摘要：This work examines the influences of relative dispersion (the ratio of the standard deviation
to the mean radius of the cloud droplet size distribution) on cloud albedo and cloud
radiative forcing, derives an analytical formulation that accounts explicitly for the
contribution from droplet concentration and relative dispersion, and presents a new
approach to parameterize relative dispersion in climate models. It is shown that
inadequate representation of relative dispersion in climate models leads to an
overestimation of cloud albedo, resulting in a negative bias of global mean shortwave cloud
radiative forcing that can be comparable to the warming caused by doubling
CO2 in magnitude, and that this dispersion bias is likely near its maximum for ambient clouds.
Relative dispersion is empirically expressed as a function of the quotient between cloud
liquid water content and droplet concentration (i.e., water per droplet), yielding an
analytical formulation for the first aerosol indirect effect. Further analysis of the new
expression reveals that the dispersion effect not only offsets the cooling from
the Twomey effect, but is also proportional to the Twomey effect in magnitude.
These results suggest that unrealistic representation of relative dispersion in cloud
parameterization in general, and evaluation of aerosol indirect effects in particular, is
at least in part responsible for several outstanding puzzles of the aerosol–cloud
conundrum: for example, overestimation of cloud radiative cooling by climate models
compared to satellite observations; large uncertainty and discrepancy in estimates of
the aerosol indirect effect; and the lack of interhemispheric difference in cloud
albedo.
