The aerosol-cloud interactions due to black carbon (BC) aerosols, as well as the implied climate responses, are examined using an aerosol module in the coupled atmosphere-ocean general circulation model MPI-ESM. BC is simulated to enhance cloud droplet number concentration (CDNC) by 10-15% in the BC emission source regions, especially in the Tropics and mid-latitudes. Higher CDNC and reduced auto-conversion from cloud water to rain water explains the increased cloud water path over the tropical regions (30 degrees S-30 degrees N) in the model. In the global mean, the cloud water-as well as precipitation changes are negligibly small. The global-mean effective radiative forcing due to aerosol-cloud interactions for BC is estimated at -0.13 +/- 0.1 Wm(-2), which is attributable to the increase in CDNC burden and (regionally) cloud water in the model. Global mean temperature and rainfall response were found to be -0.16 +/- 0.04 K and -0.004 +/- 0.004 mm day(-1), respectively, with significantly larger regional changes mainly in the downwind regions from BC sources.

Black carbon (BC) induced indirect radiative forcing and cloud albedo effect has been studied for the first time over northeast India. Measurements of BC and cloud microphysical parameters were carried out during Phase-I of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over northeast India (Guwahati) in 2009. Liquid water path (LWP) in the cloud layers coherent with BC on different experimental days was found to be 206-327 g m(-2) over the region. Black carbon aerosol indirect effect (BCIE) for fixed LWP is found to be 0.32-0.48 on different days of observations. The indirect forcing corresponding to this BCIE has been estimated using a radiative transfer model for fixed LWP by altering the derived BC-AOD (aerosol optical depth from measured BC profiles) and cloud effective radius (R-e) combinations. The estimated average BC-induced indirect forcing (BCIF) was -24 to -37.1 W m(-2) at the surface and +2.5 to +14.8 W m(-2) at the top of the atmosphere (TOA). The average albedo due to BCIF at TOA was 0.49-0.61. BCIF is found to reduce the cloud reflection by 1.5-2% over the region. The sensitivities of cloud parameters to BCIF and the albedo effect are illustrated.