Simulating the Greenland atmospheric boundary layer

A three-dimensional dynamic downscaling model of the Greenland atmospheric boundary layer, with a horizontal resolution of 20 km, is descibed and applied to the Greenland ice sheet for the 1998 ablation season. The model uses ECMWF analysis data fields of synoptic pressure, free atmospheric temperature, cloud cover, humidity and sea surface temperature to force the model. The model calculates the perturbation component of the temperature and pressure field to describe the atmospheric boundary layer dynamics. The aim of this study, the first of two papers, is to investigate the role of the turbulent heat fluxes in the surface energy balance of the ice sheet and their response to changes in atmospheric temperature. In this first paper, results from the simulation are compared with observations from six automatic weather stations situated on the ice sheet, three in the ablation zone and three in the accumulation zone. The comparison shows that the boundary layer model can reproduce the nearsurface meteorological variables of wind, temperature and specific humidity quite well and improve significantly on 2 m values taken directly from the ECMWF analysis. The increased spatial resolution of the model is essential in order to model accurately katabatically forced winds near the margin of the ice sheet. The calculated and observed melts at two sites in the ablation zone are also compared. At one site close to the margin, which is situated in a well drained ice region, the comparison with observations is very good, within 1%. At a higher site, where subsurface processes not included in the model are important for the total ablation, the calculated melt is 35% larger than the observed ablation