Biomass burning aerosol effects on the diurnal cycle of convection: a numerical study in the dry season of South America

A fully coupled meteorology-chemistry-aerosol mesoscale model (WRF-Chem) is used to study the effects of biomass burning on the diurnal cycle of convection in the dry season of South America. Aura Microwave Limb Sounder (MLS) measurements of upper tropospheric ice water content (IWC), CO and other satellite products such as MODIS aerosol and TRMM precipitation are used to evaluate a 9-day WRF-Chem simulation from 15 to 24 September 2006. It is shown that the model simulations driven by emissions from MOZART approximately reproduce the distributions of aerosols and chemical tracers in response to convection, albeit with weaker magnitude than observations. The modeled clouds and precipitation are also in approximate agreement with satellite measurements.

Sensitivity tests are conducted to assess the effects of biomass burning-induced aerosols (mainly black carbon and organic carbon) on the clouds and precipitation processes. The aerosol effects are found to modulate the diurnal cycle of convection. In the afternoon, aerosols cool the surface by scattering solar radiation while they warm the middle-layer atmosphere by absorbing solar radiation. Surface precipitation is suppressed primarily by the aerosol radiative effect. At night, precipitation is enhanced resulted from the aerosol microphysical effect.