The intricate role of clouds in the Arctic surface energy budget: A look at the recent Greenland surface melting event

In July 2012, a rare period of extended surface melting was observed covering almost the entire Greenland Ice Sheet (GIS). By combining various observational datasets and a surface energy balance model, we show that low-level, liquid water clouds, via their radiative effects, played a key role in this melt event by modulating near-surface temperatures at high elevations. At the critical surface melt time, the clouds were optically thick enough and low enough to significantly 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 and push surface temperatures above the melting point. Outside of this narrow range in cloud optical thickness the radiative contribution to the surface energy budget would have been diminished, and the extent of this melting event would have been much smaller. Such thin, low-level clouds occur frequently, both over Greenland and across the Arctic, with a frequency of occurrence around 50%. The strong sensitivity of the surface energy budget to small changes in cloud parameters provides new insights into the important role that clouds play in Arctic weather and climate. It further helps explain the difficulties that Global Climate Models have in simulating the Arctic surface energy budget, as forming and maintaining thin, liquid water clouds at supercooled temperatures has proven to be a major challenge for models at all scales.