Evaluation of black carbon semi-direct radiative effect in a climate model

Abstract

The semi-direct effect of black carbon (BC) is studied by using a newly proposed optical property parameterization for cloud droplets with BC inclusions. Based on Atmospheric Model Intercomparison Project-type climate model simulations, it is found that the cloud amount can be either enhanced or reduced when BC is included in clouds. The decrease of the global annual mean total cloud amount is only about 0.023%. The 3-D cloud fraction distribution, however, shows larger changes which vary with latitude. A correlation between the changes of the cloud fraction and the vertical velocity is found. The cloud water path is mainly affected by low clouds and so the impact of BC on the cloud water path is particularly strong. It is shown that the BC above clouds tends to stabilize the atmosphere and enhance the cloud amount in the boundary layer. This can be used to explain the relationship between aerosol optical depth and cloud amount according to satellite data. For BC in clouds and above, the global annual mean enhancement of solar absorption is about 0.049 W m -2 and 0.57 W m -2, respectively. The BC semi-direct radiative forcing is estimated by subtracting the BC direct forcing from the BC total radiative forcing. The global annual mean of BC direct forcing and semi-direct forcing at the top of the atmosphere are 0.264 W m -2 and 0.213 W m -2, respectively. Key PointsBC inclusion in cloud dropletsrelationship between BC and cloudRe-investigate the BC semi-direct forcing ©2013. American Geophysical Union. All Rights Reserved.