Extreme Events at Summit, Greenland: 3 days, 3 years, and 33 summers

The melt extent of the Greenland Ice Sheet (GIS) has been increasing in recent decades. Satellite observations show that the extent of superficial ice melt was anomalously high at 97% aerial extent in mid-July 2012, including rare melting at high altitude locations such as Summit Station. The atmospheric conditions during this event were measured by ground-based remote sensors and radiosoundings from Summit Station that were collected by the ICECAPS (Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit) project. The observational data recorded increased variability in the atmospheric state for July 2012 compared to the July 2010 and 2011. The conditions on 11 July 2012 produced temperatures high enough to cause surface melting at Summit. Temperature retrievals from a microwave radiometer (MWR) observed a warm front arriving on 10 July 2012. Elevated precipitable water vapor levels suggest suitable conditions for liquid water clouds. A low level liquid cloud was observed by a millimeter cloud radar (MMCR) from 10-12 July 2012 with elevated liquid water content retrieved from the MWRs. Observations from an infrared spectrometer indicate increased downwelling longwave fluxes during this time. The liquid water cloud effectively traps the heat thus limiting the surface's ability to cool radiatively. Surface-based inversions are common when the sky is clear, but liquid bearing clouds often weaken these inversions. In combination with solar heating, this can create surface temperatures warmer than the air aloft. To gain insight into the climatological context for such conditions, the 33 year record from ERA-Interim is used. First, ERA-Interim temperature and moisture profiles are validated over the three summers during which the ICECAPS measurements were recorded at Summit Station. Then, using the ERA-Interim data, trends in the summer means and variability are investigated over a 33-year period with attention given to years with high variability. Finally, the atmospheric mechanisms that lead to warm events in previous summers are compared to the factors that lead to the extremely warm surface temperatures in July 2012.