A numerical study of plume dispersion motivated by a mesoscale atmospheric flow over a complex terrain

A three-dimensional time-dependent mesoscale meteorological model, called HOTMAC, is applied to study the complex terrain airshed of El Paso/Ciudad Juarez staring from November 29 and ending at December 2, 1998. The model numerically solves the equations for mass, momentum, heat and moisture in terrain following coordinates using the alternating direction implicit (ADI) finite difference scheme. The model accounts for solar and terrestrial radiation effects, whereby the lower boundary conditions are defined by a surface energy balance and surface layer similarity theory, and the soil heat flux is obtained by solving a 5-level heat condition equation in the soil. Surface properties are defined for 14 landuse classes and the model includes urban and forest canopy parameterizations. For the Paso del Norte region, four nested meshes (1, 2, 4 and 8 km resolution) horizontally and 22 vertical grid levels were used, with the top of the modeling domain being 6000 m. To account for synoptic-scale weather variations, a nudging scheme was employed. The numerical wind results are compared with observations made in the Paso del Norte area. Finally, using the wind and turbulence output from HOTMAC, the particle plume trajectory information was obtained using the 'puff dispersion' model, RAPTAD, to capture details of the pollutant motions.