NASA - National Aeronautics and Space Administration

A fully coupled meteorology‐chemistry‐aerosol mesoscale model (WRF‐Chem) is used to simulate a multiday biomass burning event in the dry season of South America. The effects of biomass burning aerosols on clouds and precipitation are described at both 36 km and 4 km horizontal resolutions. The dominant effect of the aerosols is to reduce the diurnal amplitude of convection by decreasing clouds and precipitation in the afternoon but increasing them at night, with the afternoon decrease greater than the nighttime increase on the daily mean. On average, the decrease of surface precipitation is about 5% (3%) and the amplitude of diurnal cycle is reduced by about 11% (5%) in the 36 km (4 km) simulations. Such a modulation of clouds and precipitation is primarily contributed by the aerosol radiative effect, i.e., their ability to scatter and absorb solar radiation. The aerosol microphysical effect as cloud condensation nuclei tends to act oppositely to the aerosol radiative effect but with a smaller magnitude, especially in the simulations at 36 km horizontal resolution. The 4 km resolution runs exhibit similar behaviors to the 36 km simulations, with a slightly stronger role of the aerosol microphysical effect relative to the aerosol radiative effect. We find another important effect of biomass burning aerosols. When uplifted into the upper troposphere by deep convection, they can significantly warm the upper troposphere through their local radiative heating effect and result in significant moistening in the upper troposphere, potentially affecting the water vapor transport from the troposphere to the stratosphere.