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A MULTI-STREAM MODEL FOR VERTICAL MIXING OF A PASSIVE TRACER IN THE CONVECTIVE BOUNDARY LAYER
Citation:
Han, J. AND D W. Byun. A MULTI-STREAM MODEL FOR VERTICAL MIXING OF A PASSIVE TRACER IN THE CONVECTIVE BOUNDARY LAYER. Presented at 11th Joint Conference on the Applications of Air Pollution Meteorology with the AWMA, Long Beach, CA, January 9-14, 2000.
Impact/Purpose:
The objectives of this task are to continuously develop and improve the Community Multiscale Air Quality (CMAQ) modeling system, which is the science implementation within the Models-3 system framework for air quality simulation. 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. It relies on Models-3 I/O API to support machine independent data access and maintains simple interfaces among science processor modules to provide a high-level of modularity.
Description:
We study a multi-stream model (MSM) for vertical mixing of a passive tracer in the convective boundary layer, in which the tracer is advected by many vertical streams with different probabilities and diffused by small scale turbulence. We test the MSM algorithm for investigating the effects of inhomogeneous mixing and vertical transport of multiple pollutants at different source heights in a three-dimensional air quality model. All input parameters in the MSM such as the probability density function and the mean updraft and downdraft are prescribed from large-eddy simulation (LES) data and the mixed-layer similarity theory.
Applicability of the MSM is tested against the LES results for both near surface and elevated tracer sources. Results from the MSM compare well with those from LES for the overall temporal behavior of concentration distributions. However, a somewhat poor agreement between the MSM and the LES is seen in the magnitude of the concentrations at the surface and in the entrainment zone. It suggests that a further improvement of the MSM is needed to account for the frictional effects at the surface and the diffusion by small scale turbulence in the entrainment zone