Modelling and observations of the katabatic flow dynamics over Greenland

Abstract

The katabatic wind system over the Greenland ice sheet is studied using simulations of the hydrostaticNorwegian 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 casestudy of the aircraft-based experiment KABEG (Katabatic wind and boundary layer front experimentaround Greenland) in the area of southern Greenland in April/May 1997. Monthly mean structures andindividual contributions of the momentum budget integrated over the boundary layer are examinedfor one winter month. The NORLAM is able to simulate realistically the structures of the katabaticwind system in the lowest 400 m. The comparison with KABEG aircraft measurements for a katabaticwind case with strong synoptic forcing shows good agreement for the momentum budget terms. Thepure katabatic force represents the main mechanism for the boundary layer wind field, but a considerableinfluence of the large-scale synoptic forcing is found as well. Acceleration components from theNORLAM forecasts are also presented for the whole month of January 1990. The monthly mean fieldsshow significant regional differences because of different inversion strengths and synoptic forcings. Inparticular, Southeast Greenland is influenced by transient synoptic cyclones and the associated cloudpatterns. All other areas of the slopes of the Greenland ice sheet are characterized by a downslopekatabatic acceleration. The pressure gradient force over the northwestern part of the Greenland icesheet points in the direction of the local katabatic force, which explains the relatively strong monthlymean 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 pressuregradient is even opposed to the katabatic force in some regions.