Ice in Southern Ocean Clouds With Cloud Top Temperatures Exceeding −5°C

This study documents the presence of ice in stratocumulus clouds with cloud top temperatures (CTT) > −5 °C in the cold sector of extratropical cyclones over the Southern Ocean (SO) during ten SO Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) research flights. Case studies are presented showing ice signatures within clouds when CTT were between −2 and −5°C, evidenced in Doppler radar radial velocity changes observed during high-altitude flight legs as ice particles melted across the 0°C isotherm. Ice on these legs was found to contribute to precipitation 3.8% of the time from clouds with −5°C < CTT <0°C. Clouds observed with a distinct melting level on high-altitude flight legs overall had greater cloud depths, tops with higher reflectivities, and higher linear depolarization ratios, compared to clouds without a melting level. In situ flight legs were also analyzed when Himawari-8 CTT were between 0 and −5°C and the aircraft was sampling in cloud within that temperature range. It was found that 3% of clouds sampled in situ with −5°C < CTT <0°C were mixed phase with a mean number concentration of 2.35 L⁻¹ for nonspherical particles with maximum diameters >100 μm and 1.13 L⁻¹ for nonspherical particles with maximum diameters >200 μm.