Effects of Soil Moisture on Temperatures, Winds, and Pollutant Concentrations in Los Angeles

This paper examines the effects of soil moisture initialization in a coupled air quality–meteorological model on temperature profiles, wind speeds, and pollutant concentrations. Three simulations, each with different initial soil moisture fields, were run. In the baseline simulation, predicted temperatures, wind speeds, and gas/aerosol pollutant concentrations accurately matched observations. In the other two simulations, soil moisture contents were initialized about 4% lower and higher, respectively, than in the baseline simulation. In the low-moisture case, predicted temperature profiles were hotter, near-surface wind speeds were faster, and near-surface pollutant concentrations were lower than observations and baseline predictions. In the high-moisture case, predicted temperatures were colder, wind speeds were slower, and pollutant concentrations were higher than observations and baseline predictions. Initial soil moisture contents affected vertical temperature profiles up to 600-mb altitude after two days. Elevated temperature changes were due in part to changes in sensible heat fluxes from the surface and in part to changes in elevated heat advection fluxes. Changes in temperature profiles affected wind speeds and boundary layer depths, which affected times and magnitudes, respectively, of peak concentrations. Slower wind speeds, associated with high soil moisture contents, delayed times of peak concentrations in the eastern Los Angeles basin. Faster wind speeds, associated with low soil moisture contents, advanced times of peak concentrations. High soil moisture contents resulted in thinner boundary layer depths, increasing average nearsurface pollutant concentrations, including that of ozone. Low soil moisture contents resulted in thicker boundary layer depths, decreasing average concentrations, including that of ozone. At some locations, changes in the magnitude of peak ozone concentrations depended on how changes in soil moisture affected ozone precursors and destroyers.