Radiative and Cloud Responses to Absorbing Aerosols in a Global Climate Model (Invited Presentation)

Estimates of semi-direct radiative effects of absorbing aerosols have “very low confidence” in sign and magnitude. From a global modeling perspective, uncertainties may arise from model representation of low clouds and total aerosol absorption. In the present study, we revisit the question about the low cloud responses to absorbing aerosols and radiative impacts with a recent version of the global climate model CAM5.3. This model has a few key improvements in clouds and aerosols relevant to semi-direct effects, including the explicitly simulated stratus-radiation-turbulence interactions and the four-mode Modal Aerosol Module (MAM4) that treats the freshly emitted primary carbons from aged carbonaceous aerosols. Additionally, increased light absorption by brown carbon (BrCabs), which is often treated as non-absorbing organic carbon, is considered. The improved global model predicts spectrally dependent aerosol absorption and vertical profiles in agreement with the AERONET and CALIOP measurements, especially over the regions influenced by biomass burning. BrCabs inserts an additional positive radiative effect that is comparable to about 11% of the estimated black carbon (BC) direct effect, as shown amplifying the aerosol heating in the lower atmospheric. On the other hand, the enhancement of atmospheric heating by BrCabs occurs at altitudes where the background BC is less concentrated due to internal mixtures of aerosols. This leads to different cloud responses to BrCabs vs BC in the sign and magnitude, depending on the total aerosol heating profiles and simulated cloud regimes. We will also discuss the model-estimated radiative and cloud responses in comparison with observationally based analyses on the regional scales and implications based on the global model estimates.