Directional dependence and spatio-temporal variability of aerodynamic roughness length over the East Antarctic Ice Sheet

Context : Over the East Antarctic Ice Sheet (EAIS), strong and persistent near-surface winds continuously reshape the snow surface, forming wind-carved microreliefs such as sastrugi and snow dunes (e.g., Poizat et al., 2024). These surface roughness elements develop according to the prevailing wind direction at the time of their formation. The combination of their geometry, spatial arrangement and orientation relative to the wind - that is, their morphological parameters - determine, to first order, the aerodynamic roughness length (z0). Because these microreliefs are inherently anisotropic, z₀ exhibits a pronounced directional dependence, varying with the angle between the wind and the dominant alignment of microreliefs (e.g., Vignon et al., 2017). z0 plays a central role in the parameterization of turbulent fluxes and the surface energy balance of glaciers and ice sheets (e.g., Favier et al., 2011), thereby influencing surface melt and ice mass loss. However, due to the scarcity of direct observations, z0​ remains poorly constrained in climate and surface mass/energy balance models, where it is often assumed constant or empirically tuned (Van Tiggelen et al., 2023).

Snow microreliefs undergo continuous transformation under the combined effects of accumulation, erosion, and snow metamorphism. As a result, z0 varies substantially in both space and time across the Antarctic continent. Its spatial variability reflects differences in the type and morphology of microreliefs, which are primarily controlled by local wind and accumulation regimes and the redistribution of snow by wind (Amory et al., 2015). For instance, in high-wind areas, erosional features such as sastrugi can lead to z₀ values several orders of magnitude higher than those observed over smooth blue-ice surfaces. Over time, the morphology of these microreliefs evolves in response to changing meteorological conditions through processes such as aerodynamic adjustment (Amory et al., 2016), burial by accumulation, or even surface deflation following rain or melt events, while enhanced snow cohesion due to sintering and wind hardening can inhibit erosion and hinder the aerodynamic streamlining of the surface (Amory et al., 2017).

Leveraging a unique, multi-year meteorological dataset collected across five sites spanning from the interior plateau to the windy coastal margins, this internship aims to investigate the physical mechanisms controlling the spatial and temporal variability of z0 across a latitudinal sector of the EAIS.

Objectives :
i) Estimation of z0 : Derive z0 from in situ wind speed profiles at multiple sites and over different periods.
ii) Methodological sensitivity : Assess how z0 ​estimates depend on data processing parameters such as minimum fitting height, measurement height, number of sensors, and atmospheric stability corrections.
iii) Directional dependency : Quantify the dependence of z0 on wind direction and relate it to the presence, preferential orientation and morphology of snow microreliefs.
iv) Spatio-temporal variability : Characterize how z0​ varies across sites and through time, and link these variations to changes in local wind and accumulation regimes.
v) Implications for modeling : Discuss how the observed variability in z0​ could be used to improve the representation of surface turbulent fluxes in climate and surface mass/energy balance models.

Required skills : Background in climate science, experience in handling meteorological datasets (preferably NetCDF format), proficiency in programming and data analysis, ideally in Python.

How to apply : Please send a CV and a short motivation letter (≤ 1 page) to vincent.favier univ-grenoble-alpes.fr and charles.amory univ-grenoble-alpes.fr

References :
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Amory, C., Gallée, H., Naaim-Bouvet, F., Favier, V., Vignon, E., Picard, G., Trouvilliez, A., Piard, L., Genthon, C., and Bellot, H. : Seasonal Variations in Drag Coefficient over a Sastrugi-Covered Snowfield in Coastal East Antarctica, Bound.-Lay. Meteorol., 164, 107–133, https://doi.org/10.1007/s10546-017-0242-5, 2017.

Amory, C., Naaim-Bouvet, F., Gallée, H., and Vignon, E. : Brief communication : Two well-marked cases of aerodynamic adjustment of sastrugi, The Cryosphere, 10, 743–750, https://doi.org/10.5194/tc-10-743-2016, 2016.

Amory, C., Trouvilliez, A., Gallée, H., Favier, V., Naaim-Bouvet, F., Genthon, C., Agosta, C., Piard, L., and Bellot, H. : Comparison between observed and simulated aeolian snow mass fluxes in Adélie Land, East Antarctica, The Cryosphere, 9, 1373–1383, https://doi.org/10.5194/tc-9-1373-2015, 2015.

Favier, V., Agosta, C., Genthon, C., Arnaud, L., Trouvillez, A., and Gallée, H. : Modeling the mass and surface heat budgets in a coastal blue ice area of Adelie Land, Antarctica, J. Geophys. Res., 116, F03017, https://doi.org/10.1029/2010JF001939, 2011.

Poizat, M., Picard, G., Arnaud, L., Narteau, C., Amory, C., and Brun, F. : Widespread longitudinal snow dunes in Antarctica shaped by sintering. Nat. Geosci. 17, 889–895, https://doi.org/10.1038/s41561-024-01506-1, 2024.

van Tiggelen, M., Smeets, P. C. J. P., Reijmer, C. H., van den Broeke, M. R., van As, D., Box, J. E., & Fausto, R. S. : Observed and parameterized roughness lengths for momentum and heat over rough ice surfaces. Journal of Geophysical Research : Atmospheres, 128, e2022JD036970. https://doi.org/10.1029/2022JD036970, 2023.

Vignon, E., Genthon, C., Barral, H., Amory, C., Picard, G., Gallée, H., Casasanta, G., and Argentini, S. : Momentum-and Heat-Flux Parametrization at Dome C, Antarctica : A Sensitivity Study, Bound.-Lay. Meteorol., 162, 341–367, https://doi.org/10.1007/s10546-016-0192-3, 2017.