Brief communication: Two well-marked cases of aerodynamic adjustment of sastrugi

In polar regions, sastrugi are a direct manifestation of drifting snow and form the main surface roughness elements. In turn, sastrugi alter the generation of atmospheric turbulence and thus modify the wind field and the aeolian snow mass fluxes. Little attention has been paid to these feedback processes, mainly because of experimental difficulties. As a result, most polar atmospheric models currently ignore sastrugi over snow-covered regions. This paper aims at quantifying the potential influence of sastrugi on the local wind field and on snow erosion over a sastrugi-covered snowfield in coastal Ad,lie Land, East Antarctica. We focus on two erosion events during which sastrugi responses to shifts in wind direction have been interpreted from temporal variations in drag and aeolian snow mass flux measurements during austral winter 2013. Using this data set, it is shown that (i) neutral stability, 10aEuro-m drag coefficient (C-DN10) values are in the range of 1.3-1.5 x 10(-3) when the wind is well aligned with the sastrugi, (ii) as the wind shifts by only 20-30A degrees away from the streamlined direction, C-DN10 increases (by 30-120aEuro-%) and the aeolian snow mass flux decreases (by 30-80aEuro-%), thereby reflecting the growing contribution of the sastrugi form drag to the total surface drag and its inhibiting effect on snow erosion, (iii) the timescale of sastrugi aerodynamic adjustment can be as short as 3aEuro-h for friction velocities greater than 1aEuro-maEuro-s(-1) and during strong drifting snow conditions and (iv) knowing C-DN10 is not sufficient to estimate the snow erosion flux that results from drag partitioning at the surface because C-DN10 includes the contribution of the sastrugi form drag.