Abstract. Aerosol–cloud radiative effects are determined and quantified in simulations
of deep open-cell stratocumuli observed during the VAMOS Ocean-Cloud-Atmosphere-Land
Study Regional Experiment (VOCALS-REx) campaign off
the west coast of Chile. The cloud deck forms in a boundary
layer 1.5 km deep, with cell sizes reaching 50 km in diameter. Global databases of ship
tracks suggest that these linear structures are seldom found in boundary
layers this deep. Here, we quantify the changes in cloud radiative properties
to a continuous aerosol point source moving along a fixed emission line
releasing 1017 particles per second. We show that a spatially coherent
cloud perturbation is not evident along the emission line. Yet our model
simulates an increase in domain-mean all-sky albedo of 0.05, corresponding to
a diurnally averaged cloud radiative effect of 20 W m−2, given the
annual mean solar insolation at the VOCALS-REx site. Therefore, marked
changes in cloud radiative properties in precipitating deep open cells may be
driven by anthropogenic near-surface aerosol perturbations, such as those
generated by ships. Furthermore, we demonstrate that these changes in cloud radiative properties
are masked by the naturally occurring variability within the organised cloud
field. A clear detection and attribution of cloud radiative effects to a
perturbation in aerosol concentrations becomes possible when sub-filtering of
the cloud field is applied, using the spatio-temporal distribution of the
aerosol perturbation. Therefore, this work has implications for the detection
and attribution of effective cloud radiative forcing in marine stratocumuli,
which constitutes one of the major physical uncertainties within the climate
system. Our results suggest that ships may sometimes have a substantial
radiative effect on marine clouds and albedo, even when ship tracks are not
readily visible.
Surface Area and Conductivity of Open-Cell Carbon Foams