Dynamical Constraints on Katabatic Wind Cessation in Adélie Land, Antarctica

Abstract

The katabatic wind events observed in the coastal zone of Adélie Land, Antarctica, on 27 November and 3 December 1985 are simulated with a hydrostatic mesoscale atmospheric model coupled to a snow model. The diurnal cycle of insolation is strong. The main difference in the forcing between the two events is the large-scale wind, which is weak on 27 November and moderate on 3 December. In both cases temperature and wind are characterized by well-marked diurnal cycles. In particular, katabatic winds blow during nighttime and upslope winds during daytime. In both cases the katabatic airstream slows down progressively over the ocean. Consequently, continental air piles up and this generates a pool of cold air responsible for a pressure gradient force opposing the katabatic wind. An amplification of the slowing down results. When, in the morning, insolation increases, the surface inversion weakens but the influence of the cold air pool increases. The katabatic flow starts to decay over the coastal zone and then retreats progressively toward the ice sheet interior. When the large-scale wind is weak, the surface warming is sufficient for generating an additional upslope bouyancy force, and anabatic flow develops over the ice sheet in the afternoon. When the large-scale wind is moderate and downslope, the pilling up of cold air is important and this has a dramatic impact on the flow. A sharp spatial transition is generated between downslope and upslope winds over the ocean. This discontinuity moves toward the ice sheet interior in the morning and is responsible for the sudden cessation of the katabatic flow seen by static observers. The results of the simulations are used in order to refine a simple parameterization of Antarctic katabatic winds.