Planetary boundary layer structure of the Paso del Norte airshed: a numerical study

The mesoscale meteorological model MM5 was employed to simulate the synoptically influenced local winds in the Paso del Norte Airshed (PdN) in support of air-quality studies conducted in the summer of 1996 and the winter of 1998 under the auspices of the Southwest Center for Environmental Research and Policy (SCERP). These experiments were designed to understand physico-chemical characteristics, transformations, and transports of major atmospheric pollutants in the area, in particular, aerosols and ozone. The PdN airshed is known to violate National Ambient Air Quality Standards for several regulated air pollutants. Model predictions of the synoptically influenced local mountain/valley circulation showed a good agreement with the observations. Details of the three-dimensional structure and the temporal evolution of the atmospheric boundary layer were also well-captured by the model. The correlation coefficients between model predictions and observations at approximately 250m-agl (meters above ground level) were as high as 0.92 for temperature and 0.80 and 0.61, respectively, for u- and v-wind components. The correlation coefficients, in general, depended on the variable and the height. The performance of three turbulence closure schemes was evaluated vis-à-vis the observations. These closures include the Blackader non-local mixing scheme, Hong-Pan non-local closure scheme and the Gayno-Seaman 1.5-order scheme. Predictions based on the three schemes were found to be almost the same, except that some noticeable differences could be seen during the daytime of clear days, compared to days with relatively strong synoptic forcing. The computational efficiency of the Hong-Pan scheme was found to be 27% to 29% higher than the other two schemes. The present study corroborates the notion that mesoscale meteorological models can provide useful basic information on flow structure for the better understanding of air quality problems