Explicit simulations of aerosol physics in a cloud-resolving model: a sensitivity study based on an observed convective cloud

[摘要] The role of convection in introducing aerosols and promoting the formationof new particles to the upper troposphere has been examined using acloud-resolving model coupled with an interactive explicit aerosol module. Abaseline simulation suggests good agreement in the upper troposphere betweenmodeled and observed results including concentrations of aerosols indifferent size ranges, mole fractions of key chemical species, andconcentrations of ice particles. In addition, a set of 34 sensitivitysimulations has been carried out to investigate the sensitivity of modeledresults to the treatment of various aerosol physical and chemical processesin the model. The size distribution of aerosols is proved to be an importantfactor in determining the aerosols' fate within the convective cloud.Nucleation mode aerosols (here defined by 0≤d≤5.84 nm) arequickly transferred to the larger modes as they grow through coagulation ofaerosols and condensation of H₂SO₄. Accumulation mode aerosols(here defined by d≥31.0 nm) are almost completely removed bynucleation (activation of cloud droplets) and impact scavenging. However, asubstantial part (up to 10% of the boundary layer concentration) of theAitken mode aerosol population (here defined by 5.84 nm≤d≤31.0 nm)reaches the top of the cloud and the free troposphere. These particlesmay continually survive in the upper troposphere, or over time form icecrystals, both that could impact on the atmospheric radiative budget. Thesensitivity simulations performed indicate that critical processes in themodel causing a substantial change in the upper tropospheric numberconcentration of Aitken mode aerosols are coagulation of aerosols,condensation of H₂SO₄, nucleation scavenging, nucleation ofaerosols and the transfer of aerosol mass and number between differentaerosol bins. In particular, for aerosols in the Aitken mode to grow to CCNsize, coagulation of aerosols appears to be more important than condensationof H₂SO₄. Less important processes are dry deposition, impactscavenging and the initial vertical distribution and concentration ofaerosols. It is interesting to note that in order to sustain a vigorousstorm cloud, the supply of CCN must be continuous over a considerably longtime period of the simulation. Hence, the treatment of the growth ofparticles is in general much more important than the initial aerosolconcentration itself.

[学科分类] 大气科学