TROPOSPHERIC CLOUDS IN ANTARCTICA David H. Bromwich,1,2 Julien P. Nicolas,1,2 Keith M. Hines,1 Jennifer E. Kay,3 Erica L. Key,4 Matthew A. Lazzara,5 Dan Lubin,6 Greg M. McFarquhar,7 Irina V. Gorodetskaya,8 Daniel P. Grosvenor,9,10 Thomas Lachlan‐Cope,11 and Nicole P. M. van Lipzig8 Received 1 May 2011; revised 18 October 2011; accepted 18 October 2011; published 12 January 2012. [1] Compared to other regions, little is known about clouds main topics: (1) observational methods and instruments, in Antarctica. This arises in part from the challenging (2) the seasonal and interannual variability of cloud deployment of instrumentation in this remote and harsh amounts, (3) the microphysical properties of clouds and environment and from the limitations of traditional satellite aerosols, and (4) cloud representation in global and regional passive remote sensing over the polar regions. Yet clouds numerical models. Aside from a synthesis of the existing lit- have a critical influence on the ice sheet’s radiation budget erature, novel insights are also presented. A new climatol- and its surface mass balance. The extremely low tempera- ogy of clouds over Antarctica and the Southern Ocean is tures, absolute humidity levels, and aerosol concentrations derived from combined measurements of the CloudSat and found in Antarctica create unique conditions for cloud for- Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite mation that greatly differ from those encountered in other Observation (CALIPSO) satellites. This climatology is used regions, including the Arctic. During the first decade of to assess the forecast cloud amounts in 20th century global the 21st century, new results from field studies, the advent climate model simulations. While cloud monitoring over of cloud observations from spaceborne active sensors, and Antarctica from space has proved essential to the recent improvements in cloud parameterizations in numerical mod- advances, the review concludes by emphasizing the need els have contributed to significant advances in our under- for additional in situ measurements. standing of Antarctic clouds. This review covers four Citation: Bromwich, D. H., et al. (2012), Tropospheric clouds in Antarctica, Rev. Geophys., 50, RG1004, doi:10.1029/2011RG000363. 1. INTRODUCTION tudes. As the source of precipitation, clouds are integral to the replenishment of the Antarctic Ice Sheet by snowfall and, [2] Understanding Antarctic clouds and properly repre- senting them in climate models is paramount to ensure the thus, to its state of equilibrium and contribution to global mean realism of future climate projections in high southern lati- sea level. Through their shortwave and longwave radiative properties, clouds influence the temperature of the atmo- sphere. This, in turn, can have direct effects on the cryosphere 1 Polar Meteorology Group, Byrd Polar Research Center, Ohio when temperatures reach the melting point or through State University, Columbus, Ohio, USA. 2Atmospheric Sciences Program, Department of Geography, Ohio changes in the atmospheric moisture content (Clausius‐ State University, Columbus, Ohio, USA. Clapeyron law) and their possible impact on snowfall. Sim- 3 Climate and Global Dynamics Division, National Center for ilarly, clouds exert important control over the heat and Atmospheric Research, Boulder, Colorado, USA. 4Arctic Division, Office of Polar Programs, National Science freshwater budgets of the Southern Ocean, a key component Foundation, Arlington, Virginia, USA. of the global ocean circulation and global carbon cycle. 5 Antarctic Meteorological Research Center, Space Science and Furthermore, modeling studies have shown that changes in Engineering Center, University of Wisconsin‐Madison, Madison, Wisconsin, USA. cloud properties over Antarctica may impact regions of the 6Scripps Institution of Oceanography, University of California, San globe well beyond high southern latitudes [Lubin et al., 1998]. Diego, La Jolla, California, USA. 7 [3] The first decade of the 21st century has seen the Department of Atmospheric Sciences, University of Illinois at Urbana‐Champaign, Urbana, Illinois, USA. growing realization that clouds play a critical role in the 8Department of Earth and Environmental Sciences, K.U. Leuven, climate system and that much is still unknown about their Heverlee, Belgium. 9 properties and response to climate change [e.g., Stephens, Centre for Atmospheric Science, SEAES, University of Manchester, Manchester, UK. 2005; Solomon et al., 2007; Dufresne and Bony, 2008]. 10Now at Department of Atmospheric Sciences, University of This statement is particularly true for the Antarctic region Washington, Seattle, Washington, USA. 11 given the paucity of cloud observations. Some integrated British Antarctic Survey, Cambridge, UK. Copyright 2012 by the American Geophysical Union. Reviews of Geophysics, 50, RG1004 / 2012 1of40 8755-1209/12/2011RG000363 Paper number 2011RG000363 RG1004 RG1004 Bromwich et al.: TROPOSPHERIC CLOUDS IN ANTARCTICA RG1004 efforts such as the Global Energy and Water Experiment [12] 4. How, and how well, are Antarctic clouds repre- (GEWEX) Cloud System Study (http://gcss‐dime.giss.nasa. sented in current climate models? (section 5). gov/) have fostered cloud‐related research and field pro- [13] The conclusion (section 6) summarizes the remaining grams. While polar clouds have been an integral component challenges and proposes future research directions. A map of this initiative, the emphasis has been largely placed on the of Antarctica (Figure 1) shows the regions and locations Arctic region, with relatively little effort in the Antarctic. referred to throughout the paper. A list of acronyms is given [4] Although commonly grouped under the same “polar” in Table 1. attribute, the Arctic and Antarctic regions exhibit stark contrasts in their respective geography, climate, and atmo- 2. OBSERVATIONAL METHODS sphere and, therefore, also in their cloud characteristics. [14] To first order, the understanding of Antarctic clouds With its extremely low temperatures and specific humidity comes from direct observations. The variety of observed and its pristine atmosphere, Antarctica represents a unique properties can in turn be applied toward empowering environment for cloud formation. Like other aspects of numerical and climate models as well as directly improving the Antarctic climate, our current knowledge of clouds in weather forecasting. Antarctic clouds can be examined from this region is mostly tied to the sparse network of staffed the point of view of the surface as well as from airborne and stations with, in particular, little insight into clouds over the space‐based platforms. Observational studies are beginning Antarctic interior. Further, the limitations of cloud obser- to capture the basic characteristics and trends that serve as ‐ vations based upon satellite passive visible infrared mea- our basis for understanding Antarctic clouds. These studies surements have represented a significant challenge to are, however, nowhere near as extensive as studies in most investigations of clouds in high southern latitudes. other regions. This section offers an overview of the [5] An overview of Antarctic clouds was given by King methods that have been and are being used to study Ant- and Turner [1997] as part of a comprehensive description arctic clouds. of the climate of Antarctica. These results were essentially based on (1) limited records of conventional cloud obser- 2.1. Surface‐Based Observations vations from Antarctic stations and (2) early versions of [15] Observations of cloud fraction, cloud type, and cloud satellite passive cloud retrievals. A recent short review by base (or cloud ceiling) are commonly reported at staffed Lachlan‐Cope [2010] has provided additional insight into stations around the Antarctic. Summer season provides the clouds’ microphysical properties (phase, size, and shape of densest data coverage, as most stations and research field cloud particles). camps are actively measuring at this time of year. Austral [6] A new era began, undoubtedly, in the early and mid‐ winter sees a reduction in visual cloud observations as sum- 2000s with the launch of active cloud sensors mounted on mer stations close, and some year‐round stations reduce the polar‐orbiting satellites. These observations have allowed observing frequency from three to six hourly periods. Visual for more reliable cloud detection over the ice sheet and observations of clouds from the surface are aided, where Southern Ocean, a three‐dimensional perspective on cloud possible, by measurements from ceilometers (Figure 2). distribution, and information about cloud microphysical These instruments employ a pulsed diode laser at near‐ properties with an unprecedented spatial coverage. infrared wavelengths to provide a vertical backscatter profile, [7] The impetus for the present review stems from the from which information about cloud height or vertical visi- International Workshop on Antarctic Clouds, held in July bility can be derived. Cloud observations are coded, per 2010 at the Byrd Polar Research Center of The Ohio State World Meteorological Organization standards [e.g., World University. The workshop participants recognized that, indeed, Meteorological Organization, 1995] and transmitted to the substantial progress had been accomplished over the last global telecommunications system for distribution. These decade but that many questions remain unanswered. As a observations are, in many cases, primarily made to support