Diagnosis of the Katabatic Wind Influence on the Wintertime Antarctic Surface Wind Field from Numerical Simulations

Abstract

Katabatic winds have long been recognized as one of the key climatic variables of the low-level Antarctic environment. Antarctic surface winds display a high degree of persistence with mean directions related to the local topographic configuration of the ice sheet, consistent with katabatic forcing. Continental orography also constrains the atmospheric boundary layer motions through blocking and cold air damming. Finally, the coastal rim about the Antarctic continent is among the most active baroclinic zones on Earth. The establishment of the low-level wind field over Antarctica is thus potentially the result of a number of interacting processes. To quantify the forcing of the wintertime surface wind field over the Antarctic continent, two numerical strategies are presented. First, idealized numerical simulations are conducted to illustrate the strong orographic control of the low-level wind field. Second, a series of daily numerical simulations using the fifth-generation Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model (MM5) has been performed for the midwinter month July 2001. The horizontal pressure gradient as depicted in MM5 was added to the standard output and an analysis was conducted to understand the forcing of the low-level wind field. Horizontal pressure gradients at the lowest sigma level (6 m above the surface) revealed a net forcing primarily down the local topographic fall line. Analyses of the katabatic forcing showed that it was a significant component of the total horizontal pressure gradient force over the interior of the continent. Near the coast and extending several hundred kilometers inland, however, effects of the ambient pressure gradient force were typically comparable to the katabatic forcing and often considerably more important. This suggests that the role of topography in shaping the Antarctic boundary layer winds through blocking and subsequent adjustment is critical to the establishment of the low-level wintertime Antarctic wind field.