Black carbon radiative heating effects on cloud microphysics and the implications for the aerosol indirect effect: a study using cloud parcel theory.

This work examines the effect of black carbon (BC) radiative heating on cloud droplet formation. For this purpose, a cloud parcel model with detailed microphysics is employed to simulate the diffusional growth of an aerosol population into cloud droplets, in the presence of radiative heating. In addition to including the modifications of the Kohler curves from BC contained in the droplets (presented in a companion paper), the heat released from the droplets (and interstitial black carbon) are considered in the parcel heat balance, thus affecting maximum parcel supersaturation, and droplet concentration. Using the cloud model, changes in cloud droplet concentration and cloud albedo due to the presence of black carbon are computed for different cases of aerosol size distributions, meteorological conditions, BC mixing state and aerosol composition. The results of these simulations are discussed within the context of aerosol indirect radiative forcing, with an emphasis on the role of each heating mechanism. Implications for the uncertainty in global circulation model estimates of indirect forcing are also discussed.