UC Irvine

 Using the Weather Research and Forecasting model with Chemistry (WRF-Chem), we explored the impacts of nonlocal aerosol plumes transported at three different altitudes on a summertime convective system developed in a clean environment over the northeastern United States. Idealized aerosol plumes from forest fire and volcano emissions, which are known to be frequently transported in this region, were prescribed at three separate altitudes on the upstream boundary of WRF-Chem. The low-altitude (1.5–2.5 km) plume characteristic of forest fire emissions intersects the water clouds, resulting in optically thicker clouds and about a 30% decrease in accumulated precipitation. The precipitation response to the idealized aerosol plume is attributed to the aerosol “second indirect effect” and aerosol-induced enhancement in evaporation efficiency. Convection also significantly impacted this low-altitude aerosol plume because wet removal scavenges up to 70% of plume aerosols over regions where deep convection and precipitation occur. In stark contrast, midaltitude (5.6–6.6 km) and high-altitude (11.5–12.5 km) plumes exerted a negligible effect on clouds and precipitation. The apparent highly nonlinear sensitivity of simulated convection to the vertical positioning of nonlocal aerosol plumes is explained in terms of the dominant controls influencing this convection regime and limitations in the microphysics currently implemented in WRF-Chem.