Invisible Ship Tracks Produce Mean-State Cloud Microphysics Perturbation in Tropical Trade Cumulus

Aerosol effects in cumulus-topped boundary layers remain relatively unconstrained and drive uncertainty in climate modeling. We assess the impacts of shipping aerosol on trade cumulus, combining satellite retrievals and large eddy simulations (LES). Over the Bay of Bengal shipping lane, sparse cumulus cloud fields rarely produce visually identifiable plume features embedded in clouds. That is, the ship tracks are generally “invisible.” Nonetheless, we show using MODIS Aqua that this shipping lane produces a shift in the probability distribution of cloud droplet number (N_d), constituting a strong mean-state perturbation in trade cumulus composed entirely of invisible ship tracks. Liquid water path and cloud fraction adjustments are undetectable. An LES ensemble of the shipping lane shows similar sensitivity of N_d and lack of consistent domain-average adjustments. We also discuss the limitations of using aerosol mass in ACI parameterizations, and evaluate the radiative response to ship emissions and fuel regulations.