Journal article

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

Lebo Z, Morrison H, Seinfeld J ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 20 (2012) pp. 9941-9964

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Three configurations of a bulk microphysics scheme in conjunction
with a detailed bin scheme are implemented in the Weather Research
and Forecasting (WRF) model to specifically address the role of the
saturation adjustment assumption (i.e., condensing/evaporating the
surplus/deficit water vapor relative to saturation in one time step)
on aerosol-induced invigoration of deep convective clouds. The bulk
model configurations are designed to treat cloud droplet condensation/evaporation
using either saturation adjustment, as employed in most bulk models,
or an explicit representation of supersaturation over a time step,
as used in bin models. Results demonstrate that the use of saturation
adjustment artificially enhances condensation and latent heating
at low levels and limits the potential for an increase in aerosol
concentration to increase buoyancy at mid to upper levels. This leads
to a small weakening of the time- and domain-averaged convective
mass flux (~-3%) in polluted compared to clean conditions. In contrast,
the bin model and bulk scheme with explicit prediction of supersaturation
simulate an increase in latent heating aloft and the convective updraft
mass flux is weakly invigorated (~5%). The bin model also produces
a large increase in domain-mean cumulative surface precipitation
in polluted conditions (~18%), while all of the bulk model configurations
simulate little change in precipitation. Finally, it is shown that
the cold pool weakens substantially with increased aerosol loading
when saturation adjustment is applied, which acts to reduce the low-level
convergence and weaken the convective dynamics. With an explicit
treatment of supersaturation in the bulk and bin models there is
little change in cold pool strength, so that the convective response
to polluted conditions is influenced more by changes in latent heating
aloft. It is concluded that the use of saturation adjustment can
explain differences in the response of cold pool evolution and convective
dynamics with aerosol loading simulated by the bulk and bin models,
but cannot explain large differences in the response of surface precipitation
between these models.

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  • Z. J. Lebo

  • H. Morrison

  • J. H. Seinfeld

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