Global simulations of aerosol processing in clouds

[摘要] An explicit and detailed representation of in-droplet and in-crystalaerosol particles in stratiform clouds has been introduced in the globalaerosol-climate model ECHAM5-HAM. The new scheme allows an evaluation of the cloudcycling of aerosols and an estimation of the relative contributions of nucleation andcollision scavenging, as opposed to evaporation of hydrometeors in the global aerosolprocessing by clouds. On average an aerosol particle is cycled through stratiform clouds 0.5 times.The new scheme leads to important changes in the simulated fraction of aerosol scavenged in clouds,and consequently in the aerosol wet deposition. In general, less aerosol is scavenged into clouds with thenew prognostic treatment than what is prescribed in standard ECHAM5-HAM. Aerosol concentrations, size distributions,scavenged fractions and cloud droplet concentrations are evaluated and compared to different observations.While the scavenged fraction and the aerosol number concentrations in the marine boundary layer are wellrepresented in the new model, aerosol optical thickness, cloud droplet number concentrations in themarine boundary layer and the aerosol volume in the accumulation and coarse modes over the oceans areoverestimated. Sensitivity studies suggest that a better representation of below-cloud scavenging,higher in-cloud collision coefficients, or a reduced water uptake by seasalt aerosols couldreduce these biases.

[学科分类] 大气科学