Validation and comparison of cloud properties retrieved from passive satellites over the Southern Ocean

Abstract. The clouds over the Southern Ocean (SO) play a vital role in defining the Earth's energy budget. The cloud properties over the SO are known to be different from their Northern Hemisphere counterparts. As a result, monitoring cloud properties over the SO, including macro- and microphysical properties, is of particular interest. We compared the Level 3 products from AVHRR CMSAF, MODIS, and CALIOP for cloud top height, cloud fraction, and cloud liquid fraction. The study found that MODIS underestimated cloud top heights, while AVHRR CMSAF overestimates them when compared to Level 3 CALIOP. The overall cloud cover for MODIS was less than that of CALIOP while AVHRR CMSAF showed higher cloud cover over mid-latitudes and lower cloud cover over land in the Level 3 product. The comparison of cloud fraction for liquid clouds against CALIOP indicates that AVHRR CMSAF tends to overestimate, whereas MODIS tends to underestimate the liquid cloud fraction. The magnitude of the difference varies with the latitude and season. Following the Level 3 analysis, we investigated three passive remote sensing satellite datasets, MODIS Collection 6.1, AVHRR CMSAF CLARA-A3, and AVHRR PATMOS-x, over the SO. We validated the Level 2 cloud mask, the cloud top height, and the cloud phase for 2015 retrieved from the passive sensors with active CloudSat-CALIOP sensors. We compared the effective radius and cloud optical depth amongst the three passive sensor datasets. This research found that there are substantial uncertainties in the cloud top height, the cloud optical depth, and the cloud thermodynamic phase, over the SO. The extent of which varies depending on the cloud property and retrieval algorithm used. The cloud mask comparison revealed varying levels of agreement between passive and active sensor observations, with a Kuiper Skill Score (KSS) of 0.71 (AVHRR CMSAF), 0.70 (MODIS), and 0.43 (AVHRR PATMOS-x). In the comparison of cloud top height, a mean absolute bias of 0.65 km (AVHRR CMSAF), 1.03 km (MODIS), and 1.27 km (AVHRR PATMOS-x) was observed for single-layer cloud scenes cases. This mean absolute bias increased to 1.87 km (AVHRR CMSAF), 3.25 km (MODIS), and 3.30 km (AVHRR PATMOS-x) for the multilayered cloud scenes. In the comparison of cloud effective radius, it was observed that the disagreement between the passive sensors was more significant in the presence of multilayer clouds. The effective radius disagreement was higher for ice clouds. Variations in ice habit assumptions, lookup table configurations, and phase classification schemes could contribute to these differences. We found that the presence of sea ice strongly influences the retrieval of cloud optical depth at high latitudes, with most passive optical depths higher over sea ice than over ocean. The new finding that contradicts previous studies may result from the difficulty of retrieving cloud optical depth over ice using the retrieval algorithms.