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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 15) reveal all three behaviors of cloud liquid water path with aerosol changes: increasing, approximately constant, or decreasing as cloud column number concentration increases. The authors find that 1) in about one-third of the cases, predominantly in warmer locations or seasons, the cloud liquid water sensitivity is negative, and the regional and seasonal variations of the negative liquid water sensitivity are consistent with other observations; 2) in about one-third of the cases, a minus onethird (1/3) power-law relation between effective droplet radius and column number concentration is found, consistent with a nearly constant cloud water path; and 3) in the remaining one-third of the cases, the cloud liquid water sensitivity is positive. These results support the suggestion that it is possible for an increase of cloud droplet number concentration to both reduce cloud droplet size and enhance evaporation just below cloud base, which decouples the cloud from the boundary layer in warmer locations, decreasing water supply from surface and reducing cloud liquid water. Results of this study also suggest that the current evaluations of the negative aerosol indirect forcing by GCMs, which are based on either the Twomey or Albrecht effects, may be overestimated in magnitude.
Han et al. (Fri,) studied this question.