Implications of surface flooding on airborne estimates of snow depth on sea ice

Anja Rösel,Gunnar Spreen,Dmitry V Divine,Jaqueline Richter-Menge,Adam Steer,Vishnu Nandan,Jean-Charles Gallet,Sebastian Gerland,Sinead Louise Farrell

doi:10.5194/tc-15-2819-2021

Abstract

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.

Highlights

Snow and sea ice thickness in a changing Arctic climate system is the matter of many recent studies (e.g., Webster et al, 2018) since the snow layer on top of the frozen ocean generates several contradictory effects on the polar climate
2.4.3 In situ snow depth sea ice thickness and freeboards the drift that occurred during the EM31 and snow probe (SP) sampling of the 2-D survey field, we followed the procedure described in Rösel et al (2018): the EM31 data were resampled onto the coordinates of the SP track, and a Gaussian filter was applied to the EM31 data
The drill-hole measurements lie within the standard deviation of all measurements collected at the 2-D survey field site; i.e., our results demonstrate very good agreement across all observation methods (Fig. 5 and Table S1 in Supplement)

Summary

Introduction

Snow and sea ice thickness in a changing Arctic climate system is the matter of many recent studies (e.g., Webster et al, 2018) since the snow layer on top of the frozen ocean generates several contradictory effects on the polar climate. In winter, snow acts as an insulator between the relatively warm ocean and the cold atmosphere and hinders. A. Rösel et al.: Implications of surface flooding on airborne estimates of snow depth on sea ice the heat exchange between ocean and atmosphere, reducing the sea ice growth rate (Sturm, 2002; Perovich, 2003). In spring and summer, snow reflects shortwave radiation with its high optical albedo in the range of 0.7–0.85 and prevents the underlying sea ice with an albedo of about 0.6 from melting (Grenfell and Maykut, 1977; Perovich, 1996). Snow cover controls the amount of transmittance of photosynthetically active radiation affecting the productivity of primary algae and phytoplankton (Mundy et al, 2007). Snow can be a positive contributor to the sea ice mass balance since snow can transform to snow ice (Granskog et al, 2017; Merkouriadi et al, 2017a) and superimposed ice (Eicken et al, 2004; Wang et al, 2015)

Methods

Results

Conclusion