Impact of aerosols on ice crystal size
2018
Abstract. The interactions between aerosols and ice clouds represent one of the largest
uncertainties in global radiative forcing from pre-industrial time to the
present. In particular, the impact of aerosols on ice crystal effective
radius ( R ei ), which is a key parameter determining ice clouds'
net radiative effect, is highly uncertain due to limited and conflicting
observational evidence. Here we investigate the effects of aerosols on
R ei under different meteorological conditions using 9-year
satellite observations. We find that the responses of R ei to
aerosol loadings are modulated by water vapor amount in conjunction with
several other meteorological parameters. While there is a significant
negative correlation between R ei and aerosol loading in moist
conditions, consistent with the “Twomey effect” for liquid clouds, a strong
positive correlation between the two occurs in dry conditions. Simulations
based on a cloud parcel model suggest that water vapor modulates the relative
importance of different ice nucleation modes, leading to the opposite aerosol
impacts between moist and dry conditions. When ice clouds are decomposed into
those generated from deep convection and formed in situ, the water vapor
modulation remains in effect for both ice cloud types, although the
sensitivities of R ei to aerosols differ noticeably between them
due to distinct formation mechanisms. The water vapor modulation can largely
explain the difference in the responses of R ei to aerosol
loadings in various seasons. A proper representation of the water vapor
modulation is essential for an accurate estimate of aerosol–cloud radiative
forcing produced by ice clouds.
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