Numerical Investigation of Mesoscale Circulations over the Los Angeles Basin. Part II: Synoptic Influences and Pollutant Transport

Abstract

This paper describes the influence of large-scale flows on mesoscale circulations and pollutant transport in the Los Angels Basin. National Meteorological Center (NMC) gridded data for twenty-six basin smog episodes were composited, and summer and winter synoptic-scale wind and temperature profiles were created. The 24-h evolution of the atmosphere in and around the basin was simulated, both with and without large-scale winds, using a three-dimensional mesoscale model. Light large-scale winds had little influence on the strong summertime mesoscale circulations, but stronger large-scale winds exerted considerable influence on the weak mesoscale wintertime circulations. During both seasons, daytime upslope flows near the mountains ventilate the basin, inducing basinwide airflow and augmenting the sea breeze. Air from the basin's planetary boundary layer (PBL) travels upslope and rises over the ridgetops, where it mixes convectively to great heights; winds aloft then transport the air from the region. Only in one simulation did the heated-slope circulations inject pollutants into the inversion layer. Significant flows through the three major mountain passes occur, with subsequent mixing in the desert PBL. Thermal contrasts between the upper portion of the deep PBL over the desert and the free atmosphere within the basin generate flows that return to the basin. In the eastern half of the basin, entertainment and upward motion cause the PBL to grow through the inversion and mix pollutants into the inversion layer.