A235-09
Mechanisms of multiple, anomalous melt events at Summit Station, Greenland in summer 2019
Wednesday, 16 December 2020: 07:25
Virtual
Von Patrick Walden, Washington State University, Pullman, WA, United States, Heather Guy, University of Leeds, School of Earth & Environment, Leeds, United Kingdom, Christopher Cox, NOAA Boulder, PSL, Boulder, CO, United States, Ryan Neely, University of Leeds, Leeds, United Kingdom, William D Neff, Cooperative Institute for Research in Environmental Sciences, NOAA/PSL, Boulder, CO, United States and Matthew Shupe, CIRES/University of Colorado/NOAA PSL, Boulder, CO, United States
Abstract:
Above freezing temperatures and melting surface snow have occurred at Summit Station (3250 m above sea level), atop the Greenland Ice Sheet, only five times in the last 800 years, including once in 2012 and twice 2019 (June 12; July 29-31). Such events are linked to southerly advection of continental air masses that cross the North Atlantic as atmospheric rivers (ARs). The specific mechanisms, including advection, that are responsible for these rare events appear to be varied and complex. While the 2012 event was supported by anomalous cloud forcing caused by thin, liquid-bearing clouds, the two events in 2019 occurred under both clear and cloudy conditions. In fact, the net surface radiation measured during the 2019 events was similar between clear (~47 Wm
2) and cloudy (~52 Wm
2) conditions. Surprisingly, these values are similar to the climatological average for net surface radiation for summer afternoons at Summit Station.
Observations from the ICECAPS-ACE project at Summit Station (including radiative and turbulent fluxes, surface skin temperature, snow pit stratigraphy) allow a process-level analysis of the mechanisms that transfer energy from the ARs into local melting. When cloudy, the forcing was achieved by longwave cloud radiative effects, while under clear skies, positive forcing occurred due to lower surface albedo. Notably, on July 31, melt was supported by an albedo of ~0.8, which decreased significantly from ~0.86 over the preceding days before melt. The difference in albedo is partially attributed to differences between clear-sky and cloudy regimes, but metamorphosis of snow grains is also likely. Other potential mechanisms include lenses of refrozen meltwater that modify the atmosphere’s coupling to the firn and suppression of conductive flux by residual warmth in the subsurface snow. As the radiative forcing during the 2019 events was much less anomalous than the 2012 event, we hypothesize that preconditioning of the snow pack may be important for certain melt events.
