Unstable near-surface boundary conditions in summer on top of the Greenland ice sheet

The surface energy balance for the interior of the Greenland ice sheet in s ummer 2000 is described. The most important feature of the energy balance i s that net all-wave radiation is positive. Peak global radiation values of 800 W m(-2) provide energy at the snow surface that results in the turbulen t fluxes of sensible and latent heat being directed into the atmosphere. He ating at the surface gives rise to unstable near-surface atmospheric condit ions, with sensible heat fluxes typically between 10-20 W m(-2). Daily mean values of the subsurface heat flux are small and directed downwards from t he surface as the snow cover warms during the summer season. Analysis-of-th e gradient Richardson number for the past three summers shows that unstable conditions are common, with frequencies between 11-43% for a 2.5 month per iod. The unstable conditions in summer appear to be important in controllin g katabatic forcing over the ice sheet.