Causes of variability in the summertime Antarctic boundary-layer climate

Abstract

A high-resolution one-dimensional atmospheric model is used to assess the contribution of various surface characteristics and external forcings on the structure and dynamics of the atmospheric boundary layer (ABL) over the Antarctic Plateau in summer. The reference run simulates the boundary layer over a mildly sloping surface (1.5 m km-1) for a clear sky near the end of the Antarctic summer (31 January-3 February). The ABL depth is approximately 100 m. At night, a low-level jet forms due to the combined effect of katabatic forcing and an inertial oscillation. During the day a convective mixed layer is present. As expected, the ABL is very sensitive to surface slope; a larger slope forces higher wind speeds and a deeper boundary layer. Over a horizontal surface, a nocturnal jet is also found as a result of the inertial oscillation. A modest change in surface albedo alters the mixed-layer temperature and the height and strength of the nocturnal jet considerably. Rotating the large-scale wind relative to the slope direction also has a large impact on ABL depth and structure. The deepest boundary layer and largest wind speed over a northward down-sloping surface are found for an easterly (cross slope) large-scale wind, as is typical for Antarctica. A very shallow ABL with low wind speed is found for the opposite large-scale wind direction. ABL sensitivity to surface roughness was found to be small. For all experiments, the ABL sensitivity is enhanced due to the positive feedback between the cooling of the ABL and katabatic wind speed.