First Field Experiment of Marine Cloud Brightening Proves and Expands the Concept

Marine cloud brightening (MCB) refers to the deliberate augmentation of atmospheric and cloud albedo through the mass production of sea spray aerosols and their release into the marine boundary layer. Cloud reflectivity enhancement follows from perturbed cloud microphysical properties caused by higher concentrations of aerosols activating to become cloud condensation nuclei (CCN). The first indirect effect of additional CCN on cloud is for a given liquid water content (LWC) an increase in cloud droplet number concentration (N_d) and reduction in cloud drop effective radius (r_e). Although various natural and anthropogenic analogues have been studied, and a great many modelling efforts undertaken, a lack of in-situ experimentation means the concept has remained entirely theoretical. Here we show that cumulus clouds forming in a relatively clean marine airmass over the Great Barrier Reef respond with increased N_d and decreased r_e for two distinctly different size spectra of sea salt aerosol generated onboard a vessel. Further, we observed the increase in N_d to be near constant with LWC suggesting that the high hygroscopicity of the additional aerosols may inhibit full evaporation at low LWC, potentially contributing an additional radiative forcing effect. Our results confirm the concept that cloud microphysical properties can be deliberately modified through the first aerosol indirect or “Twomey” effect by the artificial production of sea spray. We demonstrate the idea is not just applicable to marine stratocumulus clouds, and hypothesize there may be an additional indirect forcing effect from swollen sea spray aerosols in regions of high but sub-activation-level humidity around and within the cloud.

Figure captions: A: Drop concentrations of control (blue) and seeded (red) 100-m cloud segments as a function of liquid water content (LWC) at a height of 1200 m in an MCB-seeded case. The seeding adds similar number of drops regardless of LWC. B: Cloud drop effective radius (re) of control (blue) and seeded (red) for the same loud segments. The control re has little changes with LWC, as in extreme inhomogeneously mixed cloud. The seeded re decreases with less re. This suggests that the seeded cloud droplets are not evaporating completely with cloud mixing due to their hygroscopicity, thus adding haze to the cloud brightening.