This project will utilize data from the Nimbus spacecraft Infrared Interferometer Spectrometer (IRIS; 1969-71), Interferometric Monitor of Greenhouse Gases (IMG; 1996-97) and Atmospheric Infrared Sounder (AIRS; 2002-present) to derive cloud thermodynamic phase, effective particle size, and optical depth, over Antarctica and the Southern Ocean. The radiative transfer techniques used will exploit the full spectral resolution offered by these instruments. For this multidecadal climate study, the operational time periods of these instruments are fortunate in that they occur during greatly varying polarities of the Southern Annular Mode (SAM), which is now recognized as a major - and anthropogenically influenced - dynamical driving factor behind Antarctic climate change. For 1982-present, similar radiative transfer analysis will be applied to Advanced Very High Resolution Radiometer (AVHRR) data co-located with High-resolution Infrared Sounder (HIRS/2) data, to search for trends in Antarctic cloud properties that might parallel the observed trend in the SAM from a negative to a mainly positive polarity. The applicability of this project to the NRA lies in the direct relevance of cloud radiative properties, and their multidecadal trends, to not only the polar radiation balance, but also surface pressure fields over the Southern Ocean and, through related changes in precipitation, the long-term ice mass balance of the Antarctic ice sheet. This project is motivated by a hypothesis that is testable with these specific satellite data: during the early 1970s, the mainly negative polarity of the SAM yielded warmer temperatures over the Antarctic continent than exist today. This may have resulted in more frequent occurrences of liquid water cloud than are found today. Between then and now, trends in the SAM have yielded a cooling of the Antarctic continent and warming of the Antarctic Peninsula. There may be associated trends in cloud thermodynamic phase, and other microphysical properties, which have implications for the radiation balance, meteorology, precipitation, and ultimately the ice mass balance of Antarctica.
