Impact of cloud-borne aerosol representation on aerosol direct and indirect effects

[摘要] Aerosol particles attached to cloud droplets are much more likely to beremoved from the atmosphere and are much less efficient at scatteringsunlight than if unattached. Models used to estimate direct and indirecteffects of aerosols employ a variety of representations of such cloud-borneparticles. Here we use a global aerosol model with a relatively completetreatment of cloud-borne particles to estimate the sensitivity of simulatedaerosol, cloud and radiation fields to various approximations to therepresentation of cloud-borne particles. We find that neglecting transportof cloud-borne particles introduces little error, but that diagnosingcloud-borne particles produces global mean biases of 20% and local errorsof up to 40% for aerosol, droplet number, and direct and indirectradiative forcing. Aerosol number, aerosol optical depth and droplet numberare significantly underestimated in regions and seasons where and when wetremoval is primarily by stratiform rather than convective clouds (polarregions during winter), but direct and indirect effects are less biasedbecause of the limited sunlight there and then. A treatment that predictsthe total mass concentration of cloud-borne particles for each mode yieldssmaller errors and runs 20% faster than the complete treatment. Theerrors are much smaller than current estimates of uncertainty in direct andindirect effects of aerosols, which suggests that the treatment ofcloud-borne aerosol is not a significant source of uncertainty in estimatesof direct and indirect effects.

[学科分类] 大气科学