Ocean and are known to affect the air quality over the Western United States. To study the chemical characteristics of these plumes and thus their impact on regional air quality, it is necessary to study their history from the source through their transport and evolution. The plumes, during their history, span spatial scales ranging from meters and tens of meters near the source region to hundreds and thousands of kilometers of transport distance. Near their origin they are affected by small-scale phenomena such as urban heat islands, land and sea breeze circulations and topographically driven flows. During their transit across the Pacific, they are affected by the large-scale synoptic waves. Once they reach the U.S. coast, they are again influenced by the synoptic short waves as well as the mesoscale circulations including terrain effects, land and sea breezes, as well as mixing processes in the planetary boundary layer (PBL). Convective phenomena such as squall lines and thunderstorms that form along cold-frontal boundaries help transport pollutants from the boundary layer to the upper atmosphere in a relatively short period of time. The dominant chemical mechanisms that occur in these plumes change as their chemical compositions change during their transport.

Traditionally, nested grid models are used for this purpose, with high-resolution nests placed along the expected path of the plume. The nested grid models have the following drawbacks.

1) An a priori knowledge of the solution is required so that the nests can be placed over regions of interest.

2) Transitions from one nest to another are abrupt.

3) Even though some models allow the high-resolution nests to move with a pre-specified constant velocity, they cannot treat cases in which the high-resolution grids might intersect.

4) The terrain representation can be different in each of the grids.

All these problems are avoided by the use of unstructured, adaptive grid methods, which have been in use in the aerospace and shock hydrodynamics community for more than a decade. The primary advantages of unstructured grids are their ability to provide high resolution only where needed and to provide smooth transitions in grid resolution over the computational domain. The advantage of the adaptive grids is their ability to provide higher resolution dynamically (at run time) to regions requiring it as the solution evolves with time. These sophisticated CFD techniques have been recently adapted to the field of numerical weather prediction (NWP) by the Center for Atmospheric Physics (CAP) of Science Applications International Corporation (SAIC). The resulting Operational Multiscale Environment model with Grid Adaptivity (OMEGA) has proven to be a powerful and flexible tool for atmospheric simulation at scales ranging from cloud scale to mesoscale with indications of utility at global scales as well. OMEGA's unstructured, adaptive grid makes it uniquely capable of addressing the multiscale features of long-range plume transport.

This paper discusses the use of the OMEGA system to study the multiscale problem of local to regional to long-range transport of pollutants.

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