The NASA Global Modeling Initiative (GMI) chemical transport model is used
to assess the sensitivity of the first indirect forcing (IF) for sulfate
aerosol and autoconversion rate to the meteorological field, cloud drop
parameterization surface albedo and ice cloud temperature threshold. GMI is
driven by meteorological fields from the NASA GEOS1-STRAT, the NASA GEOS4
finite volume GCM and the NASA GISS II’ GCM. Aerosol simulation is linked to
cloud droplet number concentration (CDNC) with two empirical correlations
and two physically-based parameterizations. Autoconversion of cloud water to rain is calculated using three schemes. Depending on the meteorological fields and droplet parameterization used, the annual mean IF is found to range from -0.59 to -1.69 W m-2 while the change in autoconversion rate between preindustrial and current day simulations range between 19.42 and
45.39%. Application of different meteorological fields account for 40% of
the variability in shortwave cloud radiative forcing and up to 17%
variability in autoconversion seen in the simulations. Both first aerosol
indirect and autoconversion forcings are most sensitive to CDNC
parameterization; meteorology is of lesser importance. Given that prescribed
aerosol size distributions are used, it is likely that the uncertainty from
application of droplet parameterizations presented in this study is a lower limit.