Implications of surface flooding on airborne estimates of snow depth on sea ice
2021
Abstract. Snow depth observations from airborne snow radars, such as the
NASA's Operation IceBridge (OIB) mission, have recently been used in
altimeter-derived sea ice thickness estimates, as well as for model
parameterization. A number of validation studies comparing airborne and
in situ snow depth measurements have been conducted in the western Arctic
Ocean, demonstrating the utility of the airborne data. However, there have
been no validation studies in the Atlantic sector of the Arctic. Recent
observations in this region suggest a significant and predominant shift
towards a snow-ice regime caused by deep snow on thin sea ice. During the Norwegian
young sea Ice, Climate and Ecosystems (ICE) expedition (N-ICE2015) in the area north of Svalbard, a
validation study was conducted on 19 March 2015. This study collected
ground truth data during an OIB overflight. Snow and ice thickness
measurements were obtained across a two-dimensional (2-D) 400 m × 60 m grid.
Additional snow and ice thickness measurements collected in situ from
adjacent ice floes helped to place the measurements obtained at the gridded
survey field site into a more regional context. Widespread negative
freeboards and flooding of the snowpack were observed during the N-ICE2015
expedition due to the general situation of thick snow on relatively thin
sea ice. These conditions caused brine wicking into and saturation of the
basal snow layers. This causes the airborne radar signal to undergo more
diffuse scattering, resulting in the location of the radar main scattering
horizon being detected well above the snow–ice interface. This leads to a
subsequent underestimation of snow depth; if only radar-based information is
used, the average airborne snow depth was 0.16 m thinner than that measured
in situ at the 2-D survey field. Regional data within 10 km of the 2-D
survey field suggested however a smaller deviation between average airborne
and in situ snow depth, a 0.06 m underestimate in snow depth by the airborne
radar, which is close to the resolution limit of the OIB snow radar system.
Our results also show a broad snow depth distribution, indicating a large
spatial variability in snow across the region. Differences between the
airborne snow radar and in situ measurements fell within the standard
deviation of the in situ data (0.15–0.18 m). Our results suggest that seawater flooding of the snow–ice interface leads to underestimations of snow
depth or overestimations of sea ice freeboard measured from radar
altimetry, in turn impacting the accuracy of sea ice thickness estimates.
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