Atmospheric response and feedback to radiative forcing from biomass burning in tropical South America

Simulations are performed to understand the importance of smoke from biomass burning in tropical South America to regional radiation and climate. The National Center for Atmospheric Research (NCAR) regional climate model coupled with the NCAR column radiative model is used to estimate smoke direct radiative forcing and consequent atmospheric perturbations during a smoke season in this region. The smoke optical properties are specified based on the measurements during the smoke, clouds, and radiation-Brazil experiment. The simulations obtain a direct radiative forcing of −16.5 W m−2 over the smoke region. This magnitude, however, is substantially reduced due to atmospheric feedback. Clouds and precipitation are reduced due to smoke. The cloud reduction mainly results from smaller water vapor transport from the ground and the planetary boundary layer to the cloud layer because of the combined effects of reduced turbulent activity and the subsidence tendency. The simulated cloud reduction agrees with a recent finding from satellite measurements. Smoke also leads to the enhancement of a dominant planetary-scale high system. A two-layer structure of warmer air with ascending tendency on top of cooler air with descending tendency is formed due to smoke with strong absorption.