Cloud Responses to Increased Absorption by Brown Carbon in Biomass Burning

Description

Observational studies have suggested that a significant amount of aerosol absorption is attributable to brown carbon (BrC) in addition to black carbon (BC). On a global scale, BrC from biomass burning is estimated to insert a direct radiative effect of +0.04 to +0.25 W m-2. This direct positive forcing may cause subsequent changes in cloud and meteorological fields that have not yet been studied.Here, we present a global simulation of BrC direct and semi-direct radiative effects with the four-mode Modal Aerosol Module (MAM4) in the Community Atmosphere Model version 5 (CAM5). The spectrally resolved absorption by BrC from primary emissions is parameterized for biomass burning, and biofuel and fossil fuel sources, respectively. The CAM5 model estimates that BrC contributes about 12% increase of the fine-mode aerosol absorption at 550nm, and about 20% in biomass burning regions. In addition to the increased column absorption, inclusion of BrC also changes the vertical profiles of atmospheric extinction and absorption leading to semi-direct effects. Over the SE Atlantic Ocean, the simulated cloud fractions of subtropical low clouds are reduced resulting from a larger absorption associated with the elevated biomass burning aerosols, while the cloud base height is lowered. In contrast, increases of low clouds fractions are simulated over the Indian Ocean trade cumulus regions. It indicates that the cloud responses to aerosol absorption depend on the boundary-layer conditions as well as the vertical placement of aerosol layers. Evaluation of the simulated vertical profiles of aerosols and cloud properties against the available observations including recent field experiments, such as Layered Atlantic Smoke Interactions with Clouds (LASIC) deployment of a DOE ARM Mobile Facility, will be presented.