Aggregation and Episode Selection Scheme for EPA's Models-3 CMAQ

摘要

The development of an episode selection and aggregation approach, designed to211u001esupport distributional estimation for use with the Models-3 Community Multiscale 211u001eAir Quality (CMAQ) model, is described. The approach utilized cluster analysis of 211u001ethe 700 hPa u and v wind field components over the time period 1984-92 to define 211u001ehomogeneous meterological clusters. Alternative schemes were compared using 211u001erelative efficiencies and meteorological considerations. An optimal scheme was 211u001edefined to include 20 clusters (five per season), and a stratified sample of 40 211u001eevents was selected from the 20 clusters using a systematic sampling technique. 211u001eThe light-extinction coefficient, which provides a measure of visibility, was 211u001eselected as the primary evaluative parameter for two reasons. First, this 211u001eparameter can serve as a surrogate for PM-2.5, for which little observational 211u001edata exist. Second, one of the air quality parameters simulated by CMAQ, this 211u001evisibility parameter has one of the most spatially and temporally comprehensive 211u001eobservational data sets. Results suggest that the approach reasonably 211u001echaracterizes synoptic-scale flow patterns and leads to strata that explain the 211u001evariation in extinction coefficient and other parameters (temperature and 211u001erelative humidity) used in this analysis, and therefore can be used to achieve 211u001eimproved estimates of these parameters relative to estimates obtained using other 211u001emethods. Moreover, defining seasonally based clusters further improves the 211u001eability of the clusters to explain the variation in these parameters.