Controls of the relationships between satellite retrieved cloud drop number concentrations and surface CCN at the high latitude Southern Oceans

Recent measurements show extreme sensitivity to aerosols of the radiative effects of marine boundary layer clouds. The relatively large cloud drop concentrations (N_d) of these clouds in the Southern Oceans (SO) causes a large cloud cover and reflectance, which is underestimated in GCMs. Recent campaigns in the SO ( ACE-SPACE , MARCUS & PEGASO [2015-2018]) are used to relate the satellite retrieve N_d and supersaturation at cloud base (S) to the actually measured CCN(S) at sea surface. The main findings show that: (a) coupled clouds have good agreement between satellite retrievals and ship measurements of CCN(S); (b) the agreement is achieved when using the brightest 10% of the clouds, and accounting for their adiabatic fraction, as measured by aircraft; (c) most decoupled clouds had much lower CCN(S) than at the underlying surface. This means that most CCN originate from the surface and not from the free troposphere. These validate the satellite retrievals and allow us to further quantify the relationships between CCN(S) and cloud microphysical properties, thus gaining insights to the root cause of the GCM underestimates of the missing cooling.