Cloud Responses to Increased Absorption by Brown Carbon in Biomass Burning

Observational and modeling studies have suggested that a significant amount of atmospheric aerosol absorption is attributable to absorbing organics, i.e. brown carbon (BrC). On a global scale, BrC is estimated to insert a direct radiative effect of +0.04 to +0.25 W m^-2, and especially over regions dominated biomass burning emissions, enhanced absorption by BrC adds a strong positive forcing of more than +0.5 W m^-2, which is comparable to the black carbon forcing. The subsequent impact of BrC on cloud properties and meteorological fields has not been studied in global climate models. In this study, we present a global simulation of BrC radiative effects with the latest four-mode version of the 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, biofuel and fossil fuel sources, respectively.  The CAM5 model estimates that BrC contributes about 12% increase of the fine-mode aerosol absorption at 550nm on a global average, and about 20% in major biomass burning regions. In addition to the increased column-integrated absorption, inclusion of BrC also changes the vertical profiles of atmospheric extinction and absorption. Over the subtropical SE Atlantic Ocean, the simulated cloud fractions of marine low clouds are decreased resulting from a larger absorption associated with the biomass burning BrC aerosols aloft, while the cloud base height is lowered. In contrast, increases of marine low clouds fractions are simulated over other regions with the inclusion of BrC absorption. We will evaluate the simulated vertical profiles of aerosols and cloud properties against the available satellite and/or ground observations from recent field experiments, such as Layered Atlantic Smoke Interactions with Clouds (LASIC) deployment of a DOE ARM Mobile Facility, and the NASA Observations of Aerosols above Clouds and their Interactions (ORACLES).