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 analysed three passive remote sensing satellite datasets, the MODIS Collection 6.1, the AVHRR CMSAF CLARA-A3, and the AVHRR PATMOS, over the SO. We validated the cloud mask, cloud top height, and cloud phase for 2015 using Level 2 data retrieved from the passive sensors with active CloudSat-CALIOP sensors. We compared the effective radius and cloud optical depth amongst the three passive sensors datasets.

This research found that there are substantial uncertainties in cloud top height, cloud optical depth, and 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 that only around two-thirds of passive sensors observations agree with active sensor observations and in the case of AVHRR PATMOS the agreement is lower. In the comparison of cloud top height, a mean absolute bias of 0.65 km (AVHRR CMSAF), 1.03 km (MODIS), and 1.31 km (AVHRR PATMOS) was observed for single-layer cloud scenes cases. This mean bias increased to 1.86 km (AVHRR CMSAF), 3.22 km (MODIS), and 3.34 km (AVHRR PATMOS) for multilayered cloud scenes. Ice phase dominates the multilayer cloud top thermodynamic phase in 2015, while liquid is the dominant top phase for single-layer cases. In general, the passive sensor and active sensor phases agree for liquid phase and ice phase except for AVHRR PATMOS, which frequently misidentified liquid phase as ice phase. In the comparison of cloud effective radius, it was observed that the disagreement between the passive sensors was greater in presence of multilayer clouds. The effective radius disagreement was largely higher for ice clouds. 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. This work highlights the areas where passive cloud retrieval algorithms over the SO could be improved.