Influences of in-cloud aerosol scavenging parameterizations on aerosol concentrations and wet deposition in ECHAM5-HAM

Abstract

A diagnostic nucleation scavenging scheme, which determines stratiform cloud scav-enging ratios for both aerosol mass and number distributions, based on cloud droplet,and ice crystal number concentrations, is introduced into the ECHAM5-HAM globalclimate model. This is coupled with a size-dependent in-cloud impaction scavenging parameterization for both cloud droplet-aerosol, and ice crystal-aerosol collisions. Sen-sitivity studies are presented, which compare aerosol concentrations, and depositionbetween a variety of in-cloud scavenging approaches, including prescribed fractions,several diagnostic schemes, and a prognostic aerosol cloud processing treatment thatpasses aerosol in-droplet and in-ice crystal concentrations between model time steps. For one sensitivity study, assuming 100% of the in-cloud aerosol is scavenged into thecloud droplets and ice crystals, the annual global mean accumulation mode numberburden is decreased by 65%, relative to a simulation with prognostic aerosol cloudprocessing. Diagnosing separate nucleation scavenging ratios for aerosol number andmass distributions, as opposed to equating the aerosol mass scavenging to the number scavenging ratios, reduces the annual global mean sulfate burden by near to 10%. Theannual global mean sea salt burden is 30% lower for the diagnostic approach, whichdoes not carry aerosol in-droplet and in-crystal concentrations between model time-steps as compared to the prognostic scheme. Implementation of in-cloud impactionscavenging reduced the annual, global mean black carbon burden by 30% for the prognostic aerosol cloud processing scheme. Better agreement with observations of blackcarbon profiles from aircraft (changes near to one order of magnitude for mixed phaseclouds),210Pb surface layer concentrations and wet deposition, and the geographicdistribution of aerosol optical depth are found for the new diagnostic scavenging as compared to prescribed ratio scavenging scheme of the standard ECHAM5-HAM.