Effects of Surface Property Variations on Simulated Daytime Airflow over Coastal Southern California

Abstract

A series of simulations were performed to determine the sensitivity of a three-dimensional mesoscale model to the spatial variation of surface properties across coastal southern California, including the Los Angeles basin. Properties were allowed to vary one at a time and all at once in the various simulations, and the results were compared to a simulation in which homogeneous surface properties were prescribed. In general, the simulated atmospheric effects of the variations were small, apparently overwhelmed by the large-scale wind and by the diurnal sea-land and mountain-valley winds that occur in the absence of surface variations. Some effects, however, were significantly large. The greatest change in the lowland airflow was a weaker westerly flow in the topographic channel between the western and eastern parts of the basin just south of the San Gabriel Mountains. Quantitative comparison of observed and predicted near-surface winds shows that small but distinct improvements in the model's predictions resulted. Realistic prescriptions of greater moisture on the upper mountain slopes diminished the heated-slope flows, but lower prescribed moisture over the lower slopes increased them somewhat; the chimney-like ventilation of the basin was therefore maintained. Airflow from the basin into the surrounding, desert through Cajon Pass is somewhat decreased.