Inference of Semi-direct Radiative Impact of Absorbing Aerosols on Low-Level Clouds

Direct radiative forcing of absorbing aerosols can cause rapid changes in the surface energy budget, the atmospheric temperature profile, and cloudiness. These rapid responses collectively lead to additional radiative forcing termed the aerosol semi-direct effect because of the associated changes in cloud properties. Although there is “high confidence” that atmospheric heating by absorbing aerosols can alter clouds, there is “low confidence” in determining the sign and magnitude of the rapid adjustments in both models and observations.

Here we examine low-level cloud microphysics and optical properties, and the lower atmospheric heating, surface radiation, water vapor, and temperature for cases where strongly absorbing aerosols (with absorption coefficient >=10 Mm-1 at 528nm) were observed at Nainital (29.38N, 79.45E) from Oct, 2011 to March, 2012 during the DOE Ganges Valley Experiment. The ground-based MPL observations show that aerosol layers are concentrated predominately between the surface and 2 km. Low-level clouds with the cloud base height below 2 km altitude are identified by ceilometer data and frequently occur within the aerosol layers. The direct radiative forcing by aerosols is estimated to be -10.5 Wm-2 at the surface and +5.2 Wm-2 in the atmosphere, causing up to 1 K day-1 heating in the lower troposphere. The observed cloud base height increasing with higher aerosol absorption coefficient is consistent with the regional climate model simulations, which show that the simulated aerosols reduce the low-level cloud cover by stabilizing the lower atmosphere and reducing the boundary vertical mixing. On the other hand, the observed cloud liquid water path from MWR increases with stronger aerosol absorption. It is because these absorbing aerosols can also act as cloud condensation nuclei as suggested by their positive correlation and possibly invigorate the vertical development of clouds. The net impact on radiation due to these two aerosol-induced changes of cloud properties will be examined with a radiative transfer model mainly using the cloud and aerosol observations.