Modelling and observations of the katabatic flow dynamics over Greenland

The katabatic wind system over the Greenland ice sheet is studied using simulations of the hydrostatic Norwegian Limited Area Model (NORLAM) and measurements of an instrumented aircraft. The structure and the dynamics of the katabatic wind over the ice sheet are investigated for a case study of the aircraft-based experiment KABEG (Katabatic wind and boundary layer front experiment around Greenland) in the area of southern Greenland in April/May 1997. Monthly mean structures and individual contributions of the momentum budget integrated over the boundary layer are examined for one winter month. The NORLAM is able to simulate realistically the structures of the katabatic wind system in the lowest 400 m. The comparison with KABEG aircraft measurements for a katabatic wind case with strong synoptic forcing shows good agreement for the momentum budget terms. The pure katabatic force represents the main mechanism for the boundary layer wind field, but a considerable influence of the large-scale synoptic forcing is found as well. Acceleration components from the NORLAM forecasts are also presented for the whole month of January 1990. The monthly mean fields show significant regional differences because of different inversion strengths and synoptic forcings. In particular, Southeast Greenland is influenced by transient synoptic cyclones and the associated cloud patterns. All other areas of the slopes of the Greenland ice sheet are characterized by a downslope katabatic acceleration. The pressure gradient force over the northwestern part of the Greenland ice sheet points in the direction of the local katabatic force, which explains the relatively strong monthly mean near surface winds over the ice. Over the southwestern and northeastern parts of Greenland, however, no significant synoptic support of the katabatic winds is present, and the synoptic pressure gradient is even opposed to the katabatic force in some regions.