Local snow transport and surface evolution processes in East Antarctica

In Antarctica, direct measurements of processes shaping the surface mass balance such as precipitation, accumulation and erosion, drifting and blowing snow, and sublimation, are sparse and challenging due to the extreme environment and related difficulties in measuring the variables of interest. An ongoing project, ‘Local Surface Mass Balance in East Antarctica’, is designed to address these processes at the local scale with the objective of relating near-surface atmospheric dynamics, drifting and blowing snow, snow properties, and resulting accumulation and erosion patterns. Two snow mass balance stations at Princess Elisabeth, Antarctica, (72S, 23E, 1380m.a.s.l.) are collecting data since December 2016, including standard meteorological instruments, snow particle counters and 3-D sonic anemometers for combined analysis of wind, turbulence and mass flux. Pre- and post-storm Terrestrial Laser Scans (TLS) record the resulting surface morphological changes. This contribution presents analysis and preliminary results of the turbulence and wind data to obtain vertically integrated mass flux estimates. Calculated surface roughness and friction velocities are used for establishing formulations of vertical profiles of drifting and blowing snow. Stability corrections, auto- and cross-correlation functions of wind and mass flux, as well as the lag-time between sonic anemometers and snow particle counters are investigated. It is shown which vertical profile shape leads to consistent mass flux estimates and how magnitudes of calculated mass fluxes compare to earlier estimates. The comparisons suggest that mass transport has rather been underestimated in early and several current studies.

Local surface mass balance processes in East Antarctica

Francesco Comola, Wolf Hendrik Huwald, Michael Lehning, Eric Sauvageat, Varun Sharma, Nander Wever

In Antarctica, direct observations of key processes shaping the surface mass balance such as precipitation and snow deposition, wind transport of snow, and sublimation, are sparse due to the extreme environment and related difficulties in measuring the variables of interest. The ongoing project ‘Local Surface Mass Balance in East Antarctica’ (LOSUMEA) is designed to address these processes at the local scale with the objective of gaining insight on the dominant surface mass and energy balance components and their link to near-surface atmospheric dynamics and surface and sub-surface snow properties. To this end, two measurement stations have been installed near the Princess Elisabeth Station, Antarctica, in December 2016. Besides standard meteorological data the stations are equipped with snow particle counters and 3-D sonic anemometers relating wind, turbulence and mass flux. Pre- and post-storm Terrestrial Laser Scans (TLS) record the resulting surface morphological changes. While preliminary data is being analyzed, the full data set of the year 2017 is retrieved in early 2018. Addition of moisture sensors in 2018 is expected to provide valuable information on latent heat fluxes related to sublimation. This contribution presents an overview of the project experiments, the data obtained to date as well as their analysis and some first results.

2018

Spatially-resolved mean flow and turbulence help explain observed erosion and deposition patterns of snow over Antarctic sea ice

Marco Giovanni Giometto, Michael Lehning, Katherine Colby Leonard, Charles Vivant Ignacio Meneveau, Marc Parlange, Ernesto Trujillo Gomez

Sea ice-atmosphere interactions are major drivers of patterns of sea ice flows and deformations in Polar regions, and affect snow erosion and deposition at the surface. Here, we combine analyses of sea ice surface topography at very high-resolutions (~1-10 cm), and Large Eddy Simulations (LES) to study surface drag and snow erosion and deposition patterns from process scales to floe scales (~ 1 cm – 100 m). The snow/ice elevations were obtained using a Terrestrial Laser Scanner during the SIPEX II (Sea Ice Physics and Ecosystem eXperiment II) research voyage to East Antarctica (September-November 2012). LES are performed on a regular domain adopting a mixed pseudo-spectral/finite difference spatial discretization. A scale-dependent dynamic subgrid-scale model based on Lagrangian time averaging is adopted to determine the eddy-viscosity in the bulk of the flow. Effects of larger-scale features of the surface on wind flows (those features that can be resolved in the LES) are accounted for through an immersed boundary method. Conversely, drag forces caused by subgrid-scale features of the surface should be accounted for through a parameterization. However, the effective aerodynamic roughness parameter z0 for snow/ice is not known. Hence, a novel dynamic approach is utilized, in which z0 is determined using the constraint that the total momentum flux (drag) must be independent on grid-filter scale. We focus on three ice floe surfaces. The first of these surfaces (October 6, 2012) is used to test the performance of the model, validate the algorithm, and study the spatial distributed fields of resolved and modeled stress components. The following two surfaces, scanned at the same location before and after a snow storm event (October 20 and 23, 2012), are used to propose an application to study how spatially resolved mean flow and turbulence relates to observed patterns of snow erosion and deposition. We show how erosion and deposition patterns are correlated with the computed stresses, with modeled stresses having higher explanatory power. Deposition is mainly occurring in wake regions of specific ridges that strongly affect wind flow patterns. These larger ridges lock in place elongated streaks of relatively high speeds with axes along the stream-wise direction, and which are largely responsible for the observed erosion.

2014

Wind-driven snow distribution patterns over an Antarctic ice floe

Michael Lehning, Katherine Colby Leonard, Ernesto Trujillo Gomez

Sea ice, snow and atmosphere interactions are major drivers of the spatial distribution of snow over sea ice in polar regions. Here, we combine measurements of the wind flow, atmospheric conditions and blowing snow at two locations on an Antarctic sea ice floe, with terrestrial laser scanning to characterize a blowing snow storm and its influence on the spatial patterns of snow distribution at resolutions of 1-10 cm over an area of 100 m x 100 m. The datasets were obtained during the SIPEX II (Sea Ice Physics and Ecosystem eXperiment II) research voyage to East Antarctica (September-November 2012). The pre-storm surface (2012-10-20) exhibits multi-directional elongated snow dunes behind aerodynamic obstacles likely formed during previous snowstorms. The post-storm surface (2012-10-23) exhibits clear new deposition dunes elongated along the predominant wind direction. The new deposition areas amount to 38% of the total surveyed area. Patterns of erosion are less evident but cover a larger portion of the area. This results in a total volume of change near zero with a mean elevation difference of 0.02 m indicating that net erosion or deposition from snowfall was small despite of large mass relocation. After the storm, the statistical distributions of elevation and the 2D correlation functions remain similar to those of the pre-storm surface. The pre- and post-storm surfaces also exhibit power-law relationships in the power spectrum with little change between pre- and post-storm slopes. These observations suggest that despite the significant change observed in the snow surface patterns, the change does not translate into significant changes in the spatial statistical and scaling properties of the surface morphology. Such an observation is important for sea-ice model representations of the sub-pixel variability of sea ice surfaces, particularly between snowstorm events, although more datasets will be required to extend these results to a wider range of sea ice surface morphologies.