Cloud Radiative Forcing at Summit, Greenland

The surface energy budget plays a critical role in determining the mass balance of the Greenland Ice Sheet, which in turn has significant implications in terms of global sea-level. At Summit Station in central Greenland, the annual cycle of surface radiative fluxes is recorded using upwelling and downwelling broadband radiometers. The Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit (ICECAPS) experiment provides cloud property information, which is utilized to determine the cloud properties that affect the surface radiative fluxes. The observing capabilities include both active (radar, ceilometer, lidar, and sodar) and passive (microwave radiometer and infrared spectrometer) instruments as well as twice daily radiosondes. A clear sky model is employed to estimate the equivalent clear sky fluxes during cloudy scenes to calculate the surface cloud radiative forcing. Low-level, liquid-bearing clouds emit radiation according to their emissivity and temperature, consequently increasing the net surface longwave radiative flux and leading to surface warming. During summer this effect is modulated by the fact that clouds act to decrease the downwelling surface shortwave radiative flux, although the net shortwave flux is less affected over the snow surface with a high albedo compared to other Arctic sites. Liquid water path retrievals, based on measurements from a pair of microwave radiometers, are used to investigate the role of liquid-bearing clouds in controlling the surface radiation balance at Summit, Greenland. The seasonal cycle of the net radiative flux and surface cloud radiative forcing, in addition to the individual flux components, are reported.