The Role of InCloud Wet Removal in Simulating Aerosol Vertical Profiles and Cloud Radiative Forcing
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AbstractAmong the physical processes controlling aerosol vertical profiles, incloud wet removal is of utmost importance while its representation in global climate models (GCMs) is crude. In this study, we implement into the Community Atmosphere Model version 6 (CAM6) a physicallybased aerosol wet removal parameterization scheme that explicitly treats aerosol activation, removal and resuspension. Evaluation against insitu observations shows that the default scheme substantially overestimates the upper tropospheric black carbon (BC) and sea salt mass. Our physicallybased scheme reduces BC and sea salt mass by a factor of 10 and 1,000, respectively, in better agreement with observations. Also, the new scheme slightly increases number of aerosol particles between 12nm and 4.8m in diameter, thereby mitigating the aerosol number underestimation in the default scheme. Our new scheme reduces the overestimation of coarsemode aerosol (0.54.8m) number. Overall, the aerosol property changes (mass decrease and number increase) reduce the cloud condensation nuclei (CCN) concentration at low supersaturation (i.e., 0.02% and 0.1%), and increase CCN at high supersaturations (i.e., 0.5% and 1%). Consequently, the global annual mean cloud liquid water path increases by 1.89g m2 and the ice water path increases by 0.51g m2. The global annual mean shortwave, longwave, and net cloud radiative forcing change by 1.06, 0.57, and 0.48Wm2, respectively. Further improvement is needed to reflect the real physics that the removal efficiencies for aerosol mass and number are disproportionate and to advect cloudborne (activated) aerosols for a complete aerosol lifecycle.
