Wind-driven snow distribution patterns over an Antarctic ice floe

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.

Change in snow distribution patterns over an Antarctic ice floe following a snow storm event

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 quantify changes caused by a snow storm in the spatial distribution of snow over an Antarctic ice floe at resolutions of 1-10 cm and over 100 m x 100 m. The snow/ice elevations were obtained using a Terrestrial Laser Scanner during the SIPEX-2 in 2012. The pre-storm surface (2012-10-20) exhibits multi-directional elongated snow dunes behind aerodynamic obstacles likely formed during previous snow storms. 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. 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 snow storm events, although more datasets will be required to extend these results to a wider range of sea ice surface morphologies.

2015

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

Analysis of surface drag over Antarctic sea ice from High-Resolution measurements from Terrestrial Laser Scanning and Large Eddy Simulations

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

Sea ice/snow surface interactions with the atmosphere are major drivers of large-scale patterns of sea ice flows and deformations in Polar Regions. Climate and weather prediction models require parameterizations of the subgrid-scale surface drag caused by the interactions between the ice/snow surfaces and surface winds. Here, we combine the analysis of sea ice/snow surface topography at very high-resolutions (~1-10 cm), local wind measurements, and Large Eddy Simulations (LES) of fully developed wind flow to study surface drag and roughness from process scales to floe scales (~ 1 cm – 100 m). The snow/ice elevations were obtained using a Terrestrial Laser Scanner (TLS) 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. The effects of large-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 surface roughness parameter z0 for snow/ice is not known. Hence, a dynamic approach is utilized, in which this parameter is determined using the first-principles based constraint that the total momentum flux (drag) must be independent on grid-filter scale. LES are performed over fine scans of typical surface structures, and over larger domains filtered at coarser grid resolutions (~ 1 m). A total of six ice floes are studied, with before and after-storm TLS data at one of the floes for an analysis of the changes in the surface structure and its effects on the surface-atmosphere interactions. The spatial distributions of friction velocities (U*), wind profiles, and variations of z0 values along different directions are obtained from the LES for scenarios with variations in wind forcings and predominant wind directions relative to the general alignment of surface features. LES results are compared to spatial statistics of the surface at the locations to determine relationships with the general characteristics of the floes, and analyzing the variations between the different floes studied.

2014

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

2014

Analysis of surface drag over Antarctic sea ice from High-Resolution measurements from Terrestrial Laser Scanning and Large Eddy Simulations

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

2014