Effects of soil moisture on temperatures, winds, and pollutant concentrations in Los Angeles

This paper examines the effects of soil moisture initialization in a couple d air quality-meteorological model on temperature profiles, wind speeds, an d pollutant concentrations. Three simulations, each with different initial soil moisture fields, were run. In the baseline simulation, predicted tempe ratures, wind speeds, and gas/aerosol pollutant concentrations accurately m atched observations. In the other two simulations, soil moisture contents w ere initialized about 4% lower and higher, respectively, than in the baseli ne simulation. In the low-moisture case, predicted temperature profiles wer e hotter near-surface wind speeds were faster, and near-surface pollutant c oncentrations were lower than observations and baseline predictions. In the high-moisture case, predicted temperatures were colder, wind speeds were s lower, and pollutant concentrations were higher than observations and basel ine predictions. Initial soil moisture contents affected vertical temperatu re profiles up to 600-mb altitude after two days. Elevated temperature chan ges were due in part to changes in sensible heat fluxes from the surface an d in part to changes in elevated heat advection fluxes. Changes in temperat ure profiles affected wind speeds and boundary layer depths, which affected times and magnitudes, respectively, of peak concentrations. Slower wind sp eeds, associated with high soil moisture contents, delayed times of peak co ncentrations in the eastern Los Angeles basin. Faster wind speeds, associat ed with low soil moisture contents, advanced times of peak concentrations. High soil moisture contents resulted in thinner boundary layer depths, incr easing average nearsurface pollutant concentrations, including that of ozon e. Low soil moisture contents resulted in thicker boundary layer depths, de creasing average concentrations, including that of ozone. At some locations , changes in the magnitude of peak ozone concentrations depended on how cha nges in soil moisture affected ozone precursors and destroyers.