Effect of Aerosol on Cloud-Environment Interactions in Trade Cumulus

This study examines the role of aerosol in mediating interactions between a warm trade cumulus cloud system and the environment that spawns it. Numerical simulations of the observed and well-studied Rain in Cumulus over the Ocean (RICO) field experiment are performed. The results draw on simulations of 34-h duration so as to avoid conclusions based on transients. Simulations show that, on average, aerosol-perturbed clouds are initially deeper and more vigorous but that after about 14 h there is a reversal in this trend, and unperturbed clouds deepen relative to the perturbed clouds. Differences in cloud depth are about 100 m, and differences in vertical velocity variance are about 30%. After about 20 h, most cloud fields are statistically similar with the exception of rain rate and optical depth, which are lower and higher, respectively, in the high-aerosol conditions. By sampling the model output at various points in the cloud system evolution, the mechanisms responsible for the initial differences and then convergence of most of the cloud field properties are addressed. Sensitivity tests indicate that responses are driven primarily by temperature profiles, rather than by humidity profiles, and that the general trend to homogenization of the bulk cloud field properties is robust for different forcings. Finally, the paper shows that even transient aerosol perturbations may endure beyond the duration of the perturbation itself, provided they persist long enough. Short-duration aerosol perturbations are unlikely to have much influence on the system.