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

by A. M. L. Ekman, C. Wang, J. Wilson, J. Ström
Atmospheric Chemistry and Physics ()

Abstract

The role of convection in introducing aerosols and promoting the formation\nof new particles to the upper troposphere has been examined using\na cloud-resolving model coupled with an interactive explicit aerosol\nmodule. A baseline simulation suggests good agreement in the upper\ntroposphere between modeled and observed results including concentrations\nof aerosols in different size ranges, mole fractions of key chemical\nspecies, and concentrations of ice particles. In addition, a set\nof 34 sensitivity simulations has been carried out to investigate\nthe sensitivity of modeled results to the treatment of various aerosol\nphysical and chemical processes in the model. The size distribution\nof aerosols is proved to be an important factor in determining the\naerosols' fate within the convective cloud. Nucleation mode aerosols\n(here defined by 0<=d<=5.84 nm) are quickly transferred to the larger\nmodes as they grow through coagulation of aerosols and condensation\nof H2SO4. Accumulation mode aerosols (here defined by d>=31.0 nm)\nare almost completely removed by nucleation (activation of cloud\ndroplets) and impact scavenging. However, a substantial part (up\nto 10% of the boundary layer concentration) of the Aitken mode aerosol\npopulation (here defined by 5.84 nm<=d<=31.0 nm) reaches the top\nof the cloud and the free troposphere. These particles may continually\nsurvive in the upper troposphere, or over time form ice crystals,\nboth that could impact on the atmospheric radiative budget. The sensitivity\nsimulations performed indicate that critical processes in the model\ncausing a substantial change in the upper tropospheric number concentration\nof Aitken mode aerosols are coagulation of aerosols, condensation\nof H2SO4, nucleation scavenging, nucleation of aerosols and the transfer\nof aerosol mass and number between different aerosol bins. In particular,\nfor aerosols in the Aitken mode to grow to CCN size, coagulation\nof aerosols appears to be more important than condensation of H2SO4.\nLess important processes are dry deposition, impact scavenging and\nthe initial vertical distribution and concentration of aerosols.\nIt is interesting to note that in order to sustain a vigorous storm\ncloud, the supply of CCN must be continuous over a considerably long\ntime period of the simulation. Hence, the treatment of the growth\nof particles is in general much more important than the initial aerosol\nconcentration itself.

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