How Greenland melts under cloudy and clear skies

Michael Gallagher , Matthew D Shupe, Von P Walden, Anne Sledd, Christopher J Cox, Heather Guy, Claire Pettersen, Ryan Neely, Andrew S Martin, Robert L. Hawley, Michael S Town, Erik Olson, Hans-Peter Marshall, Derek Pickell, Laura D. Riihimaki, Ruth Helen Mottram, Kyle Mattingly, Laura Wilson, David Clemens-Sewall, Catherine Hebson

Greenland ice loss has risen sharply since industrialization and will likely contribute 20 cm to sea level rise before 2100. On the highest emissions pathway the ice sheet could melt by the year 3000, submerging coasts by 7.2 meters. Greenland surface melt now dominates cryospheric mass loss. To constrain the driving processes of this melt, here we present high-fidelity surface observations of clouds, energy, and melt on the lower southwestern Greenland ice sheet from summer 2024. Contrasting cloudy and clear (cloud-free) conditions, we find that firn energy storage primarily increases under clouds, driven by the cloudy state's effect on surface radiation. Frequent optically thin clouds (LWP < 30 g m-2, 55% of observations) result in 26.5 W m-2 more absorbed radiation at the surface than clear conditions on average. Near solar noon, when melt is most likely, peak absorbed radiation is 80% larger under cloudy skies (63 W m-2) than under clear (35 W m-2). A co-located automatic weather station underestimates the daily mean cloud effect by 30% and overestimates the peak clear-sky flux by a factor of two at solar noon, with implications for interpreting the in situ record that anchors Greenland's surface energy budget.