A Climatology of the Phase of Southern Ocean Clouds based on MODIS Imagery

Trenberth and Fasullo (2010) have noted “disproportionately large biases that exist in both reanalysis and global coupled models in the energy budgets of the SH that is directly linked to the simulation of clouds.” In particular, they note biases in the absorbed shortwave radiation in the clouds over the Southern Ocean. Much of this cloud cover has been observed by CloudSat (Mace et al. 2007) to be of low-level clouds (cloud tops below 3km) with a fractional cloud cover of 70-90%. Further observations (Morrison et al. 2010) suggest that such clouds commonly exist in a phase of supercooled liquid water rather than being glaciated or mixed phase. Based on the emissions of the 8.5 and 11micron channels, the MODIS instruments aboard the Aqua and Terra satellites are able to define both the cloud-top temperature (CTT) and cloud-top phase (CTP). Combined, these retrievals offer some insight in to the phase of these low-level clouds that are ubiquitous over the Southern Ocean. In this work a climatology of the CTP, as defined by MODIS, is presented for Southern Ocean clouds. This climatology is then sorted by cloud-top temperature to reveal that the majority of the clouds with cloud-tops in the range of 0 to -20°C actually remain in the liquid state and are thus supercooled. This is consistent with earlier field observations of large quantities of SLW (Ryan and King, 1997). Perhaps more surprising is that MODIS observes virtually no glaciation in this range of cloud-top temperatures. The seasonal variability of these clouds suggests a very simple shift in the cloud cover through the year. There is little glaciation observed in the low-level clouds, even during the winter at latitudes of 50-60°S. In comparison, the North Pacific is found to offer a much stronger annual cycle. In particular the clouds over the North Pacific are found to have more cloud cover at colder temperatures (mid-level cloud tops) and glaciated during the winter. In the highest latitudes, MODIS detects ample mixed phase clouds over the Bering Sea during the winter, which is not evident over the Southern Ocean. Given the ultra-pristine state of the clouds over the Southern Ocean during winter (Yum & Hudson, 2003) the MODIS imagery would be well served to be specifically validated by field observations.