Supersaturation and critical size of cloud condensation nuclei in marine stratus clouds

Observations of marine stratus clouds in clean air off the Californian coast reveal a functional relationship between the number of cloud condensation nuclei (CCN) and supersaturation. Satellite-derived liquid droplet density estimates the number density of CCN. Combining the estimated supersaturation using Kohler theory, global maps of supersaturation and the critical activation size of CCN are estimated. Here, we show that high supersaturation >0.5% persists over the oceans with a critical CCN size of 25-30 nm, which is smaller than the conventional wisdom of 60 nm. Independent support for such high supersaturation in the marine cloud is obtained from CCN measurements provided by the "Atmospheric Tomography Mission". Higher supersaturation implies smaller activation size for CCN making cloud formation more sensitive to changes in aerosol nucleation.

Methods
The present data set is the critical size of cloud condensation nuclei of low clouds over the oceans and the corresponding supersaturations. 
Satellite retrieval of Liquid Water Path, Optical Thickness, and cloud fraction makes it possible to estimate the average number density of warm cloud droplets over the oceans. An empirical relationship between effective supersaturation and cloud number density allows for determining the effective supersaturation in a 1x1 deg pixel. Finally, Kohler's theory and the obtained supersaturation give a critical CCN size.