Semi-direct radiative forcing of absorbing aerosols over clouds

Satellite observations confirm the results of large-eddy simulations indicating that the cloud liquid water path of marine stratocumulus clouds increases in cases where a layer of absorbing smoke aerosol resides above the cloud-topped marine boundary layer. The cloud thickening results from enhancement of the temperature inversion capping the boundary layer owing to aerosol shortwave radiative heating in the smoke layer. The empirical relationship between cloud albedo and liquid water path derived from satellite observations is used to compare the magnitudes of the positive direct radiative heating of absorbing smoke overlaying bright clouds with the negative semi-direct radiative forcing owing to cloud thickening. The radiative forcing owing to observed changes in cloud fraction is also explored. Finally, we assess the utility of global daily passive remote sensing data for determining the radiative forcing of absorbing aerosol over clouds throughout the global tropics.