PNNL

Abstract

Among the physical processes controlling aerosol vertical profiles, in-cloud wet removal is of great importance while the fidelity of its representation in global climate models (GCMs) is not well constrained. We implement into the Community Atmosphere Model version 6 (CAM6) a physically-based aerosol wet removal parameterization scheme that includes the explicit treatments of aerosol activation, removal and resuspension. Evaluations against in-situ observations show that the default scheme substantially overestimates the upper tropospheric black carbon (BC) and sea salt mass. Our new scheme reduces BC mass mixing ratio by a factor of 10 and sea salt mass concentration by a factor of 1000, in better agreement with observations. Also, the new scheme slightly increases total aerosol number and thus mitigates the underestimation of tropospheric aerosol number concentration for the aerosol size ranging from 12 nm to 4.8 um. The aerosol mass decrease and number increase leads to a decrease in the cloud condensation nucleus (CCN) concentration at low supersaturations (i.e., 0.02 % and 0.1 %) and an increase in the CCN concentration at high supersaturations (i.e., 0.5 % and 1 %). Consequently, the global annual mean cloud liquid water path increases by 1.89 g/m2 (3%) and the ice water path increases by 0.51 g/m2 (4%). The global annual mean shortwave, longwave, and net cloud radiative forcing change by -1.06, 0.57, and -0.48 W m-2, respectively. Despite promising progress gained from our new scheme, further effort to improve aerosol wet removal should consider that aerosol mass removal efficiency is larger than that of aerosol number, and construct a closed treatment of complete cloud-borne (i.e., activated) aerosol lifecycle.