Cloud responses to surface-based sources of aerosol perturbation depend
in part on the characteristics of the aerosol transport to cloud base
and the resulting spatial and temporal distribution of aerosol. However,
interactions among aerosol, cloud, and turbulence processes complicate
the prediction of this aerosol transport and can obscure diagnosis of
the aerosols' effects on cloud and turbulence properties. Here,
scenarios of plume injection below a marine stratocumulus cloud are
modeled using large eddy simulations coupled to a prognostic bulk
aerosol and cloud microphysics scheme. Both passive plumes, consisting
of an inert tracer, and active plumes are investigated, where the latter
are representative of saltwater droplet plumes such as have been
proposed for marine cloud brightening. Passive plume scenarios show a
spurious in-plume cloud brightening due solely to the connections
between updrafts, cloud condensation, and scalar transport. Numerical
sensitivities are first assessed to establish a suitable model
configuration. Then sensitivity to particle injection rate is
investigated. Trade-offs are identified between the number of injected
particles and the suppressive effect of droplet evaporation on plume
loft and spread. Furthermore, as the in-plume brightening effect does
not depend significantly on injection rate given a suitable definition
of perturbed versus unperturbed regions of the flow, plume area is a key
controlling factor on the overall cloud brightening effect of an aerosol
perturbation.