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Are simulated aerosol-induced effects on deep convective clouds strongly dependent on saturation adjustment?

by Z. J. Lebo, H. Morrison, J. H. Seinfeld
Atmospheric Chemistry and Physics ()
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Three configurations of a bulk microphysics scheme in conjunction\nwith a detailed bin scheme are implemented in the Weather Research\nand Forecasting (WRF) model to specifically address the role of the\nsaturation adjustment assumption (i.e., condensing/evaporating the\nsurplus/deficit water vapor relative to saturation in one time step)\non aerosol-induced invigoration of deep convective clouds. The bulk\nmodel configurations are designed to treat cloud droplet condensation/evaporation\nusing either saturation adjustment, as employed in most bulk models,\nor an explicit representation of supersaturation over a time step,\nas used in bin models. Results demonstrate that the use of saturation\nadjustment artificially enhances condensation and latent heating\nat low levels and limits the potential for an increase in aerosol\nconcentration to increase buoyancy at mid to upper levels. This leads\nto a small weakening of the time- and domain-averaged convective\nmass flux (~-3%) in polluted compared to clean conditions. In contrast,\nthe bin model and bulk scheme with explicit prediction of supersaturation\nsimulate an increase in latent heating aloft and the convective updraft\nmass flux is weakly invigorated (~5%). The bin model also produces\na large increase in domain-mean cumulative surface precipitation\nin polluted conditions (~18%), while all of the bulk model configurations\nsimulate little change in precipitation. Finally, it is shown that\nthe cold pool weakens substantially with increased aerosol loading\nwhen saturation adjustment is applied, which acts to reduce the low-level\nconvergence and weaken the convective dynamics. With an explicit\ntreatment of supersaturation in the bulk and bin models there is\nlittle change in cold pool strength, so that the convective response\nto polluted conditions is influenced more by changes in latent heating\naloft. It is concluded that the use of saturation adjustment can\nexplain differences in the response of cold pool evolution and convective\ndynamics with aerosol loading simulated by the bulk and bin models,\nbut cannot explain large differences in the response of surface precipitation\nbetween these models.

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