Seasonal Variations in Drag Coefficient over a Sastrugi-Covered Snowfield in Coastal East Antarctica

The surface of windy Antarctic snowfields is subject to drifting snow, which leads to the formation of sastrugi. In turn, sastrugi contribute to the drag exerted by the snow surface on the atmosphere and hence influence drifting snow. Although the surface drag over rough sastrugi fields has been estimated for individual locations in Antarctica, its variation over time and with respect to drifting snow has received little attention. Using year-round data from a meteorological mast, seasonal variations in the neutral drag coefficient at a height of 10 m (C_{D N 10}) in coastal Adelie Land are presented and discussed in light of the formation and behaviour of sastrugi based on observed aeolian erosion patterns. The measurements revealed high C_{D N 10} values (≥ 2 × 10^{- 3}) and limited drifting snow (35% of the time) in summer (December–February) versus lower C_{D N 10} values (≈ 1.5 × 10 ^{- 3}) associated with more frequent drifting snow (70% of the time) in winter (March–November). Without the seasonal distinction, there was no clear dependence of C_{D N 10} on friction velocity or wind direction, but observations revealed a general increase in C_{D N 10} with rising air temperature. The main hypothesis defended here is that higher temperatures increase snow cohesion and the development of sastrugi just after snow deposition while inhibiting the sastrugi streamlining process by raising the erosion threshold. This increases the contribution of the sastrugi form drag to the total surface drag in summer when winds are lighter and more variable. The analysis also showed that, in the absence of erosion, single snowfall events can reduce C_{D N 10} to 1×10-3 due to the burying of pre-existing microrelief under newly deposited snow. The results suggest that polar atmospheric models should account for spatial and temporal variations in snow surface roughness through a dynamic representation of the sastrugi form drag.