Characteristics of Summertime Circulations and Pollutant Ventilation in the Los Angeles Basin

Abstract

A cross-sectional numerical primitive-equation model is used to simulate the summertime airflow pattern in the Los Angeles basin for calm synoptic-scale wind conditions. The contributions of the sea breeze, the urban heat island effect and the mountain-valley wind are quantified. The mountain-valley and sea-breeze circulations are of the same sense (landward at the surface, toward water aloft) and strength (maximum of 5-10 m s?1 at surface), but the urban heat island effect is negligible. Correct specification of the land surface characteristics is found to be important to the quality of the simulation. Model output is then used to calculate estimates of the space and time variation of boundary-layer ventilation. Ventilation, defined as the product of the height of the planetary boundary layer and the mean wind speed therein, is found to be enhanced in the vicinity of the sea breeze front, and generally increases with distance from the ocean. In the stable marine air layer behind the front, the ventilation is especially low.