Enhanced vertical transport efficiency of aerosol in convective clouds due to increases in tropospheric aerosol abundance

Abstract

A numerical model is used to show that decreases in the precipitation efficiency of a midlatitude continental convective cloud in response to increases in aerosol concentration can significantly affect the efficiency with which aerosol is transported from the lower to the upper troposphere (UT). We use a 2-D nonhydrostatic cloud and aerosol microphysics model to show that a moderately deep mixed phase convective cloud can significantly perturb aerosol in the UT: The cloud causes an increase in UT aerosol mass when the initial UT aerosol abundance is low and a decrease when the initial UT abundance is high. Realistic increases in cloud condensation nucleus (CCN) concentrations reduce the precipitation efficiency and thereby the scavenging efficiency of aerosol and allow more aerosol material to be transported to the UT. The enhancement of the UT aerosol mass after a cloud event therefore increases in clouds with higher CCN concentration, a positive feedback driven by the response of scavenging rates to aerosol abundance. Convective clouds are the principal mechanism by which lower tropospheric aerosol material is transported into the UT. The enhancement in the efficiency of this transport in high aerosol conditions may mean that in some locations, UT aerosol will be particularly sensitive to the aerosol content of the lower troposphere. Copyright 2006 by the American Geophysical Union.