MODIS Aerosol and Low-Cloud Retrievals: Orographic Effects in the Wake of Macquarie Island

Visible/infrared imagery from passive satellites is commonly relied upon to study low cloud microphysics over oceanic regions, 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 (r_e), cloud droplet number concentration (N_d) and cloud liquid water path (LWP) retrievals from the NASA Moderate Imaging Spectroradiometer (MODIS) with surface measurements collected during the Macquarie Island Cloud and 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 <65° on spatial scales of 50–100 km. However, the low overall bias in 3.7-μm band effective radius (r_{e 3.7}) retrievals partly results from cancellation of errors: r_{e 3.7} is overestimated for non-to-lightly precipitating clouds, and underestimated for heavier drizzling clouds by ∼1–1.5 μm. 1.6-μm and 2.1-μm band r_e retrievals are biased high. N_d 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 to 2020 show a distinct region of enhanced cloud cover and N_d (and lower r_e) 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.