Shipboard observational evidence of supercooled liquid water clouds in the mid-troposphere over the Southern Ocean.

Clouds are crucial to regulating incoming and outgoing radiation over the ocean. However, there are still large discrepancies between climate models and observations, particularly over the Southern Ocean, which plays a major role in global climate projections. Using shipboard ceilometer and radiometer data, we examine the relationship between the cloud-base temperature and the occurrence frequency of supercooled liquid water (SLW) clouds. Our results indicate that SLW clouds dominate as thin (< 200 m thick) altocumulus in the mid-troposphere with a fraction of about 95%, even under cloud-base temperatures of less than [Formula: see text] C. These clouds are therefore difficult to reproduce in models with coarse vertical resolution. An underestimation of surface downward longwave radiation was found in atmospheric reanalysis due to the misrepresentation of the mid-tropospheric clouds near 4 km where phase transitions occur. Based on air mass backward trajectory analysis, the clouds associated with the local transport during the non-cyclone period seemed essential for the surface radiation budget.