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portion of this paper employ historical radiative forcing from the years 1860–2000. Model comparisons use the ensemble mean and time mean from a five-member ensemble of historical experiments in CM2.1 and CM3 over the years 1981–2000. Each ensemble member was initialized from a different point during the 1860 radiatively forced spinups for the two respective models, with CM2.1 ensemble members initialized 40 years apart and CM3 ensemble members initialized 50 years apart. For our purposes, the
portion of this paper employ historical radiative forcing from the years 1860–2000. Model comparisons use the ensemble mean and time mean from a five-member ensemble of historical experiments in CM2.1 and CM3 over the years 1981–2000. Each ensemble member was initialized from a different point during the 1860 radiatively forced spinups for the two respective models, with CM2.1 ensemble members initialized 40 years apart and CM3 ensemble members initialized 50 years apart. For our purposes, the
used to calculate the maximum number of nucleated drops. The equation represents the formulation of the activation source term in the cloud drop prognostic equation. Model configuration is similar to the fixed sea surface temperature (SST) simulation presented in Donner et al. (2011 ), except that interannual variability in boundary conditions and forcings are removed to enable shorter simulations. Interannual monthly SSTs are replaced with monthly climatologies for the period 1980
used to calculate the maximum number of nucleated drops. The equation represents the formulation of the activation source term in the cloud drop prognostic equation. Model configuration is similar to the fixed sea surface temperature (SST) simulation presented in Donner et al. (2011 ), except that interannual variability in boundary conditions and forcings are removed to enable shorter simulations. Interannual monthly SSTs are replaced with monthly climatologies for the period 1980
, therefore causing the two fields not to be coupled in the way that they are in the atmosphere. Detailed atmospheric behavior in the vicinity of the tropopause is also poorly simulated as the ozone climatology used for model forcing tends to be described as a function of pressure, rather than relative to the tropopause. As a result, cooling rates in the vicinity of the tropopause are poorly simulated in comparison with measurements ( Forster et al. 2007 ). Second, about half of the CMIP3 models
, therefore causing the two fields not to be coupled in the way that they are in the atmosphere. Detailed atmospheric behavior in the vicinity of the tropopause is also poorly simulated as the ozone climatology used for model forcing tends to be described as a function of pressure, rather than relative to the tropopause. As a result, cooling rates in the vicinity of the tropopause are poorly simulated in comparison with measurements ( Forster et al. 2007 ). Second, about half of the CMIP3 models
convective systems, and lateral entrainment for shallow cumulus relative to Donner (1993) and Bretherton et al. (2004) account for implementation issues in AM3 and simulation deficiencies using the referenced values. The Donner entrainment coefficients for deep cumulus updrafts were selected to yield cumulus vertical velocities in agreement with observations on a finer vertical grid for the cumulus updrafts than is used in AM3. Among other simulation characteristics, shortwave cloud forcing is
convective systems, and lateral entrainment for shallow cumulus relative to Donner (1993) and Bretherton et al. (2004) account for implementation issues in AM3 and simulation deficiencies using the referenced values. The Donner entrainment coefficients for deep cumulus updrafts were selected to yield cumulus vertical velocities in agreement with observations on a finer vertical grid for the cumulus updrafts than is used in AM3. Among other simulation characteristics, shortwave cloud forcing is
