| Summary: | Low-level cloud cover over the Southern Ocean (SO) has a significant influence on global radiative balance. Climate models have had difficulty in the past reproducing shortwave radiation reflected by these clouds because of the inability to represent low-level cloud top properties, more specifically, they underrepresent cloud top supercooled liquid water concentrations. Past in-situ and satellite studies over the region have emphasized the abundance and importance of supercooled liquid water in cold sector SO clouds, especially at or near cloud top. SO supercooled clouds contribute between 27% and 38% of the total reflected solar radiation between 40ºS and 70ºS and represent a significant source of uncertainty in global climate models. Understanding these cloud’s properties and processes is critical to accurately simulating them in the climate system. Past global climate model studies have found that replacing all mixed-phase clouds with supercooled clouds at temperatures between -35ºC and 0ºC would result in 17 W m-2 more radiation reflected back to space. Ice crystals are typically thinner for the same liquid water content present as a pure supercooled liquid cloud. This study uses collocated airborne radar, lidar, and thermodynamic data from twelve high-altitude flight legs during the SO Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) field campaign to characterize SO cold sector cloud top phase (top 96 m) and cloud precipitation properties as a function of cloud top temperature. Training datasets were developed to create probabilistic phase classifications based on High Spectral Resolution Lidar data and HIAPER Cloud Radar data. These classifications were used to identify cloud top phase. Results indicate that nearly 88.7% of the cloud tops contained liquid, 1.1% contained ice, 0.8% could not be identified based on the training datasets, and 9.4% were unknown (meaning they fell outside of the training datasets). 75.6% of cloud cover had cloud top temperatures less than 0ºC. Considering only these clouds we found that 89.9% had supercooled liquid water at cloud top, 0.4% contained ice, 0.1% could not be identified based on the training datasets and 9.6% were unknown (meaning they fell outside the training datasets). Case studies are also presented illustrating examples of low-level cold sector cloud top phase identification. Ice production in clouds with warm cloud top temperatures (greater than -5ºC) occurred in several locations during 8 research flights. Case studies showed the clear presence of a melting level even when cloud top temperatures were as warm as -3ºC. 41.0% of clouds sampled had cloud top temperatures between -5ºC and 0ºC. 8.5% of those clouds were precipitating through the 0ºC isotherm. 23.5% of clouds with cloud top temperatures between -5ºC and 0ºC that precipitated through the 0ºC isotherm had a definitive melting level present, implying that primary and/or secondary ice production was occurring. U of I Only Author requested U of Illinois access only (OA after 2yrs) in Vireo ETD system
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