Satellite retrieval of cloud condensation nuclei concentrations in marine stratocumulus by using clouds as CCN chambers

Abstract

A new methodology for the satellite retrieval of cloud condensation nuclei (CCN) in shallow marine boundary layer clouds is developed and validated in this study. The methodology is based on retrieving cloud base drop concentrations (N d) and updrafts (W b), which are used for calculating the supersaturation (S). The N d is then defined as the CCN at that S. The accuracy of the satellite retrievals was validated against ship-borne measurements of CCN done in recent campaigns in the Southern Oceans (ACE-SPACE, MARCUS, & PEGASO [2015-2018]) and in the subtropics (MAGIC [2012-2013]). The satellite-retrieved N d and S at cloud base were validated against the measured CCN at sea surface. The main findings show that (a) coupled clouds have good agreement between satellite retrievals and ship measurements of CCN; (b) the best agreement is achieved when using the brightest 10% of the clouds and accounting for their adiabatic fraction; (c) most of the decoupled clouds had much lower CCN than were present at the underlying surface. This means that most CCN in the coupled clouds originate from the surface and not from the free troposphere. This study validates the satellite retrievals of CCN and allows us to further quantify the relationships between CCN and cloud microphysical properties. Plain Language Summary Invisibly small particles serve as sites for condensation of cloud droplets; they are called cloud condensation nuclei (CCN). In this study we present a newly developed satellite methodology to quantify these CCN particles which are the basis for the development of low-level oceanic clouds. This method was validated against near-surface measurements of particles from ships in the Southern Oceans, close to Antarctica and in the east mid-Pacific Ocean between California and Hawaii. The results show that this method works best for the brightest clouds that are coupled with the surface, that is, forming by air that originates at the sea surface. The rest of the clouds are formed with mostly lower drop concentration due to smaller CCN concentrations than at the surface. This indicates that most of the CCN particles in the coupled clouds originate from the sea surface and not from above the clouds. This validation of the satellite retrievals allows us to further quantify the relationships between particles in the atmosphere and cloud microphysical properties.

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