NSSL, partners: Thin, low Arctic clouds played an important role in the massive 2012 Greenland ice melt

Thin low clouds over Greenland caused unusual melting.
Thin low clouds over Greenland caused unusual melting.

Better understanding of Arctic clouds will help improve climate and weather forecasts

Clouds over the central Greenland Ice Sheet last July were “just right” for driving surface temperatures there above the melting point, according to a new study by scientists at NOAA and the Universities of Wisconsin, Idaho and Colorado. The study, published today in Nature, found that thin, low-lying clouds allowed the sun’s energy to pass through and warm the surface of the ice, while at the same time trapping heat near the surface of the ice cap. This combination played a significant role in last summer’s record-breaking melt.

“Thicker cloud conditions would not have led to the same amount of surface warming,” said Matthew Shupe, research meteorologist with NOAA’s Cooperative Institute for Research in Environmental Sciences at the University of Colorado and the NOAA Earth System Research Laboratory. “To understand the region’s future, you’ll need to understand its clouds. Our finding has implications for the fate of ice throughout the Arctic.”

Scientists around the world are trying to understand how quickly Greenland is warming because ice melt there contributes to sea level rise globally. The Greenland Ice Sheet is second only to Antarctica in ice volume. In July, more than 97 percent of the Greenland Ice Sheet surface experienced some degree of melting, including at the National Science Foundation’s Summit Station, high atop the ice sheet. According to ice core records, the last time the surface at Summit experienced any degree of melting was in 1889, but it is not known whether this extended across the entire ice sheet.

To investigate whether clouds contributed to, or counteracted, the surface warming that melted the ice, the authors modeled the near-surface conditions. The model was based on observations from a suite of sophisticated atmospheric sensors operated as part of a study called the Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit.

“The July 2012 ice melt was triggered by an influx of unusually warm air sweeping in from North America, but that was only one factor,” said David Turner, research meteorologist with the NOAA National Severe Storms Laboratory and one of the lead investigators. “In our paper, we show that low-lying clouds containing a low amount of condensed water were instrumental in pushing surface air temperatures up above freezing and causing the surface ice to melt.”

Clouds can cool the surface by reflecting solar energy back into space, and can warm it by radiating heat energy back down to the surface. The balance of those two processes depends on many factors, including wind speed, turbulence, humidity and cloud “thickness,” or liquid water content.

In certain conditions, these clouds can be thin enough to allow some solar radiation to pass through, while still “trapping” infrared radiation at ground level. That is exactly what happened last July: the clouds were just right for maximum surface warming. Thicker clouds would have reflected away more solar radiation; thinner ones couldn’t have trapped as much heat, and in either of those cases, there would have been less surface warming.

The researchers also found these thin, low-lying liquid clouds occur 30 to 50 percent of the time in summer, both over Greenland and across the Arctic. Current climate models tend to underestimate their occurrence in the Arctic, which limits those models’ ability to predict how clouds and their warming or cooling effects may respond to climate change.

“The cloud properties and atmospheric processes observed with the Summit Station instrument array provide a unique dataset to answer the large range of scientific questions we want to address,” said Turner. “Clouds play a big role in the surface mass and energy budgets over the Greenland Ice Sheet. Melting of the world’s major ice sheets can significantly impact human and environmental conditions via its contribution to sea-level rise.”

Better understanding of clouds also improves climate and weather models.

“Our results may help to explain some of the difficulties that current global climate models have in simulating the Arctic surface energy budget, including the contributions of clouds,” said Ralf Bennartz, lead author for the study and professor at the University of Wisconsin-Madison. “Above all, this study highlights the importance of continuous and detailed ground-based observations over the Greenland Ice Sheet and elsewhere. Only such detailed observations will lead to a better understanding of the processes that drive Arctic climate.”

NOAA’s mission is to understand and predict changes in the Earth’s environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources.

Contact:

Keli Pirtle   405-325-6933

keli.pirtle@noaa.gov

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High and Dry – Probing Greenland’s Atmosphere and Clouds

ICECAPS1

– by Matthew Shupe (Cooperative Institute for Research in Environmental Studies)

High atop the Greenland Ice Sheet, cloudy skies portend warmer temperatures and higher winds.  These clouds alter the surface energy budget, diminish the strong near-surface atmospheric stability, and precipitate ice crystal to the surface.  Together these processes comprise the focus of the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit (ICECAPS) project that has been underway at Summit, Greenland since summer 2010.  Exciting initial results are rolling out, providing the first detailed look at cloud and atmosphere properties and processes over the Greenland Ice Sheet.  The action observed by the extensive, ground-based instrument suite can be followed via daily imagery available at www.esrl.noaa.gov/psd/arctic/observatories/summit.

Playing key roles in the U.S. Arctic Observing Network (AON) and the International Arctic Systems for Observing the Atmosphere (IASOA) network, ICECAPS is a collaborative project between the Universities of Colorado, Idaho, and Wisconsin, with substantial support from the National Science Foundation, the National Oceanic and Atmospheric Administration, the Department of Energy, and Environment Canada.  Principle Investigators Von Walden (University of Idaho), Matthew Shupe (ESRL/CIRES), David Turner (NSSL), and Ralf Bennartz (University of Wisconsin) lead a large team of field technicians, engineers, graduate students, and collaborators as they endeavor to make year-round measurements of the atmosphere and clouds in the extreme Greenland Ice Sheet environment.  The instrument suite, housed in a movable facility, includes highly complementary observational perspectives from microwave and infrared radiometers, lidars, radar, ceilometer, sodar, precipitation sensor, and twice-daily radiosonde profiles (see Figure1).  These measurements can be jointly used to characterize the diurnal and seasonal variability of atmospheric structure, cloud microphysical and radiative properties, and precipitation.  ICECAPS provides a new and unique observational examination of these climatically-important aspects of the ice sheet environment and will offer important context for ongoing precipitation and surface energy budget measurements at the site.

At Summit, the atmosphere is extremely dry and cold with strong near-surface static stability predominating throughout the year, particularly in winter.  This low-level thermodynamic structure, coupled with frequent moisture inversions, conveys the importance of advection for local cloud and precipitation formation.  Cloud liquid water is observed in all months of the year, even in the particularly cold and dry winter, while annual cycle observations indicate the largest atmospheric moisture amounts, cloud water contents, and snowfall occur in summer and under southwesterly flow.  Atmospheric ice crystals, or diamond dust, readily form as advecting air masses cool over the ice sheet, leading to outstanding optical displays.  Surprisingly, many of the basic structural properties of clouds observed at Summit, and particularly the low-level stratiform clouds, are very similar to their counterparts in other Arctic regions in spite of the unique environment encountered on top of the ice sheet.  The ICECAPS observations and accompanying analyses will be used to improve the understanding of key cloud–atmosphere processes and the manner in which they interact with the GIS. Furthermore, they will facilitate model evaluation and development in this data-sparse but environmentally unique region.

Related Article:  Shupe, M. D., D. D. Turner, V. P. Walden, R. Bennartz, M. Cadeddu, B. Castellani, C. Cox, D. Hudak, M. Kulie, N. Miller, R. R. Neely III, W. Neff, and P. Rowe, 2013:  High and Dry:  New observations of tropospheric and cloud properties above the Greenland Ice Sheet.  Bull. Amer. Meteor. Soc., 94, 169-186, doi:10.1175/BAMS-D-11-00249.1.

 

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