July 2012 Greenland melt extent enhanced by low-level liquid clouds

Abstract

Melting of the world’s major ice sheets can affect human and environmental conditions by contributing to sea-level rise. In July 2012, an historically rare period of extended surface melting was observed across almost the entire Greenland ice sheet1,2, raising questions about the frequency and spatial extent of such events. Here we show that low-level clouds consisting of liquid water droplets (‘liquid clouds’), via their radiative effects, played a key part in this melt event by increasing near-surface temperatures. We used a suite of surface-based observations3, remote sensing data, and a surface energy-balance model. At the critical surface melt time, the clouds were optically thick enough and low enough to enhance the downwelling infrared flux at the surface. At the same time they were optically thin enough to allow sufficient solar radiation to penetrate through them and raise surface temperatures above the melting point. Outside this narrow range in cloud optical thickness, the radiative contribution to the surface energy budget would have been diminished, and the spatial extent of this melting event would have been smaller. We further show that these thin, low-level liquid clouds occur frequently, both over Greenland and across the Arctic, being present around 30–50 per cent of the time3,4,5,6. Our results may help to explain the difficulties that global climate models have in simulating the Arctic surface energy budget7,8,9, particularly as models tend to under-predict the formation of optically thin liquid clouds at supercooled temperatures6—a process potentially necessary to account fully for temperature feedbacks in a warming Arctic climate.

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Figure 1: Observed and simulated temporal evolution of the July 2012 surface melting event at Summit.
Figure 2: Observed spatial distribution of clouds over Greenland on 11 July 2012 and their effect on surface temperature.
Figure 3: Frequency of occurrence of thin, liquid-bearing clouds.

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Acknowledgements

ICECAPS is supported by the US National Science Foundation (grants ARC-0904152, 0856773 and 0856559) as part of the Arctic Observing Network (AON) programme. Two-metre temperature and radar measurements were provided by the NOAA Earth System Research Laboratory. Lidar and ceilometer measurements were provided by the Department of Energy Atmospheric Radiation Measurement programme. We thank the extended ICECAPS team of scientists, engineers, students and field technicians for obtaining high-quality atmosphere and cloud observations at Summit.

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Contributions

R.B. conceived this study, developed the surface temperature model, and performed most of the data analysis; M.D.S. coordinated ICECAPS measurement streams and contributed to interpretation of cloud-surface interactions and Arctic cloud context; D.D.T. performed the physical retrievals to derive precipitable water vapour and liquid water path from the microwave radiometers, and analysed the cloud height distributions over Summit for the three July periods; V.P.W. served as Principal Investigator for the ICECAPS project, and oversaw the calculation of longwave downwelling clear-sky and all-sky fluxes; K.S. provided the broadband radiative flux observations; C.J.C. calculated longwave downwelling clear-sky fluxes from radiosonde observations and all-sky fluxes from spectrally highly resolving infrared observations; M.S.K. provided support developing multi-frequency microwave radiometer retrievals and retrieving/analysing operational datasets; N.B.M. provided ceilometer derived cloud fraction calculations and collected various operational datasets; and C.P. helped operate the radar, and helped identify the MODIS data and other observational datasets.

Corresponding author

Correspondence to R. Bennartz.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data: 1) Observational data and information flow overview; 2) Detailed description of the prognostic surface energy balance model a) Parameterization of temperature Profile in the surface layer b) Representation of physical processes c) Parameter choices i) Atmospheric heat fluxes and relaxation time scale ii) Ground heat flux relaxation time scale iii) Advection d) Simplifications and assumptions made; 3) Parameterization of radiative fluxes at the surface a) Longwave fluxes b) Shortwave fluxes; 4) Additional information on observations at Summit a) Cloud fraction statistics at Summit b) Liquid water path statistics c) Comparison of observed and simulated fluxes; 5) Comparison of simulated and observed surface temperatures for additional months; 6) Information on additional observational sites; 7) Acronyms and Symbols a) Acronyms b) Symbols 8) Additional References. This file also contains Supplementary Figures 1-10, Supplementary Tables 1-3 and additional references. (PDF 2082 kb)

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Bennartz, R., Shupe, M., Turner, D. et al. July 2012 Greenland melt extent enhanced by low-level liquid clouds. Nature 496, 83–86 (2013). https://doi.org/10.1038/nature12002

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