The dynamics of idealized katabatic flow over a moderate slope and ice shelf

Authors

  • Ian A. Renfrew

    Corresponding author
    1. British Antarctic Survey, Cambridge, UK
    • Physical Sciences Division, British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
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Abstract

A non-hydrostatic numerical weather prediction model has been employed to simulate idealized katabatic flows over a moderate slope and adjoining ice shelf. The topography of Coats Land and the adjoining Brunt Ice Shelf, Antarctica, has been used; this is typical of much of the Antarctic coastline. The Regional Atmospheric Modeling System Version 4.3 has been adapted for simulations over compacted snow, most notably through changes to the multi-layer soil model. The simulations are initialized using clear-sky conditions and at rest. On the slope, a shallow katabatic flow develops, the winds becoming approximately steady on the slope by ∼12 h. The peak downslope winds are about 7 m s−1 at 30 m above the snow surface. The katabatic flow depth ranges from 50 to 100 m down the slope. Over the ice shelf the katabatic flow peters out, while a pool of cold air develops, primarily through sensible-heat loss into the surface and partially balancing the net radiative-heat loss to space. Near-surface and sounding data from the model simulations compare well with archetypal and typical katabatic flow observations, especially after some tuning of the model's turbulence parametrization.

An analysis of the downslope flow dynamics shows the buoyancy force is generally balanced by the inertial force, except towards the foot of the slope where it is balanced by upslope forces related to gradients in the potential-temperature deficit and katabatic-layer height, caused by the pool of cold air over the ice shelf. Over time, the cooling of the ice shelf boundary layer leads to an apparent retreat of the katabatic flow from the ice shelf and some way up the slope. The dynamical analysis explains the surface climatology observed, such that the persistent katabatic winds of Coats Land rarely reach the Brunt Ice Shelf. The simulated katabatic flow moves from ‘shooting’ to ‘tranquil’ towards the foot of the slope. This transition acts to trigger a train of internal gravity waves which propagate energy upwards away from the katabatic flow jump. Previous studies have also found shooting to tranquil katabatic flow transitions, so to generalize these findings would suggest that internal gravity-wave generation is ubiquitous around much of coastal Antarctica and Greenland. Copyright © 2004 Royal Meteorological Society.

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