In this study, wintertime in-situ observations of cloud effective radius (reff), cloud droplet number concentration (Nd) and cloud thermodynamic phase from eleven flights over the mid-latitude Southern Ocean (SO) (43–45oS, 145–148oE) are compared against retrieval products derived from the MODerate-resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations. The eleven missions were designed to make observations underneath the A-Train constellation overpasses with the collocated in-situ observations being constrained to a 30-minute window. During these eleven flights, open mesoscale cellular convection (MCC) was observed to be predominant, although closed MCC was observed during two flights. For open MCC, clouds were commonly observed to be precipitating intermittently, to be patchy in spatial extent, and to be intermittently mixed-phase. As such, these clouds were rarely ideal for satellite retrievals.
Compared to the in-situ observations of the cloud thermodynamic phase, the CALIOP cloud-top phase and MODIS cloud phase optical property (CPOP, collection 6) products are found to consistently underestimate the frequency of occurrence of mixed-phase clouds, whereas the MODIS cloud-top phase (infrared based) product shows a better qualitative agreement despite the relative large fraction of uncertain class.
Focusing on the liquid phase clouds only, the MODIS retrieved reff_2.1 (reff from the 2.1-μm channel) overestimates the in-situ reff for non-drizzling cloud samples (by ~13 μm, on average) and, to a lesser extent, for lightly drizzling samples. Conversely, reff_2.1 underestimates the in-situ observations for heavily drizzling samples by ~10 μm, on average. Overall, the MODIS reff shows much weaker variability than the in-situ observations. The overestimation of reff for non-drizzling cloud samples is much greater than that reported for stratocumulus clouds over South East Pacific from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The retrieved cloudy pixel population for the 2.1-μm channel is higher than those with 3.7-μm channel and this is outstanding in most drizzling cases. Overall, reff_3.7 was in a better agreement with the in-situ observations than reff_2.1 for non-drizzling and light drizzling conditions, but reff_2.1 was in a better agreement than reff_3.7 for heavily drizzling conditions. No specific bias was evident for MODIS retrievals made at large solar zenith angle (> 65o), although the sample size is quite limited. In the broken or patchy cloud field with a mixed presentation of ice, liquid and large drizzling particles, the large inhomogeneity and 3-D effect are expected to be the primary sources of errors for the reff retrievals, although the differences between the effective variance prescribed in the precomputed look-up table and the actual range of variability are not negligible. Ice contamination and sub-pixel variability may also be important factors for the retrievals bias.
The computed Nd based on MODIS retrievals, however, is largely consistent with the in-situ observations, regardless of the presence of drizzle. However, the Nd of the outstanding two high Nd cases featuring with closed MCC observed during the flights is highly underestimated. These biases may be associated with the overestimated reff combined with the errors in the retrieved cloud optical thickness, as well as uncertainties in the algorithms used for computing Nd.