Publisher: Copernicus Publications
Journal: Atmospheric Chemistry and Physics
Subjects: Chemistry, QD1-999, Physics, QC1-999
How do changes in the amount and properties of aerosol affect warm clouds?
Recent studies suggest that they have opposing effects. Some suggest that an
increase in aerosol loading leads to enhanced evaporation and therefore
smaller clouds, whereas other studies suggest clouds' invigoration. In this
study, using an axisymmetric bin-microphysics cloud model, we propose a
theoretical scheme that analyzes the evolution of key processes in warm
clouds, under different aerosol loading and environmental conditions, to
explain this contradiction.
Such an analysis of the key processes reveals a robust reversal in the trend
of the clouds' response to an increase in aerosol loading. When aerosol
conditions are shifted from superpristine to slightly polluted, the clouds
formed are deeper and have larger water mass. Such a trend continues up to
an optimal concentration (Nop) that allows the cloud to achieve a
maximal water mass. Hence, for any concentration below Nop the cloud
formed contains less mass and therefore can be considered as aerosol-limited,
whereas for concentrations greater thanNop cloud periphery
processes, such as enhanced entrainment and evaporation, take over leading
to cloud suppression. We show that Nop is a function of the
thermodynamic conditions (temperature and humidity profiles). Thus, profiles
that favor deeper clouds would dictate larger values of Nop, whereas for
profiles of shallow convective clouds, Nop corresponds to the pristine
range of the aerosol loading.
Such a view of a trend reversal, marked by the optimal concentration,
Nop, helps one to bridge the gap between the contradictory results of
numerical models and observations. Satellite studies are biased in favor of
larger clouds that are characterized by larger Nop values and therefore
invigoration is observed. On the other hand, modeling studies of cloud
fields are biased in favor of small, mostly trade-like convective clouds,
which are characterized by low Nop values (in the pristine range)
and, therefore, cloud suppression is mostly reported as a response to an increase
in aerosol loading.
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