Short ship-track lifetimes explain limited liquid-water path responses

Activity: Talk or presentationTalk or presentation at a conference


hip tracks provide striking illustration of aerosol-cloud interactions. They are a widely known example of an inadvertent aerosol-perturbation experiment. Such experiments are valuable for the quantification of aerosol-cloud interactions because they provide clear causation between aerosol perturbation and cloud response. At the same time, it is not clear to what extent assessments from such experiments can be generalized to constrain the climatological forcing of anthropogenic aerosol. For the liquid-water path response to aerosol perturbations in stratocumulus, this uncertainty manifests in a stark discrepancy between assessments based on ship tracks, for which a response is almost absent, as opposed to climatological satellite studies, which imply a significant reduction in liquid-water path. A recent study of a shipping lane shows an intermediate response. We demonstrate that these discrepancies can be reconciled when accounting for the specific duration of the respective aerosol perturbations.

Our analysis builds on an ensemble of ~150 large-eddy simulations of non-precipitating stratocumulus. The scope of our ensemble resembles a satellite dataset in that it samples a broad range of liquid water paths (LWP) and cloud-droplet number concentrations (N). From Gaussian-process regression we obtain an emulator for the temporal co-evolution of LWP and N. This emulator allows us to show that the LWP response to perturbations in N, dlnLWP/dlnN, becomes increasingly negative over time. It specifically approaches an asymptotic value of dlnLWP/dlnN = -0.64 with a characteristic timescale of ~20h. This timescale is longer than the typical lifetime of a ship track (~3h) but shorter than the Lagrangian lifetime of a stratocumulus deck (~48h). The limited LWP response in ship tracks consequently follows from an insufficient response time. A climatological Sc sampling, in contrast, captures a well-developed cloud response.

These timescales need to be taken into account if we are to constrain the climatological aerosol forcing based on natural, inadvertent, and targeted aerosol-perturbation experiments. Furthermore, the LWP response and its timescales have implications for the design of cloud-brightening experiments.
Period15 Dec 2020
Event titleAGU Fall Meeting 2020
Event typeConference
Degree of RecognitionInternational