Arctic Mixed-Phase Cloud Properties Derived from Surface-Based Sensors at SHEBA

Abstract

Arctic mixed-phase cloud macro- and microphysical properties are derived from a year of radar, lidar, microwave radiometer, and radiosonde observations made as part of the Surface Heat Budget of the Arctic Ocean (SHEBA) Program in the Beaufort Sea in 1997?98. Mixed-phase clouds occurred 41% of the time and were most frequent in the spring and fall transition seasons. These clouds often consisted of a shallow, cloud-top liquid layer from which ice particles formed and fell, although deep, multilayered mixed-phase cloud scenes were also observed. On average, individual cloud layers persisted for 12 h, while some mixed-phase cloud systems lasted for many days. Ninety percent of the observed mixed-phase clouds were 0.5?3 km thick, had a cloud base of 0?2 km, and resided at a temperature of ?25° to ?5°C. Under the assumption that the relatively large ice crystals dominate the radar signal, ice properties were retrieved from these clouds using radar reflectivity measurements. The annual average ice particle mean diameter, ice water content, and ice water path were 93 ?m, 0.027 g m?3, and 42 g m?2, respectively. These values are all larger than those found in single-phase ice clouds at SHEBA. Vertically resolved cloud liquid properties were not retrieved; however, the annual average, microwave radiometer?derived liquid water path (LWP) in mixed-phase clouds was 61 g m?2. This value is larger than the average LWP observed in single-phase liquid clouds because the liquid water layers in the mixed-phase clouds tended to be thicker than those in all-liquid clouds. Although mixed-phase clouds were observed down to temperatures of about ?40°C, the liquid fraction (ratio of LWP to total condensed water path) increased on average from zero at ?24°C to one at ?14°C. The observations show a range of ?25°C at any given liquid fraction and a phase transition relationship that may change moderately with season.