Albrecht effect

Abstract: The 'indirect' radiative cooling of climate due to the role of anthropogenic aerosols in cloud droplet formation processes (which affect cloud albedo) is potentially large, up to -1.5 W m^(-2) (ref. 1). It is important to be able to determine the number concentration of cloud droplets to within a few per cent, as radiative forcing as a result of clouds is very sensitive to changes in this quantity, but empirical approaches are problematic. The initial growth of a subset of particles known as cloud condensation nuclei and their subsequent 'activation' to form droplets are generally calculated with the assumption that cloud droplet activation occurs as an equilibrium process described by classical Kohler theory. Here we show that this assumption can be invalid under certain realistic conditions. We conclude that the poor empirical correlation between cloud droplet and cloud condensation nuclei concentrations is partly a result of kinetically limited growth before droplet activation occurs. Ignoring these considerations in calculations of total cloud radiative forcing based on cloud condensation nuclei concentrations could lead to errors that are of the same order of magnitude as the total anthropogenic greenhouse-gas radiative forcing.

Abstract: The component of the indirect aerosol effect related to changes in precipitation efficiency (the second indirect or Albrecht effect) is presently evaluated in climate models by taking the difference in net irradiance between a present-day and a preindustrial simulation using fixed sea surface temperatures (SSTs). This approach gives a “quasi forcing,” which differs from a pure forcing in that fields other than the initially perturbed quantity have been allowed to vary. It is routinely used because, in contrast to the first indirect (Twomey) effect, there is no straightforward method of calculating a pure forcing for the second indirect effect. This raises the question of whether evaluation of the second indirect effect in this manner is adequate as an indication of the likely effect of this perturbation on the global-mean surface temperature. An atmospheric global climate model (AGCM) is used to compare the evaluation of different radiative perturbations as both pure forcings (when available) and...

Abstract: Estimates of the indirect aerosol effect in GCMs assume that either cloud liquid water path is constant (Twomey effect) or increases with increased droplet number concentration (drizzle-suppression or Albrecht effect). On the other hand, if cloud thermodynamics and dynamics are considered, cloud liquid water path may also decrease with increasing droplet number concentration, which has been predicted by model calculations and observed in ship track and urban influence studies. This study examines the different changes of cloud liquid water path associated with changes of cloud droplet number concentration. Satellite data (January, April, July, and October 1987) are used to determine the cloud liquid water sensitivity, defined as the ratio of changes of liquid water path and changes of column droplet number concentration. The results of a global survey for water clouds (cloud-top temperature >273 K, optical thickness 1 ≤ τ < 15) reveal all three behaviors of cloud liquid water path with aerosol ch...