Visible/infrared imagery from passive satellites is commonly relied upon to study low cloud microphysics over ocean, including for the Southern Ocean (SO), but relatively little validation has been undertaken for the SO. In this article, we compare low-cloud effective radius (re), cloud droplet number concentration (Nd) and cloud liquid water path (LWP) retrievals from the NASA Moderate Imaging Spectroradiometer (MODIS) with surface measurements collected during the Macquarie Island Cloud & Radiation Experiment (MICRE). MODIS 3.7-μm band retrievals show little bias and moderately good correlation relative to MICRE retrievals for liquid-phase low clouds when restricted to Solar Zenith Angles < 65o on spatial scales of 50 to 100 km. However, the low overall bias in 3.7-μm band effective radius (re 3.7) retrievals partly results from cancellation of errors: re 3.7 is overestimated for non-to-lightly precipitating clouds and underestimated for heavier drizzling clouds by ~1 to 1.5 μm. In contrast, 1.6-μm, and 2.1-μm band re retrievals are biased high. Nd may likewise be slightly under- or overestimated depending on the concentration, but there is insufficient data to provide confidence in this result. Interestingly, a composite of MODIS retrievals from 2002-2020 shows a distinct region of enhanced cloud cover and Nd (and lower re) in the wake of Macquarie Island associated with orographic cloud formation. MODIS retrievals of aerosol optical depth (AOD) and Angstrom Exponent (AE) upwind and downwind of the island do not differ significantly. Comparison with MICRE measurements suggests that MODIS Collection-6 AOD retrievals are reasonable, while AE is problematically large.

Shallow cloud decks residing in or near the boundary layer cover a large fraction of the Southern Ocean (SO) and play a major role in determining the amount of shortwave radiation reflected back to space from this region. In this article, we examine the macrophysical characteristics and thermodynamic phase of low clouds (tops < 3 km) and precipitation using ground-based ceilometer, depolarization lidar and vertically-pointing W-band radar measurements collected during the Macquarie Island Cloud and Radiation Experiment (MICRE) from April 2016-March 2017. During MICRE, low clouds occurred ~65% of the time on average (slightly more often in austral winter than summer). About 2/3 of low clouds were cold-topped (temperatures < 0°C); these were thicker and had higher bases on average than warm-topped clouds. 83-88% of cold-topped low clouds were liquid phase at cloud base (depending on the season). The majority of low clouds had precipitation in the vertical range 150 to 250 meters below cloud base, a significant fraction of which did not reach the surface. Phase characterization is limited to the period between April 2016 and November 2016. Small-particle (low-radar-reflectivity) precipitation (which dominates precipitation occurrence) was mostly liquid below-cloud, while large-particle precipitation (which dominates total accumulation) was predominantly mixed/ambiguous or ice phase. Approximately 40% of cold-topped clouds had mixed/ambiguous or ice phase precipitation below (with predominantly liquid phase cloud droplets at cloud base). Below-cloud precipitation with radar reflectivity factors below about -10 dBZ were predominantly liquid, while reflectivity factors above about 0 dBZ were predominantly ice.