Clouds Enhance Greenland Ice Sheet Meltwater Runoff

dave_summit_greenland_73_belowNSSL’s Dave Turner has co-authored a paper entitled “Clouds enhance Greenland ice sheet meltwater runoff,” which appears in Nature Communications this month. The research is an international effort, which has been coordinated by KU Leuven in Belgium.

The study was conducted using satellite observations over Greenland from 2007 to 2010. With these observations, researchers were able to determine that cloud cover directly impacts the melting rate of the Greenland ice sheet. Previously, there had been no calculation of snow and ice loss related to clouds, and theoretical climate models do not agree its significance.

How was NSSL involved? Dave Turner, a Research Scientist with NSSL’s Forecast Research & Development Division, acted as a Principal Investigator on the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit project. This project, funded primarily by the National Science Foundation, with additional support from NOAA and the Department of Energy, deployed an advanced atmospheric research station at Summit (in the middle of the Greenland ice sheet) in June 2010, and has since been collecting data continuously. Dr. Turner was involved in the derivation of the microphysical properties of the clouds above Summit and offered his expertise in radiative transfer to help interpret study results.

“We have known for some time that clouds that contain liquid water have a big impact on the energy balance at the surface, and hence on the amount of melt of the ice sheet,” Dr. Turner said. “However, this study showed that clouds that contain only ice have about the same impact on the surface melt over Greenland as the liquid-bearing clouds, which is surprising.”

This research is noteworthy because it promotes understanding of the properties and evolution of the Arctic atmosphere, a significant scientific challenge that NOAA is helping to address. In addition, the ICECAPS observations at Summit are being used to examine the properties and evolution of stable boundary layer. Very stable boundary layers are difficult to properly represent in numerical weather prediction models, and we are hopeful this research will improve our knowledge.

Overall, the study brings to light the importance of clouds in climate modeling. For accurate meltwater estimates, it is crucial to consider cloud cover. Future climate projections will be far more reliable when these impacts are understood.

For the full press release from KU Leuven, click here.

 

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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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